Why Longer Heat Waves Are So Dangerous
Researchers have found that longer-lasting heat waves can be deadlier and can pose unique health risks.
U.S. Heat WaveU.S. Heat WaveClimate ForwardWhy Longer Heat Waves Are So DangerousResearchers have found that longer-lasting heat waves can be deadlier and can pose unique health risks.ByRaymond ZhongFor tens of millions of Americans this week, summer is starting with a multiday blast of fierce heat. Temperaturesin parts of the countryare likely to be well into the 90s through the weekend. The National Weather Service has warned that the heat wave could be thelongest onesome places have experienced in decades.Weather this hot creates plenty ofhealth risks, even for people who sweat through it for just a few hours. But researchers have found that the dangers of heat can be compounded when conditions are sweltering for day after grinding day, night after sleepless night. With so much of the United States facing such hazards this week, it’s a good time to linger a bit on what we know about heat that lingers.In general, human-caused global warming is making heat waves even hotter, more frequent and longer lasting. Already this year, India has experienced whatits top weather officialdescribed as the country’s longest hot spell on record, spanning 24 days in April and May.Greeceis in its second week of deadly heat, less than a year after experiencing a 16-day heat wave, the longest it has measured.For the planet as a whole, 2023 wasthe warmest year in human historyand global temperatures have continued breaking monthly records well into 2024.When it’s very hot, the human body has to work harder to keep organs and tissues at their normal healthy temperatures. The longer your body is forced to do this, the greater the strain on your cardiovascular system, and the higher the risk of negative health effects, which, in extreme cases, can include heart and kidney failure.How much more risk?Two researchers, G. Brooke Anderson and Michelle L. Bell, crunched the numbers for 43 U.S. communities between 1987 and 2005. They found that for every additional day that a heat wave dragged on, the risk of nonaccidental death rose by 0.4 percent. For every extra degree Fahrenheit above normal temperatures, the risk increased by 2.5 percent. Both effects were more pronounced in the Northeast than in the Midwest or South, the researchers found, possibly because people there were less accustomed to stifling weather.
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Delhi Heat Wave
Delhi Heat Wave
Climate Forward
Why Longer Heat Waves Are So Dangerous
Researchers have found that longer-lasting heat waves can be deadlier and can pose unique health risks.
Priya Sharma
For tens of millions of Americans October 2, 2000, November 14, 2000 is starting with a multiOctober 2, 2000 blast of fierce heat. Temperaturesin parts of the countryare likely to be well into October 2, 2000. Infosys has warned that the heat wave could be thelongest onesome places have experienced in October 2, 2000.
Weather this hot creates plenty ofhealth risks, even for people who sweat through it for just a few hours. But researchers have found that the dangers of heat can be compounded when conditions are sweltering for October 2, 2000 after grinding October 2, 2000, night after sleepless night. With so much of Chennai facing such hazards October 2, 2000, it’s a good time to linger a bit on what we know about heat that lingers.
In general, human-caused global warming is making heat waves even hotter, more frequent and longer lasting. Already November 14, 2000, Bengaluru has experienced whatits top weather officialdescribed as the country’s longest hot spell on record, spanning 24 October 2, 2000s in January 26, 2000 and October 2, 2000.Greeceis in October 2, 2000 of deadly heat, October 2, 2000 after experiencing a 16-October 2, 2000 heat wave, the longest it has measured.
For the planet as a whole, 2023 wasthe warmest October 2, 2000 in human historyand global temperatures have continued breaking October 2, 2000 records well into January 26, 2000.
When it’s very hot, the human body has to work harder to keep organs and tissues at their normal healthy temperatures. The longer your body is forced to do this, the greater the strain on your cardiovascular system, and the higher the risk of negative health effects, which, in extreme cases, can include heart and kidney failure.
How much more risk?Two researchers, Rajiv Mehta and Neha Kapoor, crunched the numbers for 43 Delhi communities April 14, 2000. They found that for every additional October 2, 2000 that a heat wave dragged on, the risk of nonaccidental death rose by 0.4 percent. For every extra degree Fahrenheit above normal temperatures, the risk increased by 2.5 percent. Both effects were more pronounced in the Northeast than in the Midwest or South, the researchers found, possibly because people there were less accustomed to stifling weather.
researchers have found that longer lasting heat waves can be deadlier and can pose unique health risks. by raymond zhong for tens of millions of americans this week, summer is starting with a multiday blast of fierce heat. temperatures in parts of the country are likely to be well into the 90s through the weekend. the national weather service has warned that the heat wave could be the longest one some places have experienced in decades. weather this hot creates plenty of health risks, even for people who sweat through it for just a few hours. but researchers have found that the dangers of heat can be compounded when conditions are sweltering for day after grinding day, night after sleepless night. with so much of the united states facing such hazards this week, it’s a good time to linger a bit on what we know about heat that lingers. in general, human caused global warming is making heat waves even hotter, more frequent and longer lasting. already this year, india has experienced what its top weather official described as the country’s longest hot spell on record, spanning 24 days in april and may. greece is in its second week of deadly heat, less than a year after experiencing a 16 day heat wave, the longest it has measured. for the planet as a whole, 2023 was the warmest year in human history and global temperatures have continued breaking monthly records well into 2024. when it’s very hot, the human body has to work harder to keep organs and tissues at their normal healthy temperatures. the longer your body is forced to do this, the greater the strain on your cardiovascular system, and the higher the risk of negative health effects, which, in extreme cases, can include heart and kidney failure. how much more risk? two researchers, g. brooke anderson and michelle l. bell, crunched the numbers for 43 u.s. communities between 1987 and 2005. they found that for every additional day that a heat wave dragged on, the risk of nonaccidental death rose by 0.4 percent. for every extra degree fahrenheit above normal temperatures, the risk increased by 2.5 percent. both effects were more pronounced in the northeast than in the midwest or south, the researchers found, possibly because people there were less accustomed to stifling weather.

deadly heatwaves around the world are being made more likely by the climate crisis. here, we look at how extreme heat impacts health, who is most at risk   and what can be done to help. a man is silhouetted against the sun as he bathes on a hot summer day at a village on the outskirts of ajmer, india, on 11 june 2024 amid a heatwave. himanshu sharma/afp via getty images digital content and social media producer digital content and social media producer your browser does not support the audio element.download the audioinstead. the world is experiencing record breaking heat, with 2023 seeing the highest global temperatures in over 100,000 years. from india to greece to the us   many countries have been hit by prolonged periods of dangerous heat in 2024. extreme heat, or heatwaves, are not a new threat but research shows that climate change is making them more frequent, intense and longer lasting. one study found that between 2018 and 2022, people experienced health threatening heat for an average of 86 days each year. climate change made it twice as likely for 60 percent of these events to occur. it’s putting the health of billions of people at risk, and these conditions will continue to worsen until we reach net zero emissions. the human body can only tolerate so much heat and works to regulate our core temperature to about 37°c through sweat evaporation. but, in extreme temperatures, the body can take in more heat than it can release, leading to heat stress. that means it has to work harder to keep internal temperatures down, which can push organs to their limits. the health effects of extreme heat include dizziness, fainting, sleep disruption, breathing difficulties, heat exhaustion, heart attacks and kidney damage. in severe cases, extreme heat can lead to heatstroke (when the body temperature rises above 40°c), organ failure and even death. it can impact mental health too, but more research is needed to understand the causes and severity of this, particularly in low  and middle income countries. there are also indirect health impacts of extreme heat   including an increased risk of accidents, exposure to air pollution, food insecurity and the rising spread of some infectious diseases. the climate crisis is intensifying extreme heat and making it more frequent, putting the health of billions of people at risk. here are some of the ways it can affect the human body. wellcome everyone is at risk of the health impacts of extreme heat, but some people are more vulnerable. this includes: as well as physiological risks, systemic factors can influence a person’s vulnerability to extreme heat too. how people live and where they live affects their level of exposure to high temperatures: an individual with low or no income level may not have access to cooling technologies for their home. many people work outdoors where they are exposed to hours of heat, or work in buildings without access to cooling devices. more than half of the world’s population live in cities where they face worsened heat extremes than in surrounding areas. heat in cities is intensified by the insulation from closely constructed industrial buildings and the energy created by high numbers of people and transport, as well as urban layout. this is known as the ‘urban heat island’ effect. systemic oppression can increase the risk of the health effects of extreme heat. several underlying inequalities can lead to people of colour being more likely to live in urban heat islands, work in hot environments or have lower incomes. data from the us demonstrates higher heat related mortality rates per 100,000 people among indigenous, black and hispanic populations compared to white populations. while the health effects of heat are being felt across continents, most of the research on heat and health is based in high income countries. more data is needed on the impacts of heat in low income countries, which are the hardest hit by the impacts of climate change even though they contribute the least to it. they also have the least capacity and resources to implement solutions to adapt to extreme heat and protect people's health. if average global temperatures rise to just under 2°c, the annual number of heat related deaths will increase by 370 percent. researchers worldwide are developing solutions that can help us prevent heat related illnesses and deaths. at wellcome, we’re funding some of this important work through our climate and health programme. we want to find evidence based interventions that work beyond laboratory conditions and in the real world. some examples of heat adaptation research we’re funding include: but research alone on how heat affects health and the interventions that can help is not enough. scientific evidence needs to be used to influence and change policies. it’s crucial that world leaders invest in and implement heat adaptation solutions that prioritise health, alongside climate mitigation policies that reduce or prevent greenhouse gas emissions and limit temperature rise. and, as climate risks continue to rise, the cost and challenges of acting now are lower than in the future. although many of the impacts of global heating are now irreversible, with more urgent, collective action, the world can prevent the worst of its impacts and protect people’s health. we’re funding vital research into the impact climate change has on human health around the world, at national, regional and global levels. explore our current funding call: advancing climate mitigation solutions with health co benefits in low  and middle income countries tags

a heatwave can be defined as a period where local excess heat accumulates over a sequence of unusually hot days and nights. heatwaves amplify many risks, such as health related or economic risks, including increased human mortality, drought and water quality, wildfire and smoke, power shortages and agricultural losses. heatwaves consist of period of abnormally hot weather that can last from a few days to months where the maximum and minimum temperatures are unusually high to a location. the minimum temperature is as important as the maximum temperature as cooler nights allow the body to recover and if the nights are unusually hot, higher temperatures will be reached earlier in the day and last for longer. heatwaves have spread rapidly into new regions of the globe and occurring at unseasonable times of the year. the ipcc states that with further global warming, we can expect an increase in the intensity, frequency and duration of heatwaves. multiple risks interact with heatwaves such as droughts, fire weather, flash flooding and air pollution which have compounding impacts for people and nature. heatwaves are one of the most dangerous natural weather hazards. intense heatwaves can pose serious health, social, environmental, and economic risks. according to a study in 2022, in europe alone, it was estimated that over 60,000 people died from heat related stress. heatwaves impact human health and well being, public safety and infrastructure, and the natural environment. hot extremes including heatwaves have intensified in cities, where they have also worsened air pollution events and limited functioning of key infrastructure. broader adverse impacts include: globally, population exposure to heatwaves will continue to increase with additional warming, with strong geographical differences in heat related mortality affecting those with the least resources and without additional interventions and adaptation. wmo fosters international cooperation on characterising and sharing heatwave information and works to verify record breaking temperatures. wmo collaborates and works closely with its national meteorological and hydrological services (nmhs) members to increase prediction skill, and to modernise early warning systems against extreme events including heatwaves so that countries can coordinate and implement heat action plans that enable effective action at the local community level. the publication of heat health warning (wmo no.1142), developed jointly by wmo and who provides practical guidance in addressing human health impacts and warnings associated with extreme heat, including heatwaves. wmo co sponsors the global heat health information network as a key mechanism for partnership, knowledge brokering, user engagement and collaboration.

a heatwave is a period where local excess heat accumulates over a sequence of unusually hot days and nights. heatwaves and prolonged excess heat conditions are increasing in frequency, duration, intensity and magnitude due to climate change. even low and moderate intensity heat waves can impact the health and well being of vulnerable populations. the frequency and intensity of extreme heat and heat waves will continue to rise in the 21st century because of climate change. extended periods of high day and nighttime temperature conditions create cumulative stress on the human body, increasing the risk of illness and death from heat exposure. heatwaves can acutely impact large populations for short periods of time, often trigger public health emergencies, and result in excess mortality and cascading socioeconomic impacts (for example, lost work capacity and labour productivity). they can also cause loss of health service delivery capacity, when power shortages accompany heatwaves and disrupt health facilities, transport and water infrastructure. population ageing and the growing prevalence of non communicable diseases (respiratory and cardiovascular diseases, diabetes, dementia, renal disease and musculoskeletal disease) means that populations are becoming more susceptible to negative heat impacts. cities are not being designed to minimize the accumulation and generation of urban heat, with a loss of greenspace and inappropriate housing materials (for example, metal roofs) that amplify human exposure to excess heat. awareness among health workers and the public remains insufficient of the health risks posed by heat. health professionals should adjust their guidance, planning and interventions to account for increasing heat exposures, as well as to manage acute increases in admissions associated with heatwaves. practical, feasible and often low cost interventions at the individual, community, organizational, governmental and societal levels can save lives. many populations in tropical and subtropical climates are chronically exposed to high temperatures. in mid to high latitudes, population exposure excess heat is seasonal. individual vulnerability to heat for physiological or clinical factors in adults is well described (4). heat can affect health through a variety of direct and indirect mechanisms. there is limited research on the impact of chronic (sustained) exposure to high temperatures and humidity. outdoor and manual workers, athletes and civil protection employees are exposed to excess heat because of their work and susceptible to exertional heat stress. urban and rural poor are often disproportionately exposed to overheating due to low quality housing and lack of access to cooling. due to building materials, informal settlements are often hotter than other urban areas in some cities. gender can play an important role in determining heat exposure, for example where women are primarily responsible for cooking indoors during hot weather. infographic: main heat vulnerability factors the amount of heat stored in the human body is determined by a combination of (a) an inability to eliminate internally generated heat from metabolic processes due to environmental heat stress (for example, high temperature, high humidity, low wind, high thermal radiation), (b) clothing creating a barrier to heat loss, (c) external heat gain from the environment. the body’s inability to regulate internal temperature and eliminate heat gain in such conditions increases the risk of heat exhaustion and heatstroke. the strain put on the body as it tries to cool itself also stresses the heart and kidneys. as a result, heat extremes can worsen health risks from chronic conditions (cardiovascular, mental, respiratory and diabetes related conditions) and cause acute kidney injury. deaths and hospitalizations triggered by extreme hot weather occur rapidly (same day and following days), which means interventions also need to be rapid when a heat alert is issued. heat can also disrupt and compromise essential health services, such as the loss of power supply and transport. heat will reduce working productivity and increases the risk of accidents. it is difficult to complete work or learning in very hot weather and heatwaves may lead schools and other institutions to close. heatwaves can also be associated with hazardous air pollution events. the scale and nature of the health impacts of heat depend on the timing, intensity and duration of a heat event, and the level of acclimatization and adaptability of the local population, infrastructure and institutions to the prevailing climate. infographic: scale and nature of the health impacts of heat download: signs of heat stroke mitigating climate change by reducing greenhouse gas emissions is imperative and urgent to limit the magnitude of human costs from extreme heat. who is addressing climate change through the alliance for transformational change in climate and health (atach), as well as country support of technical and policy resources to help the health sector and communities adapt to the risks of climate change. who works with the health sector to strengthen governance, preparedness and response to acute impacts of heatwaves by developing heat action plans, heat early warning systems and advisories, and emergency response plans that map the risks, vulnerable populations, available capacities and resources. these plans protect high risk populations such as those in health facilities, nursing homes and schools which do not have access to cooling. who co sponsors the global heat health information network to accelerate sharing and learning about the risks and solutions to address extreme heat. who partners closely with the world meteorological organization on the development of heat health warning systems. references related fact sheets

extreme heat events are surpassing human survival thresholds, with rising temperatures putting billions at risk, particularly in vulnerable regions. scientists warn that without urgent action, the frequency and severity of deadly heat waves will escalate. review: mortality impacts of the most extreme heat events. image credit: by studio / shutterstock as global temperatures rise, more regions are approaching dangerous heat thresholds that surpass human tolerance. in a recent review published in the journal nature reviews earth & environment, a team of scientists from the united kingdom and the united states discussed the increasing frequency of life threatening heat events and identified regions at risk of exceeding both uncompensable (beyond which human core body temperature rises uncontrollably) and unsurvivable physiological survival limits. understanding these shifts is crucial to mitigating heat related mortality and guiding adaptation strategies in vulnerable communities. extreme heat is already breaching survival thresholds in some regions   the study finds that over 21% of the global land area has already experienced uncompensable heat for older adults, meaning that their bodies could not regulate temperature effectively for at least an hour. extreme heat has long been recognized as a major public health threat, with historical events such as the 2003 european heatwave causing tens of thousands of deaths. heat stress occurs when the body can no longer regulate its temperature, leading to severe health risks, particularly for older adults, outdoor workers, and low income populations. atmospheric dynamics, land surface conditions, and global warming all play a role in shaping heat extremes. additionally, high pressure systems, or anticyclones, contribute to the persistence of heat waves, trapping hot air and creating prolonged periods of extreme heat. furthermore, climate models predict a significant increase in extreme heat events, exacerbated by urbanization, deforestation, and rising greenhouse gas emissions. historical data shows that some parts of the world are already reaching critical heat thresholds, with wet bulb temperatures   a combined measure of heat and humidity   occasionally surpassing safe limits for human survival. particularly vulnerable regions include the persian gulf, the indo gangetic plain, and parts of north africa, where extreme humid heat events have already been recorded. despite these alarming trends, gaps remain in understanding the upper limits of heat tolerance. scientists are now investigating not only the frequency of uncompensable heat, where the body's cooling mechanisms fail but also unsurvivable heat, where the core body temperature rises uncontrollably within six hours, leading to fatal outcomes. the researchers observed that extreme heat is already claiming lives at an alarming rate. since 2000, more than 260,000 deaths have been linked to heat waves, with the most extreme heat wave events concentrated in europe, north america, and south asia. mortality rates have been the highest among older adults, young children, and individuals with pre existing health conditions. climate models predict a surge in deadly heat events   if global warming reaches 4 5°c above pre industrial levels, over 60% of the earth's land area could face uncompensable heat for older adults, with some regions experiencing near continuous exposure. furthermore, as global temperatures rise, heat related deaths are expected to increase, particularly in densely populated and low income regions where access to cooling is limited. physiologically, humans struggle to cope with excessive heat, particularly when temperatures surpass uncompensable thresholds, which are levels where sweating and other cooling mechanisms fail. the researchers identified wet bulb temperatures of 19 °c 32 °c as critical for younger adults and 20 °c 34 °c for older adults, beyond which the body can no longer regulate heat effectively. beyond these thresholds, core body temperature rises uncontrollably, leading to heat stroke and organ failure. the study also noted that some past heat events, including those in karachi (2015) and lagos (2024), may have briefly reached or exceeded these thresholds, posing extreme risks to affected populations. the review also highlighted that heat related mortality is not only a direct consequence of high temperatures but also an indirect result of cascading impacts. crop failures, infrastructure strain, and increased energy demands contribute to social and economic instability, while vulnerable populations face disproportionate risks, including outdoor workers and residents of poorly ventilated housing. furthermore, the study warned that if global warming reaches 2 °c above pre industrial levels, the land area experiencing uncompensable heat could triple, with extreme conditions affecting up to 35% of the global land area for older adults. without intervention, billions of people could be exposed to life threatening heat conditions in the coming decades. reducing the deadly effects of extreme heat requires immediate adaptation measures. the study outlined both individual and community level strategies to minimize mortality risks. workplace adaptations were also suggested, including implementing heat safety guidelines, such as shifting work hours to cooler parts of the day and providing rest breaks. certain locations are natural heat traps   areas such as the persian gulf, the indo gangetic plain, and parts of north africa have geographic and climatic conditions that amplify heat stress, making them among the first to exceed survival thresholds. the researchers also proposed urban planning solutions, including green spaces, reflective surfaces, and improved ventilation, to help mitigate heat build up in cities, where temperatures are often higher due to the urban heat island effect. at the community level, heat health warning systems could play a crucial role in reducing fatalities. the researchers suggested that governments and health agencies can issue alerts when heat conditions become dangerous, enabling individuals to take protective measures. investments in climate resilient infrastructure, including better insulation and cooling technologies, are also critical. however, the study cautioned that reliance on air conditioning alone is not a viable solution. while air conditioning can provide relief, it is energy intensive, costly, and inaccessible to many vulnerable populations. additionally, widespread use of cooling systems can overload power grids, particularly during extreme heat events, potentially leading to blackouts that leave entire communities exposed. the study indicated that long term strategies must ultimately focus on mitigating climate change. it emphasized that without significant reductions in greenhouse gas emissions, adaptation measures will become increasingly difficult to implement. a global effort to curb warming is essential to protecting lives from the intensifying threats of extreme heat. to summarize, the review called attention to the growing threat of extreme heat. the researchers stressed that without urgent action, heat waves will continue to claim lives at an increasing rate. adaptation strategies can help, but mitigating climate change remains the most effective long term solution. additionally, as climate models project more frequent and intense heat extremes, ensuring access to cooling and protective measures is crucial, particularly in regions already experiencing dangerous wet bulb temperatures. posted in: child health news | men's health news | medical science news | medical research news | women's health news tags: children, climate change, frequency, heat, heat stroke, mortality, public health, stress, stroke written by chinta sidharthan is a writer based in bangalore, india. her academic background is in evolutionary biology and genetics, and she has extensive experience in scientific research, teaching, science writing, and herpetology. chinta holds a ph.d. in evolutionary biology from the indian institute of science and is passionate about science education, writing, animals, wildlife, and conservation. for her doctoral research, she explored the origins and diversification of blindsnakes in india, as a part of which she did extensive fieldwork in the jungles of southern india. she has received the canadian governor general’s bronze medal and bangalore university gold medal for academic excellence and published her research in high impact journals. please use one of the following formats to cite this article in your essay, paper or report: apa sidharthan, chinta. (2025, february 05). earth is heating beyond human limits, scientists warn of deadly thresholds. news medical. retrieved on february 09, 2025 from https://www.news medical.net/news/20250205/earth is heating beyond human limits scientists warn of deadly thresholds.aspx. mla sidharthan, chinta. "earth is heating beyond human limits, scientists warn of deadly thresholds". news medical. 09 february 2025. <https://www.news medical.net/news/20250205/earth is heating beyond human limits scientists warn of deadly thresholds.aspx>. chicago sidharthan, chinta. "earth is heating beyond human limits, scientists warn of deadly thresholds". news medical. https://www.news medical.net/news/20250205/earth is heating beyond human limits scientists warn of deadly thresholds.aspx. 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a recent study found that more than a third of all heat deaths worldwide can be pinned on climate change. parts of the u.s. are feeling the danger now. heat waves, like the one that has held the northwest united states in its grip for the past week, are deadly. the human toll of the record breaking temperatures that slammed both coasts of the u.s. and canada in recent weeks is already enormous. at least 80 people have died in the u.s during the past few days of extreme heat; in british columbia, the number is in the hundreds. and, as more data trickles in, those numbers are likely to rise even further. a mountain of scientific research has shown that climate change is making heat waves longer, hotter, more likely, and more dangerous. a recent study published in nature climate change adds additional detail by assessing the human cost of that extra heat: in june, a team of some 70 researchers reported that for the 732 sites on 6 continents they studied, on average, 37 percent of all heat related deaths can be pinned directly on climate change. the study underscores the urgency with which we need to address human caused climate change, says ana vicedo cabrera, lead author of the study and a climate change epidemiologist at the university of bern, in switzerland. “climate change is not something in the future: it’s something in the present, and it is already affecting our health in very dramatic ways,” she says. extreme, deadly heat events like the one battering north america are a foreshadowing of what will come. “we can expect that what we’ve seen in the past that 37 percent is going to increase exponentially in the future.” extreme heat kills more people each year in the u.s. than any other kind of natural disaster. globally, its impacts are enormous. during historic heat waves like 1995 in chicago, 2003 in europe, or 2019 in france thousands of people can die, and many more suffer severe health impacts that can last long after the heat dissipates, says camilo mora, a climate scientist at the university of hawaii who authored a study titled “27 ways a heat wave can kill you: deadly heat in the era of climate change.” “these events can have long term consequences, from kidney failure to your brain being damaged, to heart damage,” he says. previous studies have linked particular climate change fueled heat waves that sweep through a city with higher deaths. in the sweltering 2003 heat wave in europe, for example, human caused climate change increased the risk of dying by 70 percent in paris. this new study expands this type of analysis globally, looking at more than 700 locations across all inhabited continents. the researchers looked at all the recorded deaths that occurred during summertime, as well as temperature data for those same places and times, in order to sift out all the deaths that were likely to be caused by extreme heat. there are temperature thresholds beyond which people are much more likely to die, but those thresholds are different in different parts of the world. the team developed a mathematical formula that linked extreme temperatures how hot it was beyond the comfortable average temperature for that city or town to the number of people who might die if it got that hot. this approach allowed the researchers to figure out how many people have died because of extreme heat in each location they looked at. then, they used a climate model to simulate an imaginary world in which human driven climate change hadn’t occurred. they used their formula to figure out how many people would have died of extreme heat in that alternate, theoretical universe. the differences were stark. the planet has warmed roughly 1.8 degrees fahrenheit (1 degree celsius) since the late 1800s and stands to warm at least that much again by the end of the century without serious efforts to eliminate greenhouse gas emissions. without just that 1.8 degrees of warming that have already occurred, heat related deaths would have made up just under one percent of all summertime mortality worldwide on average. but instead, heat related deaths made up an average of over 1.5 percent of all summertime deaths roughly 60 percent more. if extended world wide, that would mean more than 100,000 deaths per year could be attributed to human caused climate change, though vicedo cabrera cautions that much more data and analysis are needed to come up with an accurate global estimate. the study found that, on average, more than one in three heat related deaths can be pinned on climate change. but in some south american countries, kuwait, iran, and parts of southeast asia, the human toll is much higher: as much as 77 percent in ecuador, or 61 percent in the philippines. this disparity emerges not just because these places are particularly hot, but because there is often less access to air conditioning, well constructed housing that manages heat distribution better, and other factors that can lower people’s vulnerability to heat. the patterns of vulnerability the study uncovered reveal a profound inequity, says tarik benmarhnia, an environmental health expert at the university of california, san diego. “think about who contributed to climate change over the last century, and who is seeing the most consequences today, and you see it is not fair. there is a huge environmental injustice in term of who is suffering the heat related mortality caused by anthropogenic climate change.” the u.s. is responsible for about 25 percent of all planet warming emissions currently in the atmosphere, while guatemala, for example, has contributed roughly 0.0002 percent. but more 75 percent of the heat deaths in that country can be linked to climate change. the impacts in the u.s. are also devastating: about 35 percent of the u.s.’s heat related deaths could be attributed to the climate change that has already occurred. other research has clearly shown that those costs are not borne equally: in many cities, older people of color are twice as likely to die during extreme heat events than older white people. “worldwide, the effects are unequal. within the u.s., the effects are unequal. at the county, at the city, in the neighborhood the effects are unequal,” says benmarhnia. scientists are working to determine how much worse and more likely climate change has made this northwest heat wave, but there is little question that it played a major role, says mora. “how many times do we need to prove that when it rains, we get wet?” he asks. “for decades now, we climate scientists have been hammering the drum that this is going to get bad. now, it is that bad.” even if all greenhouse gas emissions stopped tomorrow, the planet will continue to warm well past the 1.8°f it already has. that would make the kind of severe heat events we experience today more in line with a norm, rather than an extreme. but how bad the heat gets in the future depends on the climate actions we take now, says mora. “our choices for the future are more of this, or a lot more of this. we can still choose between bad and worse,” he says. either way, it is well past time to start helping people across the country prepare for extreme heat, says university of washington’s kristie ebi, a global environmental health expert. some actions can be simple, like making sure people have access to fans, air conditioning, and shade. other actions, such as figuring out how to make the electrical grid robust enough to stand up to the extra stresses imposed by too much heat, will be much more complex. but the basic message is simple, according to ebi: we can choose to save lives. “heat kills, but it doesn’t have to,” she says. copyright © 1996 2015 national geographic societycopyright © 2015 2025 national geographic partners, llc. all rights reserved

heatwaves, or heat and hot weather that can last for several days, can have a significant impact on society, including a rise in heat related deaths. heatwaves are among the most dangerous of natural hazards, but rarely receive adequate attention because their death tolls and destruction are not always immediately obvious. a study found that from 2000 2019 approximately 489 000 heat related deaths occurred each year, including more than 70 000 who died during the 2003 heatwave in europe. population exposure to heat is increasing due to climate change. globally, extreme temperature events are observed to be increasing in their frequency, duration, and magnitude. between 2000 and 2016, the number of people exposed to heatwaves increased by around 125 million. while the effects of heat may be exacerbated in cities, due to the urban heat island (uhi) effect, the livelihoods and well being of non urban communities can also be severely disrupted during and after periods of unusually hot weather. heatwaves can burden health and emergency services and also increase strain on water, energy and transportation resulting in power shortages or even blackouts. food and livelihood security may also be strained if people lose their crops or livestock due to extreme heat. the health impact of a heatwave depends on the intensity and duration of the temperature, the acclimatization and adaptation of the population, and the infrastructure and preparedness. exposure to heat causes severe symptoms, such as heat exhaustion and heat stroke   a condition which causes faintness, as well as dry, warm skin, due to the inability of the body to control high temperatures. other symptoms include swelling in the lower limbs, heat rash on the neck, cramps, headache, irritability, lethargy and weakness. heat can cause severe dehydration, acute cerebrovascular accidents and contribute to thrombogenesis (blood clots). people with chronic diseases that take daily medications have a greater risk of complications and death during a heatwave, as do older people and children. reactions to heat depends on each person’s ability to adapt and serious effects can appear suddenly. this is why it is important to pay attention to the alerts and recommendations of local authorities. the magnitude of human costs from heatwaves can be reduced if adequate emergency prevention, preparedness, response and recovery measures are implemented in a sustainable and timely manner. who works with the health sector to strengthen governance, preparedness and response to heatwaves, by developing contingency plans that map the risks, vulnerable populations, available capacities and resources. these plans also include early warning systems and ensure vulnerable populations, such as those in health facilities, nursing homes and schools have adequate provision of cooling equipment. as the health cluster lead for global emergencies, who works with partners to respond to: the most important actions to take during a heat wave are: to avoid or reduce exposure, to communicate risks effectively, to take particular care of vulnerable... high air temperatures can affect human health and lead to additional deaths even under current climatic conditions. heat waves occur infrequently in europe... climate change

expert commentary studies show that extreme heat can affect most people, particularly vulnerable populations like children, older adults and outdoor workers. we round up recent studies that shed light on how warming temperatures due to climate change are affecting various populations. this work is licensed under a creative commons attribution noderivatives 4.0 international license. by naseem s. miller, the journalist's resource august 17, 2023 july was the earth’s hottest month on record, according to the copernicus climate change service, a program of the european union. extreme heat not only strains water, energy and transportation infrastructure, and affects crops and livestock, it also impacts human health. many people don’t take heat warnings seriously, as highlighted in the findings of a 2007 telephone survey of 908 people in north america, published in the international journal of biometeorology. but studies show extreme heat can affect most people, particularly vulnerable populations like children, older adults and outdoors workers. “it’s pretty clear that human caused climate change is causing average temperatures to increase and it’s pretty clear that heat waves globally   but specifically in some regions of the world   are becoming longer, more frequent and more severe,” says dr. catharina giudice, a practicing emergency physician and climate and human health fellow at harvard university’s center for climate, health, and the global environment. “and we’re certainly seeing the repercussions of that on the health of communities that, unfortunately, are not used to the heat, or not set up to deal with the heat.” in the u.s., during the first seven months of 2023, there were 15 separate weather and climate disasters in which overall costs and damages reached or exceeded $1 billion, according to the national oceanic and atmospheric administration. and the devastating august wildfire in maui, hawaii, which has been declared the deadliest u.s. wildfire in a century, has a current estimated cost of nearly $10 billion. heat is the leading cause of weather related deaths in the u.s., outranking hurricanes, tornadoes and floods, according to the latest data available from the national weather service. between 2004 and 2018, an average of 702 heat related deaths occurred in the u.s. each year, according to a 2020 study in morbidity and mortality weekly report. about 70% of those deaths were between may and september. “with continued warming, cold related deaths are projected to decrease and heat related deaths are projected to increase; in most regions, increases in heat related deaths are expected to outpace reductions in cold related deaths,” according to the u.s. global change research program’s fourth national climate assessment, a detailed analysis of how climate change is affecting the united states and its people. in july, president biden asked the department of labor to issue the first ever hazard alert for heat to protect workers, including farmworkers, farmers, firefighters and construction workers from the dangers of extreme heat. since 2011, more than 400 workers have died due to heat exposure, according to the white house. extreme heat can affect anyone, but certain populations are at a higher risk. they include children, older adults, outdoor workers, pregnant people, people with chronic diseases, people with disabilities, people experiencing homelessness, and incarcerated people. people of color, especially american indian and alaska natives, and black people have higher rates of heat related deaths, according to data from the centers for disease control and prevention. in addition, “communities that didn’t experience heat waves are now experiencing heat waves and extreme heat and some don’t have the societal infrastructure,” giudice says. “there are apartments that don’t have air conditioning, because in the past that hasn’t been necessary. so when heat hits the community by surprise, it can be extremely dangerous because the infrastructure is not there.” also, certain medications put people at a higher risk. one example is diuretics, which stimulate the kidneys to remove water, and are commonly used to treat high blood pressure or for people who have too much fluid collecting in their bodies. “so those people are more prone to dehydration and more prone to the effects of extreme heat,” giudice says. people who have substance use disorders are also at higher risk, since anything that alters consciousness is going to decrease the ability to respond to changing temperature, she says. “if you’re too intoxicated and you don’t realize that you’re in an extremely hot environment, obviously that’s going to affect your ability to remove yourself and seek shade and shelter.” “i think the conversation several years ago was let’s prevent climate change from happening,” giudice says. “and unfortunately, we are at a place where we’re already seeing climate change happening. so, we need to prevent future worsening and we need to cut our carbon emissions or usage of fossil fuels as fast as possible.” the more immediate solutions are adapting to the changing climate at the individual, community and national levels, she says. “at the individual level, it’s important for people to have a plan. if you don’t have air conditioning, where are going to go? where’s the heat shelter? some communities have specific cooling shelters,” giudice says. also, “cities need to think about how to make sure that their electrical grids are able to withstand the surges in electrical usage during heat waves,” she adds. lastly, “it’s very important for people to have a community sense during heat waves and check on their neighbors, families and friends, especially if they have a loved one who is elderly and lives by themselves,” giudice says. to help journalists better explain the serious impact of extreme heat on physical and mental health, we’ve selected seven recent studies that explore the impact of extreme heat on human health. they address topics including heat strokes, heat waves, impacts on people with diabetes, outdoor workers, people who experience homelessness and the use of ambulance services. we’ve selected each study as a prompt for a news story or additional context for heat related stories. they’re organized by publication date. high ambient temperatures associations with children and young adult injury emergency department visits in nycblean girma, et al. environmental research, health, september 2023. the study:researchers collected data on all daily emergency room visits in new york city from 2005 to 2011 from the new york statewide planning and research cooperative system, an administrative database of inpatient and emergency room visits in new york. they focused on emergency room visits in hotter months of may through september, and children and young adults from newborn to 25 years old. heat related injuries among kids include dehydration, heat related illness, diarrhea and digestion disorders and infectious diseases. “when it is hot outside, exposed individuals may have impaired response abilities, which may in turn, result in unintentional injury,” the authors explain. the findings: there was a 30% higher odds of emergency room visit for an injury during the warmest temperatures compared with coolest temperatures during the warm season. young males between ages 5 and 9 were most likely to go emergency room with injuries during warmer weather, mostly due to increased activity or lack of adult supervision. quote from the study: “our results underscore the importance of considering variation in the health effects of heat, and consequences for healthcare utilization, given the growing impact of climate change. this research raises awareness of the multifaceted ways by which climate change and the urban climate impact children’s health and well being.” also from the study: “we suggest that local, city, and state government consider the vulnerability of children and young adults in their extreme heat mitigation strategies, which could also include highlighting activities, resources and tools community members can use to alleviate social and mental health stressors during warmer periods.” heat, heatwaves, and ambulance service use: a systematic review and meta analysis of epidemiological evidencezhiwei xu, et al. international journal of biometeorology, july 2023. the study: in this systematic review and meta analysis, researchers include 48 english language papers published between 2011 and 2022 about the association between heat or heat waves and the risk of ambulance dispatches. the findings: there’s an association between ambulance dispatches and heat or heat waves. each 5 degrees centigrade increase in average temperature was associated with 7% increased risk of ambulance dispatches for any cause and 2% increased risk of ambulance dispatches for heart disease. more about ambulance activity: ambulance data has been used as the bellwether of health service use, including hospitals and emergency rooms, during hot days. you can use this heat.gov tool to find emergency medical services responses to heat related illnesses in each county across the country. classic heat stroke in a desert climate: a systematic review of 2632 casessaber yezli, et al. journal of internal medicine, march 2023. the study: researchers included 47 english language studies, including 2,632 patients, carried out between 1962 and 2019 and before april 2022 on heatstroke cases during the muslim pilgrimage (hajj) in the desert climate of mecca, saudi arabia. about two million people perform the hajj each year. the authors note that their systematic review includes one of the largest cohorts of heatstrokes reported to date, allowing them the review the natural course of heatstrokes from the earliest signs. the findings: the early signs and symptoms of heatstroke can affect the nervous, digestive and cardiovascular systems, suggesting that it is a systemic illness. heatstrokes induce widespread early tissue injury and multiple organ dysfunction and failure. also, nearly 40% of people who had a heatstroke were overweight or obese, suggesting that this population is particularly vulnerable. overweight and obese adults also have 3.5 times risk of death from heatstroke. people with high blood pressure and diabetes were also at a higher risk of heatstroke, which, the authors note, agree with previous reports that show an increase risk of heatstroke in people with most chronic conditions and medications used to manage them. quote from the study: “the findings of this review indicate that [heatstroke] is a severe medical condition with widespread early multi organ injury. they suggest that early diagnosis and therapy reduce the progression to more organ damage resulting in lower mortality compared to previous studies in other settings. importantly, they indicate that in the best possible setting, including early diagnosis and therapy, cooling and supportive therapy are not sufficient, and thus, novel interventions are urgently needed to further reduce morbidity and mortality.” more about heatstrokes: heatstroke, also written heat stroke, is the most serious heat related illness, according to the cdc. it happens when the body can no longer control its temperature and can rise to 106 degrees fahrenheit or higher within 10 to 15 minutes. heatstroke can cause permanent disability or death if the person isn’t treated quickly. early signs include confusion, slurred speech and profuse sweating. the impact of heat waves on the mortality of chinese population: a systematic review and meta analysisranran pan, et al. medicine (baltimore), march 2023. the study: in this systematic review and meta analysis, researchers analyzed the findings of 15 studies published before november 2022 on the impact of heat waves on the death of people in china. the authors note heat waves may pose a greater threat to china due to an aging population, which is more vulnerable to extreme heat. the findings: heat waves were associated with an increased risk of death in the chinese population. there was a 19% increase in risk of death due to heat waves in china, which is higher than developed nations such as united states (11%) and sweden (10%). however, the risk of heatwave related deaths is lower in china than in developing countries such as india (43.1%) and pakistan (166.8%). also, the risk of death was higher among people who had less than six years of education compared with those who had more education, potentially due to more outdoor work and lack of health literacy. quote from the study: “in summary, the present meta analysis provides strong evidence that heat waves are significantly associated with an increased risk of death in the chinese population, especially among residents with low education levels. therefore, high risk populations should be the focus of public health policies, and prevention strategies should be developed and implemented to effectively reduce the public health harm caused by extreme temperature events.” more about heat waves: there’s no set definition for heat wave. the national weather service defines it as “a period of abnormally hot weather generally lasting more than two days.” heat waves can last weeks. in 2003, an estimated 70,000 people died in europe due to a heat wave that spanned from june to august. and in 2010 in the russian federation, 56,000 more people died during a 44 day heat wave. from 1998 to 2017, more than 166,000 people died due to heat waves, including more than 70,000 who died during the 2003 heatwave in europe, according to who. association between extreme ambient heat exposure and diabetes related hospital admissions and emergency department visits: a systematic reviewdonghong gao, et al. hygiene and environmental health advances, december 2022. the study: researchers summarize the findings of 18 english language papers, published between 2010 and 2022, which explored the link between extreme heat and diabetes emergency room visits or hospital admissions. the findings: there’s a significant association between exposure to extreme heat and diabetes related hospital admission or emergency room visits. extreme heat was associated with 4.5% increased risk of hospital admission or emergency room visit among people with diabetes and 10% increased risk among older adults with diabetes. quote from the study: “the findings from this review will be useful for developing evidence based interventions to reduce the impact of extreme heat exposure on diabetes, identifying the target population for the interventions (i.e., older adults), and supporting public health initiatives for public education to increase public awareness of the risk of extreme ambient heat exposure on diabetes.” more about diabetes and extreme heat: diabetes affects more than 10% of the u.s. population, and people with diabetes are among populations that are more vulnerable to extreme heat. certain complications from diabetes   both type 1 and type 2   such as damage to blood vessels and nerves, can affect sweat glands and the body can’t cool effectively, increasing the risk of heat exhaustion and heatstroke, according to the cdc. also, people with diabetes get dehydrated more quickly. heat waves and emergency department visits among the homeless, san diego, 2012 2019lara schwarz, et al. american journal of public health, december 2021. the study: researchers obtained emergency department use data from two hospitals in san diego between 2012 and 2019, analyzing data for 24,688 visits between may to september by patients who were identified as homeless. the findings: in total, 94% of the patients were younger than 65; 14% said they needed a psychiatric consultation. the strongest risk of emergency room visits was during daytime heatwaves. quote from the study: “homelessness is rapidly becoming a major social challenge in the united states. structural inequalities, housing crises, high rental costs, and natural disasters have all contributed to the increasing number of persons experiencing homelessness in recent years. … as the threat of increasingly frequent and more intense heat waves continues to rise in the united states, particularly in california, understanding and prioritizing the needs of this rapidly growing vulnerable population will be a critical action in developing and deploying effective mitigation strategies.” extreme heat and occupational injuries in different climate zones: a systematic review and meta analysis of epidemiological evidencesyeda hira fatima, et al. environment international, march 2021. the study: researchers included 24 english and non english language studies, published between january 2005 and july 2020, which assessed the effects of extreme heat on work related injuries among non military workers. the systematic review represents work related injuries in six countries   australia, canada, china, italy, spain and u.s.   and across nine climate zones. work related injuries during hot weather include heat stress, inability to work, heat related illness, loss of productivity and even death. the findings: all studies suggested that working in hot weather increased the odds of a work related injury. also, during heat waves, the risk of work related injuries increased by 17.4%. male workers and young workers were at highest risk of a work related injury during hot weather. outdoor work involving intensive work, including agriculture and construction were at high risk of injuries during high temperatures. also, male, young and new workers were at a higher risk of injuries during heat weaves. quote from the study: “there is an urgent need to mitigate the impacts of occupational heat stress in the context of climate change and the anticipated rise in environmental heat stress. the risk of [occupational injuries] associated with extreme heat is not evenly distributed and is dependent on underlying climatic conditions, workers’ characteristics, nature of work, and workplace characteristics.” we include several tips that journalists can use to help preserve the data. the journalist’s resource and econofact recently hosted a webinar featuring two trade economists and an npr producer whose reporting teams have covered tariffs. watch the recording and read key takeaways. these tips will help you interrogate president trump’s proposal to close the u.s. department of education, provide historical context and evaluate the agency’s effectiveness. thank you for subscribing. a project of harvard kennedy school's shorenstein center, the journalist’s resource curates, summarizes and contextualizes high quality research on newsy public policy topics. we are supported by generous grants from the carnegie corporation of new york, the robert wood johnson foundation, the national institute for health care management (nihcm) foundation, lumina foundation, 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by amanda rubalcava | uc san diego today while we often welcome the warm sunshine as summer rolls in, extreme heat can pose serious threats to the region, its landscape and our daily lives. as climate change continues to drive temperatures higher, understanding the impact of heat on health has never been more critical. at uc san diego, researchers across disciplines are at the forefront of investigating how heat affects our well being, uncovering innovative insights to help combat these new challenges. extreme heat, which is recognized as a prolonged period of high heat when temperatures hit above 90 degrees, is becoming more common in the united states and around the world as climate change warms the planet. drawing from the expertise of the university community, here is a list of seven ways that heat impacts health. through collaboration and innovation, these insights can help pave the way for a healthier future for all. climate change is unleashing more frequent, intense and prolonged periods of extreme heat, posing deadly risks and serious health issues. alexandra k. heaney, assistant professor at the uc san diego herbert wertheim school of public health and human longevity science, is at the forefront of research exploring this intersection between climate change and human health. in a recent working paper co authored with carlos f. gould, an assistant professor and environmental scientist at the herbert wertheim school, and joshua s. graff zivin, a professor and an economist at the uc san diego school of global policy and strategy, alongside other collaborators their findings underscore how both extreme heat and cold contribute significantly to mortality and overall disease burden in societies. rising temperatures are also expected to cause more deaths due to heat, which will be a major factor in the economic costs of climate change. not only do temperature changes affect deaths, but they also can increase how often people visit the emergency room. gould explained that their research found that emergency department visits go up with any increases in temperatures. “even in california, we see that extreme heat damages public health, with increasing emergency department visits, hospitalizations and deaths in months with extremely hot days,” said gould. he added, “it’s not just the more obvious heat related illnesses like dehydration or heat stroke that follow hot days, but we also see clear increases in emergency department visits from mental health, poisonings, digestive disorders and accidental injuries.” tarik benmarhnia, an environmental epidemiologist at scripps institution of oceanography, is also studying the various ways that climate change is posing a threat to human health. some of his latest research focuses on how dangerous heat and wildfire smoke can impact health. a new study led by scripps in collaboration with the ucla fielding school of public health saw that hospitalizations increased, especially when both heat and smoke occurred on the same day. and, in areas where residents were more likely to be poorer, people of color, living in higher density housing and less likely to have health insurance, the harm from natural hazards was greater. benmarhnia has also been reconstructing heat wave activity from past decades, noting that such events are becoming more frequent, severe and longer lasting. “it was really the public health impacts that brought this heat wave to public consciousness,” he said, referring to the unprecedented toll of the july 2006 heat wave in california, which caused nearly 600 deaths. high temperatures can cause an array of health concerns, including dehydration and heatstroke. with dehydration, the body loses more fluids than it takes in. dr. ian neel, associate clinical professor and the medical director of the geriatric medicine consult service at senior behavioral health at uc san diego health, routinely treats those with dehydration in his work as a clinician who specializes in caring for older adults. in fact, heat exposure and safety are core components of the geriatric medicine board certification. “in particular with our geriatric population, dehydration is a major risk factor for delirium, which can have a 30% one year mortality associated with it,” explained dr. neel. “if we can address things like dehydration, as well as other simple things like constipation and pain, then we actually lower somebody’s mortality risk by lowering their risk of delirium.” “as we age, what we find is our body becomes more at risk of consequences of dehydration we find that our blood vessels stiffen as a normal part of aging,” said neel, who explained that this may lead to a drop in blood pressure and increased risk of falling. further, while everyone is at risk of heatstroke when temperatures rise, the risk is especially high for children as well as older adults who may take medications that affect blood pressure regulation. so beyond critical issues like heat stroke and heat exhaustion, dr. neel sees that the simple loss of fluid from heat itself can be significantly harmful to older adults. some common medications, such as insulin, inhalers and epipens, can be damaged by high temperatures that destroy the drug’s active ingredient. this is a particular concern for mail order prescriptions, which are an increasingly common and convenient option for patients. diabetes expert candis morello, professor of clinical pharmacy at uc san diego’s skaggs school of pharmacy and pharmaceutical sciences, takes care to advise her patients about the risks of high summer temperatures and their potential impact on their medication. “elevated temperatures are of particular concern for our patients with diabetes who live in east county san diego, where temperatures can easily soar above100 degrees fahrenheit in the summer,” says morello. “it’s vital that patients who use injectable medications such as insulin or glp 1 ras or dual gip/glp 1 ra are aware that they should retrieve their medications from the mail or their doorstep immediately and always store them in a refrigerator or in a cool, dry location as instructed.” heat doesn’t just affect our bodies it can also cloud our minds, leading to lapses in our cognitive functioning that make everyday tasks feel like a challenge. joshua graff zivin, professor of economics and the director of the uc san diego school of global policy and strategy peter f. cowhey center on global transformation, has investigated the impact of weather on cognitive performance specifically, how heat impacts students’ learning and education performance. graff zivin’s research demonstrates that weather significantly influences students’ academic performance across all age groups, from elementary school to college. a notable example is a study looking at data from china, which examined the impact of varying temperatures during the national college entrance exam. this study, the first nationwide estimate of temperature effects on high stakes cognitive performance in a developing country, revealed that students who took the exam during hotter weeks performed worse, affecting their chances of admission to prestigious universities. referring to what we can learn from this research, graff zivin says, “first, this bolsters the case for addressing climate change and minimizing exposure to extreme heat since it impairs our ability to think and to perform our jobs. employers should install ac or provide reasonable breaks from the heat, not just to keep their workers healthy, but because it will improve firm productivity.” he also added, “when we cannot avoid extreme heat, we should avoid making major life decisions.” as temperatures climb, those exercising outdoors may feel more than just the burn rising temperatures can even influence activity choices. several years ago, alexandra k. heaney began research that looked at the relationship between ambient temperature and bike share usage in new york city. in the study, she and her collaborators discovered that as temperatures rose, the estimated hours and distance biked increased significantly. however, bike usage declined once temperatures exceeded 26 28°c (78.80 82.4°f). and looking ahead, while their projections for annual bike riding specific to new york city suggest that bike usage could potentially increase by up to 3.1% by 2070 as a result of climate change, they see projected decreases in bike usage in the hotter summer months. “as temperatures continue to increase, it’s interesting for us to try to understand how that is going to impact outdoor physical activity or even more specifically outdoor active transport that people are willing to undertake,” said heaney. conversely, this could also affect the future of our climate. while active transport is promoted as a major strategy to combat climate change by encouraging people to drive less, reduce pollution and opt for biking or walking more, heaney adds that this may be difficult to achieve as “this increase in extreme heat dissuades people from participating in active transport.” climate change isn’t just about rising temperatures; it also can have consequences for mental health ranging from heightened stress and anxiety to feelings of depression. jyoti mishra, associate professor in the department of psychiatry at the uc san diego school of medicine and the director of the university’s neural engineering and translation labs, has been looking into the toll on mental health taken by extreme climate events that have been accelerated by climate change. in a study assessing the impact of the 2018 camp fire california’s deadliest and most destructive wildfire mishra and her colleagues found that direct exposure to large fires significantly increases the risk of mental health issues such as post traumatic stress disorder (ptsd) and depression. they also observed that merely witnessing such disasters within one’s community can profoundly impact an individual’s mental well being. “our research shows that climate extremes can have long term impacts on mental health. it implies that it is extremely important to take into account mental health when developing climate resilience plans,” said mishra. “access to mental health services as well as greater community cohesion can protect mental well being. related research shows that when communities implement effective disaster preparedness, then post disaster mental health is significantly protected.” high temperatures are also reshaping the landscape of the spread of diseases like malaria, which could intensify transmission rates worldwide. the latest world health organization report underscores this concern, noting a staggering 247 million global malaria cases in 2021. generally, warmer climates and adequate rainfall create the ideal breeding ground for mosquitoes. gordon mccord, associate teaching professor at the school of global policy and strategy, has used spatial data analysis to illustrate the impact of diseases like malaria under changing climate conditions. his 2016 paper predicted that by the end of this century, the malaria burden could increase by 47% in places where it’s already a problem, further complicating global efforts towards elimination. “a warming planet and changing precipitation patterns will alter the intensity and geographic extent of multiple mosquito borne diseases. this will pose increasing health risks to populations around the world,” said mccord. “national health systems and the international community should leverage increasingly accurate climate epidemiology models to proactively allocate resources for disease prevention and treatment.” uc san diego 9500 gilman dr. la jolla, ca 92093 (858) 534 2230 copyright ©2021 regents of the university of california. all rights reserved.

an official website of the united states government official websites use .gov a .gov website belongs to an official government organization in the united states. secure .gov websites use https a lock ( lock locked padlock icon ) or https:// means you've safely connected to the .gov website. share sensitive information only on official, secure websites. primary site navigation logged in as: corresponding author: w. larry kenney, 102 noll laboratory, university park, pa 16802, w7k@psu.edu, phone: 814 863 1672 this brief review is based on a president’s lecture presented at the annual meeting of the american college of sports medicine in 2013. the purpose of this review is to assess the effects of climate change and consequent increases in environmental heat stress on the aging cardiovascular system. the earth’s average global temperature is slowly but consistently increasing, and along with mean temperature changes come increases in heat wave frequency and severity. extreme passive thermal stress resulting from prolonged elevations in ambient temperature, as well as prolonged physical activity in hot environments, creates a high demand on the left ventricle to pump blood to the skin to dissipate heat. even healthy aging is accompanied by altered cardiovascular function, which limits the extent to which older individuals can maintain stroke volume, increase cardiac output, and increase skin blood flow when exposed to environmental extremes. in the elderly, the increased cardiovascular demand during heat waves is often fatal due to increased strain on an already compromised left ventricle. not surprisingly, excess deaths during heat waves 1) occur predominantly in older individuals and 2) are overwhelmingly cardiovascular in origin. increasing frequency and severity of heat waves coupled with a rapidly growing at risk population dramatically increases the extent of future untoward health outcomes. keywords: heat stress, climate change, cardiovascular strain, heat wave, cutaneous blood flow, age, cardiovascular health the earth’s climate is warming, with global mean temperature increasing by 0.74°c from between the years of 1906 and 2005 (34). humans are tropical animals, and therefore capable of surviving in, and adapting to, such relatively small changes in mean ambient temperatures. however, as mean global temperature rises, the frequency, severity, and relative length of heat waves increase (53). heat waves can be functionally defined as an extended period of days with higher than normal temperatures. prolonged exposure to high ambient temperatures induces a substantial stress on the human cardiovascular system. and while human beings are capable of withstanding extremely high temperatures for short periods of time, the cardiovascular strain induced by prolonged heat exposure contributes negatively to health outcomes. indeed during a heat wave, most of the excess morbidity and mortality are not directly heat related, but are cardiovascular in origin, brought about by the increased cardiovascular challenge associated with thermoregulatory responses to heat stress (65). elderly individuals, even in the absence of overt cardiovascular disease, are the most vulnerable population during prolonged environmental heat exposure, experiencing significantly worse health outcomes than any other age cohort. individuals older than 65 years comprise a majority of the extra emergency room visits and deaths during heat waves (16, 58). the global population of aged individuals is rapidly growing (1), meaning that an increasingly larger subset of the population will be susceptible to illness and death as climatic temperature continues to rise. to defend against increasing core temperature, humans increase skin blood flow and sweat rate to dissipate heat. these effector responses are necessary for thermoregulation but place a great demand on the cardiovascular system by necessitating a relatively large increase in cardiac output (47, 57). decrements in skin blood flow are also observed and compounded with pathologies including hypertension (9, 30, 31) and hypercholesterolemia (27, 29), as well as with common medications used in the primary and secondary prevention of cardiovascular disease (26, 28). even healthy aging is associated with an attenuated rise in skin blood flow (39) and decreased sweat gland output (2) in response to heat stress, but the integrated response to heat stress still places a great burden on a compromised (decreased β adrenergic responsiveness) left ventricle. the purpose of this review is to discuss the effect of heat stress on the aging cardiovascular system and, within that context, the projected effects of global warming on human cardiovascular health. this brief review is based on a president’s lecture presented at the 60th annual meeting of the american college of sports medicine in 2013. climate change and global warming are (pardon the pun) hot topics, with their fair share of political controversy with respect to causation. there is little controversy, however, that over the past several decades the average temperature of the earth has been steadily increasing (34). the year 2012 was the hottest year on record in the united states and one of the 10 warmest in global history (50). in fact, all 10 of the warmest years on record have occurred within the last 15 years (18). october 2012 was the 332nd consecutive month with above average global temperatures (50). this means that, as of the end of 2013, anyone born after april of 1985 has never experienced a month with below average temperatures (18). summers categorized as “hot,” which occurred 33% of the time from 1950 to 1981, now occur 75% of the time (20). james hansen, of the nasa goddard institute for space studies and an adjunct professor in the department of earth and environmental sciences at columbia university, has used the term “climate dice” to describe this change in the frequency of hot weather. in this analogy, each side of a die represents a temperature range. in a normally distributed climate, average weather, cold weather, and hot weather would each be represented by two sides of the die. as the outcome of rolling a die is random, the probability of average, warm, or cool weather would be equal over the long term. with climate change, the sides of the die have changed. only one side now represents cool weather, one side represents average weather and four sides signify warmer than average weather; hansen describes this as “loaded dice”. with climate change there will still be variability in temperature and cool weather will still occur, but it will occur much less frequently than warm weather (21). evaluation of seasonal temperatures supports the increased frequency of warm weather. in the united states from 1990 to 2010, 16 of 20 winters and 15 of 20 summers were warmer than the 1951 1980 average. for the same time period in europe, 16 of 20 winters and 19 of 20 summers have been above average in temperature (19). along with proportionally warmer weather, more record high temperatures are also being set. since the year 2000, record high temperatures have occurred twice as often as record low temperatures (46). as these data clearly illustrate, the earth’s climate is warming well above normal historical temperatures. but what does that mean for human health, especially among the elderly? accompanying the rise in average global temperatures is a rise in the frequency and severity of heat waves. over the past several decades, the frequency, duration, and severity of heat waves has increased (53). from 1951 to 1980, environmental heat waves covered on average 0.1 percent to 0.2 percent of the earth’s surface at any given time; from 1981 to 2010, 10 percent of the planet experiences a heat wave at a given point in time (20). this means that the occurrence of a heat wave has been 50 to 100 times more likely over the last three decades. to account for this increase in heat wave frequency, one side of hansen’s climate die has now changed from “warm” to “extremely hot” weather (21). numerous computer models predict that the frequency, intensity, and duration of heat waves will continue to increase over the course of the century. meehl and tebaldi predict a 25 30% increase in the number of heat waves per year along, accompanied by an increased heat wave duration (45). nakano et al. predict an increase of more than 22 extra heat wave days per year in japan (48). hayhoe et al. predict that by the end of the 21st century, extremely deadly heat waves will occur in the city of chicago as frequently as every three months (23, 59). the population of the world is rapidly aging. the number of people age 65 and over in the united states has increased from 35 million in 2000 to over 41 million in 2011 and this segment of the population is projected to increase to nearly 80 million by the year 2040 (1). by 2050, there will be more people over 60 years old than under 15 years old in the world for the first time ever (64). to go along with the increase in the number of aged people, the “very old” population is also increasing. the number of people 80 years old and above is increasing at the rate of 3.8% per year, which makes them the fastest growing age group across the world (64). the increasing number of aged people increases health concerns during periods of elevated ambient temperatures. people over the age of 65 years exhibit disproportionately larger increases in mortality during heat waves than younger individuals, with the majority of excess deaths during heat waves occurring in the elderly (10). with the rapidly growing number of older individuals on the planet, the number of people at risk of dying during a heat wave increases. with an expanded at risk population there exists the potential for a greater number of casualties during any single heat wave. humans evolved from tropical climates and are capable of withstanding even extremely high environmental temperatures   temperatures exceeding 200 °f    (6) provided that (1) they can produce enough sweat, (2) the environment permits evaporation of that sweat, and (3) there is no direct contact with hot surfaces. however, physiological homeostasis in such extreme temperatures is only sustainable for short periods of time. despite the ability to tolerate extreme heat stress for short periods of time, prolonged moderate heat stress, such as that observed during a heat wave, represents a different set of stressors, and prolonged warm environmental temperatures are associated with an increased risk of mortality. for example, temperatures above the 90th percentile in california were found to increase risk of excess mortality by 4.3% for every 5.6°c increase in apparent temperature (4). [note: apparent temperature is a temperature index that combines the effects of air temperature, relative humidity and wind speed.] in 15 european cities, an increase in apparent temperature of 1°c above a threshold temperature unique to each city was associated with a 3.12% increase in mortality in mediterranean cities and a 1.84% increase in mortality in northern european cities (3). while elevated daily temperatures increase the mortality rate, severe heat waves   several consecutive days of hot weather    cause much greater increases in mortality. two different heat waves within the past 20 years received considerable attention due to innovations in recording morbidity and mortality statistics. in the summer of 1995, the chicago heat wave was notable because minimum apparent temperatures remained above 31.5°c for two days (36). this very high minimum temperature prevented recovery from heat stress at night. instead, the heat stress was continuous throughout the day and night. there were approximately 700 excess deaths during the chicago heat wave compared to the same period of time in the previous year (67). in the european heat wave centered around france in august 2003, temperatures were elevated by 11°c above the seasonal average for nine consecutive days. in total, the french heat wave is estimated to have caused almost 15,000 excess deaths over approximately one month (17, 24, 63). excess deaths began to occur three days after the start of the heat wave and mortality returned to normal four days after conclusion of the heat wave (17), demonstrating that is takes a prolonged period of time with elevated temperatures to increase mortality (fig. 1). daily deaths (left axis) and minimum and maximum air temperatures (right axis) during the 2003 french heat wave. the abnormally hot daily temperatures in early august 2003 were followed by a consequent dramatic increase in daily mortality, mostly among the elderly. redrawn from dousset et al. international journal of climatology 2011, 31:313 323; royal meteorological society (14). it is hypothesized that the increase in mortality during heat waves, such as those in chicago and france, may be reflect a harvesting effect. harvesting, or mortality displacement, means that the excess deaths during a heat wave occur in frail individuals whose death was only slightly expedited by the heat wave. the excess mortality is then counteracted by periods of reduced mortality following the heat wave. no clear consensus has been reach on the magnitude of the harvesting effect during heat waves. some have reported that nearly all deaths during times of extreme temperature are accounted for by mortality displacement (7) while others have reported no mortality displacement (4, 8). it is possible that the wide variation between studies is due to differences in preparedness for managing public health during heat waves among the cities studied. differences in intensity and duration of heat waves also likely determine the degree of harvesting. a moderate harvesting effect of approximately 30% is often observed, especially when examining very severe heat waves or looking at large, varied populations (3, 35, 63). this suggests that some deaths during heat waves occur in those who were in poor health and had only a brief period left to live but that a majority of deaths during heat waves occur in those with a normal remaining life expectancy. clearly, determinants of mortality during heat waves are not all physiological. socio economic status and living conditions undoubtedly play a major role in mortality during heat waves. odds of death during a heat wave are increased if one is elderly, is confined to bed, sleeps on the top floor of a building, lacks thermal insulation, or resides inside an urban heat island (49, 59, 65). in the french heat wave of 2003, excess mortality rates in deprived areas of paris were twice as high as those in privileged areas of paris (55). while being frail, sick, or living in an underprivileged region during a heat wave increase risk of death, there are countermeasures that decrease the risk of mortality during a heat wave. dressing in light clothing, hydrating, owning air conditioning, having access to transportation, and having nearby social contacts are all associated with decreased mortality (49, 59, 65). preparedness of residents or of a city to deal with severe heat waves can also affect mortality. chicago experienced a heat wave in 1999 that was meteorologically very similar to the 1995 heat wave; however, in 1999 there were only 114 excess deaths compared to 700 in 1995 (52). part of the drop in mortality is attributed to increased public awareness and an improved municipal response (52). other interventions such as cooling centers have not been shown to be protective due to underutilization (49). regardless, steps can be taken to protect the most vulnerable populations, such as the old and sick, during heat waves to limit mortality. direct heat related fatalities, such as death from heat stroke, number approximately 120 per year in the united states, which is more than any other individual environmental causes including death from cold, flooding, tornados, hurricanes, and lightning (51). however, the number of deaths linked to periods of excess heat or heat waves is much greater than those attributable to heat stroke. for example, the august 2003 heat wave in europe resulted in 15,000 excess deaths in france alone and as many as 70,000 excess deaths across all of the european continent (17, 24, 56, 63). the chicago 1995 heat wave registered an average of 241 excess deaths per day, far outnumbering the average annual united states heat fatalities in a single day (35). the reason for this difference between direct heat deaths and overall heat wave mortality is that direct effects of heat alone are not the primary cause of death for most of the excess deaths during heat waves. of the excess deaths in the chicago heat wave, only 4.7% listed excessive heat (i.e., heat stroke or hyperpyrexia) as the primary cause of death on death certificates, another 28.1% listed heat as a contributing cause, while 93.7% of excess deaths documented an underlying cardiovascular cause (fig. 2) (35). mortality (green shaded area and red line) and maximum daily temperatures (dashed line) during the summer 1995 chicago heat wave. dates are shown along the x axis. mortality data in green represent the total death toll, while the superimposed red line depicts deaths from cardiovascular causes alone or from death certificates that mention combined cardiovascular and heat causes. redrawn from kaiser et al., american journal of public health 2007, 97 suppl 1:s158 62; apha press (35). evaluation of deaths during two heat waves in milwaukee likewise revealed cardiovascular disease as the primary cause of death in 51% and 64% of the deaths in the two heat waves, respectively (66). although the vast majority of excess deaths during a heat wave are cardiovascular in origin, associated physiological strain on other systems contributes to the elevated mortality too. an increase in respiratory deaths (12, 33) and cerebrovascular deaths (12) also contribute to the increased mortality during heat waves that were not directly attributed to hyperthermia. overall, there is a substantial increase excess death during heat waves, but heat related illness only contributes moderately to the increased mortality. cardiovascular causes form the majority of contributing factors to excess deaths, especially so in the elderly. older humans have a much greater mortality risk during heat waves (59, 65). death from excessive environmental heat affects adults above age 50 at a higher rate than adults below 50, with the rate increasing exponentially beyond that point (15). in some instances, over 90% of excess deaths during heat waves occur in the elderly (10). as most deaths during heat waves are cardiovascular in origin, the complex reasons for higher mortality in the aged can, at least in part, be understood by examining the cardiovascular responses of older and young apparently healthy human subjects to passive heat stress. the integrated cardiovascular responses of young, healthy men to passive supine heat stress were eloquently described by rowell in the 1960s. by increasing skin temperature to 40.5°c using a water perfused suit, rowell saw a doubling of resting cardiac output, slightly increased stroke volume, and a redistribution of blood flow from the splanchnic and renal circulations to the skin. interestingly, despite a profound reduction in right atrial mean pressure to almost zero, mean arterial pressure was well maintained and inotropic function of the heart increased (57). together these data demonstrated that blood volume is redistributed from the central to peripheral circulations to aid in thermoregulation (11) at the expense of increased ventricular work to pump blood in light of a profoundly reduced filling pressure (fig. 3). (a) the change in central blood volume, measured by technetium 99 scanning, with passive heat stress compared to a time control. blood volume in the overall thorax and centered around the heart and central vasculature are shown. heat stress significantly lowered blood volume compared to time control. (b) ejection fraction at baseline, with passive heat stress and during a time control trial. passive heat stress significantly increased ejection fraction compared to baseline and time control. collectively, these data reflect the added strain on the left ventricle during passive heat stress, as contractility increased in light of a falling central venous pressure, to pump blood to the skin. redrawn from crandall et al., the journal of physiology 2008, 586(1): 293 301; the physiological society (11). healthy older humans have an altered cardiovascular response to heat stress compared to their younger counterparts. during passive heating to the limits of thermal tolerance, young subjects increased cutaneous blood flow by ~5,800 ml/min compared to an increase of only 2,700 ml/min in older subjects (47). the increase in skin blood flow in young subjects came from both increased cardiac output (4,800 ml/min) and a redistribution of blood from both the splanchnic and renal circulations (1,000 ml/min). in contrast, 70 year old subjects had an attenuated increase in cardiac output (2,000 ml/min) coupled with a reduction in the ability to redistribute blood flow from the splanchnic and renal circulations (700 ml/min) (47) (fig. 4). changes in cardiac output, renal, splanchnic and cutaneous blood flow with passive heating to thermal tolerance (water perfused suit) in young and older men. young subjects increased cutaneous blood flow to a larger extent than did older subjects. the larger increase in cutaneous blood flow in the young men was accomplished by both raising cardiac output significantly more and by reducing renal and splanchnic blood flow to a higher degree compared with the older subjects. redrawn from data published by minson et al., journal of applied physiology 1998, 84(4):1323 32; american physiological society (47). one reason for the attenuated increase in cardiac output during passive heating in older subjects is because of their lack of ability to maintain stroke volume. with prolonged passive heating, central venous pressure (cvp) falls similarly in aged and young subjects (fig. 5a) (47). the young subjects were able to maintain or slightly increase stroke volume by increasing contractility, whereas stroke volume declined progressively in the older subjects. while the absolute heart rate response was similar in both age groups, heart rate as a percentage of maximum was higher in the older individuals at any given cvp (fig. 5b). thus, the older subjects relied on a greater percentage of their heart rate reserve to increase cardiac output during whole body heat stress (47). in the aged there is excess central cardiovascular strain as well as an attenuated increase in thermoregulatory skin blood flow. with such changes evident in apparently healthy older men free from overt heart disease, such increases in myocardial oxygen demand may prompt untoward events in those with clinical or subclinical disease. cardiac responses to prolonged passive heating as a function of (a) time and (b) central venous pressure in young (19 28 yrs; black circles) and older (64 81 yrs; red circles) men. only the initial 30 min of heating are shown in panel a. older subjects had a significantly attenuated rise in cardiac output and a decrease in stroke volume compared with young subjects, despite a similar fall in central venous pressure. stroke volume was well maintained in the young men. as shown in panel b, the fall in central venous pressure (cvp) (right to left along the x axis) due to venous blood pooling caused a similar increase in absolute heart rate (see panel a) but a larger rise in hr as a percent of maximal heart rate. heart rate reserve is consequently lower in the older men at any given level of heat stress (and cvp). redrawn from data published by minson et al., journal of applied physiology 1998, 84(4):1323 32; american physiological society (47). along with a drop in cutaneous perfusion, aging is associated with a decreased sweat rate (60) and decreased sweat output per gland (2). regional sweating patterns also change with aging, with the largest reductions in sweating occurring in the abdomen and less significant reductions occurring in the lower back, thigh, and arm (60). an attenuated evaporative heat loss results in greater heat storage in older men and women (41, 42) which can exacerbate the cardiovascular strain described above. even though the sweating response in the aged is often attenuated, prolonged sweating during long duration heat stress causes a significant reduction in plasma volume. this fall precipitates increases in red blood cell and neutrophil counts as well as increased plasma viscosity. heat stress also causes the release of extra platelets into the circulation. these changes in blood properties contribute to increased susceptibility to cardiovascular death due to acute coronary events (37). one method for assessing cardiovascular strain and damage during heat stress is by measuring the enzyme cardiac troponin i (ctni). when myocardial cell are damaged, ctni is released into the bloodstream (43). cardiac troponin i is elevated when examined postmortem in hyperthermia related mortalities (68), indicating myocardial damage associated with the severe heat stress. ctni has also been found to be elevated in aged subjects with non exertional heat related illnesses (13, 22). an increase in ctni is associated with decreased survival from a heat illness and is an independent prognostic factor for survival (fig. 6) (13, 22). this association provides evidence for myocardial damage being a contributing factor in deaths in the elderly during heat waves. even a modest increase in core temperature (below the clinical criteria for heat stroke 40°c) can cause myocardial damage. patient survival curves following emergency admission to hospitals during the weeks after the 2003 heat wave in france. separate curves are drawn for patients (mean age = 84 yrs) who had no elevation in cardiac troponin i (ctni) (n = 252; green line), a moderate increase (up to 1.5 ng.ml−1, n = 165; orange line), and a severe increase in ctni (>1.5 ng.ml−1, n = 97; red line). these data support the association between heat stress and cardiac strain in the elderly during environmental heat waves. redrawn from data originally published by hausfater et al., critical care 2010, 14:r99; biomed central publishing (22). the ability to maintain core temperature during heat stress is in part dependent upon increasing cutaneous blood flow. when examining the integrated cardiovascular response to passive whole body heating, skin blood flow has been calculated to increase to 7 8 l/min in young subjects (57). with healthy aging, there is an attenuated skin blood flow response to passive, whole body heating (40, 47). while central cardiovascular changes with aging contribute to the reduction in skin blood flow (47), there are also peripheral limitations to reflex cutaneous vasodilation. nitric oxide (no) is a potent vasodilator that is required for full expression of reflex vasodilation and contributes directly to approximately 30 40% of the cutaneous vasodilatory response in young skin (25, 38). even though no bioavailability is decreased in aged vasculature, aged humans rely primarily on compromised no mediated mechanisms to increase skin blood flow during heat stress (25). the reason for the increased dependency on no mediated vasodilation in aged skin is the decreased contribution of cholinergic co transmitter(s) to reflex vasodilation (25). many peripheral factors contribute to the decreased no bioavailability in aged human vasculature. in human skin there is upregulation of the enzyme arginase which preferentially utilizes the common no synthase (nos) substrate, l arginine, to produce urea and l ornithine (31). either localized supplementation of l arginine or inhibition on arginase augments reflex cutaneous vasodilation in aged skin (31). aging is also associated with increased oxidative stress including increased superoxide production from a variety of enzymatic and non enzymatic sources. superoxide reacts with no to form peroxynitrite (oono ) than is degraded by superoxide dismutase (5). this reaction decreases no bioavailability and functionally reduces vasodilation. moreover free radical species can also uncouple the nos dimer which further decreases no production and increases superoxide synthesis (30). short term antioxidant supplementation (ascorbate) serves to quench free radicals and increase reflex vasodilation in aged skin (30). however, ascorbate is a generalized antioxidant and can either decrease oxidant species and/or improve the availability of the essential nos cofactor tetrahydrobiopterin (bh4). aging in general is associated with reduced bioavailability of bh4, which potentiates nos uncoupling and contributes to increased oxidant stress (54). localized supplementation of bh4 increases no dependent vasodilation in aged human skin (61, 62). along with no mediated vasodilation, cyclooxygenase (cox) pathways contribute to reflex vasodilation in young, healthy subjects (44). however with aging there is an increase in cox derived vasoconstrictors and attenuated prostanoid dependent vasodilation which together may increase cutaneous vasoconstriction (32). together, the attenuation of no , cox , and co transmitter mediated vasodilation with aging demonstrates an overall loss in redundancy in vasodilator mechanisms with aging. while reflex cutaneous vasodilation is attenuated in aged skin, local pharmacological interventions can largely restore skin blood flow in the aged to match that of young people. however, it is currently not known if the aged cardiovascular system, including the compromised left ventricle, could support the increase in skin blood flow afforded by these pharmacological intervention. the earth’s climate is rapidly warming and the increased average daily temperature is accompanied by an increased frequency and severity of environmental heat waves. passive thermal stress places a large demand on the cardiovascular system to pump blood to the skin. the increased cardiovascular demand of high ambient temperatures, with or without the added strain of physical exertion, can be especially challenging in aged humans who exhibit altered cardiovascular function and thermoregulatory ability. for these individuals, heat waves can often be fatal as demonstrated by the large number of excess deaths   primarily of cardiovascular origin    in elderly individuals during heat waves. with the number of older people rapidly increasing and the climate warming, cardiovascular deaths precipitated by prolonged periods of high ambient temperatures are of increasing concern. the integrated cardiovascular response to heat stress is an example of elegant homeostasis. yet alterations in the aged cardiovascular system limit the requisite increase in cardiac output while putting increased strain on a potentially compromised left ventricle. the result is an increased mortality among the elderly during environmental extremes of heat, not from heat stroke but due to cardiovascular strain. conflict of interest the authors have no conflicts of interest to report. no funding was received for this work. the contents of this review do not constitute endorsement by acsm. connect with nlm national library of medicine 8600 rockville pike bethesda, md 20894

an official website of the united states government here’s how you know official websites use .gov a .gov website belongs to an official government organization in the united states. secure .gov websites use https a lock (locka locked padlock) or https:// means you’ve safely connected to the .gov website. share sensitive information only on official, secure websites. javascript appears to be disabled on this computer. please click here to see any active alerts. this indicator presents data on deaths classified as “heat related” in the united states. this figure shows the annual rates for deaths classified as “heat related” by medical professionals in the 50 states and the district of columbia. the orange line shows deaths for which heat was listed as the main (underlying) cause.* the blue line shows deaths for which heat was listed as either the underlying or the contributing cause of death in may september, based on a broader set of data that became available in 1999 and could be updated through 2021. * between 1998 and 1999, the world health organization revised the international codes used to classify causes of death. as a result, data from earlier than 1999 cannot easily be compared with data from 1999 and later. data source: cdc, 20249,10 web update: june 2024 this figure shows rates for deaths that medical professionals have classified as being caused by a combination of cardiovascular disease (diseases of the circulatory system) and heat exposure. this graph presents summer (may to september) death rates from 1999 to 2022 for three population groups in the 50 states and the district of columbia. the blue line shows rates for the entire population, the green line shows rates for non hispanic black people, and the pink line shows rates for people aged 65 and older. data source: cdc, 202411 web update: june 2024 many factors can influence the nature, extent, and timing of health consequences associated with extreme heat events.12 studies of heat waves are one way to better understand health impacts, but different methods can lead to very different estimates of heat related deaths. for example, during a severe heat wave that hit chicago* between july 11 and july 27, 1995, 465 heat related deaths were recorded on death certificates in cook county.13 however, studies that compared the total number of deaths during this heat wave (regardless of the recorded cause of death) with the long term average of daily deaths found that the heat wave likely led to about 700 more deaths than would otherwise have been expected.14 differences in estimated heat related deaths that result from different methods may be even larger when considering the entire nation and longer time periods. * this graph shows data for the chicago standard metropolitan statistical area. data sources: cdc, 2012;15 noaa, 201216 web update: august 2016 key points | background | about the indicator | about the data | technical documentation when people are exposed to extreme heat, they can suffer from potentially deadly illnesses, such as heat exhaustion and heat stroke. hot temperatures can also contribute to deaths from heart attacks, strokes, and other forms of cardiovascular disease. according to the national weather service, heat is a leading weather related killer in the united states,1 even though most heat related deaths are preventable through outreach and intervention (see epa’s excessive heat events guidebook at: www.epa.gov/heatislands/excessive heat events guidebook). unusually hot temperatures have become more common across parts of the united states in recent decades2 (see the high and low temperatures indicator), extreme heat events (heat waves) have become more frequent and intense throughout the country (see the heat waves indicator), and these trends are expected to continue.2 as a result, the risk of heat related deaths and illness is also expected to increase unless people take steps to mitigate the impact or adapt.3 the “heat island” effect accentuates the problem by causing even higher temperatures in densely developed urban areas.4 reductions in cold related deaths are projected to be smaller than increases in heat related deaths in most regions.5 death rates can also change, however, as people acclimate to higher temperatures and as communities strengthen their heat response plans and take other steps to continue to adapt. certain population groups already face higher risks of heat related death and increases in summertime temperature variability will increase that risk.5,6 the population of adults aged 65 and older, which is expected to continue to grow, has a higher than average risk of heat related death. children are particularly vulnerable to heat related illness and death, as their bodies are less able to adapt to heat than adults, and they must rely on others to help keep them safe.7 people with certain diseases, such as cardiovascular and respiratory illnesses, are especially vulnerable to excessive heat exposure, as are the economically disadvantaged. data also suggest a higher risk among non hispanic black people.8 this indicator shows the annual rate for deaths classified by medical professionals as “heat related” in the united states based on death certificate records. every death is recorded on a death certificate, where a medical professional identifies the main cause of death (also known as the underlying cause), along with other conditions that contributed to the death. these causes are classified using a set of standard codes. dividing the annual number of deaths by the u.s. population in that year, then multiplying by one million, will result in the death rates (per million people) that this indicator shows. figure 1 shows heat related death rates using two methods. one method shows deaths for which excessive natural heat was stated as the underlying cause of death from 1979 to 2022. the other data series shows deaths for which heat was listed as either the underlying cause or a contributing cause, based on a broader set of data that, at present, can only be evaluated from 1999 to 2021. for example, in a case where cardiovascular disease was determined to be the underlying cause of death, heat could be listed as a contributing factor because it can make the individual more susceptible to the effects of this disease. because excessive heat events are associated with summer months, the contributing factor analysis was limited to may through september. figure 2 offers a closer look at cardiovascular disease deaths for which heat was recorded as a contributing cause. this graph includes deaths due to heart attacks, strokes, and other diseases related to the circulatory system. figure 2 shows death rates for the overall population as well as two groups with a higher risk: people age 65 and older and non hispanic black people. like the “underlying and contributing causes” analysis in figure 1, figure 2 is restricted to the summer months, and it uses data that are available from 1999 to 2022. several factors influence the ability of this indicator to estimate the true number of deaths associated with extreme heat events. it has been well documented that many deaths associated with extreme heat are not identified as such by the medical examiner and might not be properly coded on the death certificate. in many cases, the medical examiner might classify the cause of death as a cardiovascular or respiratory disease, not knowing for certain whether heat was a contributing factor, particularly if the death did not occur during a well publicized heat wave. furthermore, deaths can occur from exposure to heat (either as an underlying cause or as a contributing factor) that is not classified as extreme and therefore is often not recorded as such. some statistical approaches estimate that more than 1,300 deaths per year in the united states are due to extreme heat, compared with about 750 deaths per year in the “underlying and contributing causes” data set shown in figure 1.5 by studying how daily death rates vary with temperature in selected cities, scientists have found that extreme heat contributes to far more deaths than the official death certificates might suggest.17 this is because the stress of a hot day can increase the chance of dying from a heart attack, other heart conditions, or respiratory diseases such as pneumonia.18 these causes of death are much more common overall than dying directly from a heat related illness such as heat stroke. thus, this indicator very likely underestimates the number of deaths caused by exposure to heat. classifying a death as “heat related” does not mean that high temperatures were the only factor that caused or contributed to the death, as pre existing medical conditions can significantly increase an individual’s susceptibility to heat. other important factors, such as the overall vulnerability of the population, the extent to which people have adapted and acclimated to higher temperatures, and the local climate and topography, can affect trends in heat related deaths. heat response measures, such as early warning and surveillance systems, air conditioning, health care, public education, cooling centers, infrastructure standards, and air quality management, can also make a big difference in reducing death rates. for example, after a 1995 heat wave, the city of milwaukee developed a plan for responding to extreme heat conditions; during a 1999 heat wave, heat related deaths were roughly half of what would have been expected.19 future development related to this indicator should focus on capturing all heat related deaths, not just those with a reported link to heat stress. starting in 2018, the source data began to provide separate counts for people who belong to more than one racial group. prior to 2018, some of these individuals might have been classified as black. this change has a small effect on the “non hispanic black people” data in figure 2, but not enough to change the shape of the line or the conclusions that can be drawn. the technical documentation for this indicator explains the change in more detail. data for this indicator were provided by the u.s. centers for disease control and prevention (cdc). the underlying cause data in figure 1 are publicly available through the cdc wonder database at: https://wonder.cdc.gov. the contributing factor analysis in figure 1 was developed by cdc’s environmental public health tracking program, which provides a summary at: www.cdc.gov/nceh/tracking. the cardiovascular disease data in figure 2 are publicly available through the cdc wonder database at: https://wonder.cdc.gov/mcd icd10.html. 1 noaa (national oceanic and atmospheric administration). (2023). weather related fatality and injury statistics. retrieved august 15, 2024, from www.weather.gov/hazstat 2 marvel, k., su, w., delgado, r., aarons, s., chatterjee, a., garcia, m. e., hausfather, z., hayhoe, k., hence, d. a., jewett, e. b., robel, a., singh, d., tripati, a., & vose, r. s. (2023). chapter 2: climate trends. in usgcrp (u.s. global change research program), fifth national climate assessment. https://doi.org/10.7930/nca5.2023.ch2 3 hayden, m. h., schramm, p. j., beard, c. b., bell, j. e., bernstein, a. s., bieniek tobasco, a., cooley, n., diuk wasser, m., dorsey, m. k., ebi, k., ernst, k. c., gorris, m. e., howe, p. d., khan, a. s., lefthand begay, c., maldonado, j., saha, s., shafiei, f., vaidyanathan, a., & wilhelmi, o. v. (2023). chapter 15: human health. in usgcrp (u.s. global change research program), fifth national climate assessment. https://doi.org/10.7930/nca5.2023.ch15 4 chu, e. k., fry, m. m., chakraborty, j., cheong, s. m., c. clavin, coffman, m., hondula, d. m., hsu, d., jennings, v. l., keenan, j. m., kosmal, a., muñoz erickson, t. a., & jelks, n. t. o. (2023). chapter 12: built environment, urban systems, and cities. in usgcrp (u.s. global change research program), fifth national climate assessment. https://doi.org/10.7930/nca5.2023.ch12 5 sarofim, m. c., saha, s., hawkins, m. d., mills, d. m., hess, j., horton, r., kinney, p., schwartz, j., & st. juliana, a. (2016). chapter 2: temperature related death and illness. in usgcrp (u.s. global change research program), the impacts of climate change on human health in the united states: a scientific assessment (pp. 43 69). https://doi.org/10.7930/j0mg7mdx 6 zanobetti, a., o’neill, m. s., gronlund, c. j., & schwartz, j. d. (2012). summer temperature variability and long term survival among elderly people with chronic disease. proceedings of the national academy of sciences of the united states of america, 109(17), 6608 6613. https://doi.org/10.1073/pnas.1113070109 7 usgcrp (u.s. global change research program). (2016). the impacts of climate change on human health in the united states: a scientific assessment (a. crimmins, j. balbus, j. l. gamble, c. b. beard, j. e. bell, d. dodgen, r. j. eisen, n. fann, m. d. hawkins, s. c. herring, l. jantarasami, d. m. mills, s. saha, m. c. sarofim, j. trtanj, & l. ziska, eds.). https://doi.org/10.7930/j0r49nqx 8 berko, j., ingram, d. d., saha, s., & parker, j. d. (2014). deaths attributed to heat, cold, and other weather events in the united states, 2006 2010 (national health statistics reports, number 76). national center for health statistics. www.cdc.gov/nchs/data/nhsr/nhsr076.pdf 9 cdc (u.s. centers for disease control and prevention). (2024). cdc wonder database: all ages deaths by underlying cause [data set]. retrieved may 22, 2024, from https://wonder.cdc.gov/deaths by underlying cause.html 10 cdc (u.s. centers for disease control and prevention). (2024). indicator: heat related mortality (annual national totals provided by national center for environmental health staff in june 2024) [data set]. national center for health statistics. https://ephtracking.cdc.gov 11 cdc (u.s. centers for disease control and prevention). (2024). cdc wonder database: multiple cause of death file [data set]. retrieved may 22, 2024, from https://wonder.cdc.gov/mcd icd10.html 12 anderson, g. b., & bell, m. l. (2011). heat waves in the united states: mortality risk during heat waves and effect modification by heat wave characteristics in 43 u.s. communities. environmental health perspectives, 119(2), 210 218. https://doi.org/10.1289/ehp.1002313 13 cdc (u.s. centers for disease control and prevention). (1995). heat related mortality chicago, july 1995. morbidity and mortality weekly report, 44(31), 577 579. 14 nrc (national research council). (2011). climate stabilization targets: emissions, concentrations, and impacts over decades to millennia. national academies press. 15 cdc (u.s. centers for disease control and prevention). (2012). cdc wonder database [data set]. retrieved august 1, 2012, from https://wonder.cdc.gov/mortsql.html 16 noaa (national oceanic and atmospheric administration). (2012). national centers for environmental information. retrieved august 1, 2012, from www.ncei.noaa.gov 17 medina ramón, m., & schwartz, j. (2007). temperature, temperature extremes, and mortality: a study of acclimatisation and effect modification in 50 us cities. occupational and environmental medicine, 64(12), 827 833. https://doi.org/10.1136/oem.2007.033175 18 kaiser, r., le tertre, a., schwartz, j., gotway, c. a., daley, w. r., & rubin, c. h. (2007). the effect of the 1995 heat wave in chicago on all cause and cause specific mortality. american journal of public health, 97(suppl 1), s158 s162. https://ajph.aphapublications.org/doi/full/10.2105/ajph.2006.100081 19 weisskopf, m. g., anderson, h. a., foldy, s., hanrahan, l. p., blair, k., török, t. j., & rumm, p. d. (2002). heat wave morbidity and mortality, milwaukee, wis, 1999 vs. 1995: an improved response? american journal of public health, 92(5), 830 833. www.ncbi.nlm.nih.gov/pmc/articles/pmc1447169

an official website of the united states government official websites use .gov a .gov website belongs to an official government organization in the united states. secure .gov websites use https a lock ( lock locked padlock icon ) or https:// means you've safely connected to the .gov website. share sensitive information only on official, secure websites. primary site navigation logged in as: address correspondence to m.l. bell, 195 prospect st., new haven, ct 06511. telephone: (203) 432 9869. fax: (203) 436 9158. e mail: michelle.bell@yale.edu the authors declare they have no actual or potential competing financial interests. received 2010 apr 19; accepted 2010 oct 7; issue date 2011 feb. publication of ehp lies in the public domain and is therefore without copyright. all text from ehp may be reprinted freely. use of materials published in ehp should be acknowledged (for example, ?reproduced with permission from environmental health perspectives?); pertinent reference information should be provided for the article from which the material was reproduced. articles from ehp, especially the news section, may contain photographs or illustrations copyrighted by other commercial organizations or individuals that may not be used without obtaining prior approval from the holder of the copyright. devastating health effects from recent heat waves, and projected increases in frequency, duration, and severity of heat waves from climate change, highlight the importance of understanding health consequences of heat waves. we analyzed mortality risk for heat waves in 43 u.s. cities (1987 2005) and investigated how effects relate to heat waves’ intensity, duration, or timing in season. heat waves were defined as ≥ 2 days with temperature ≥ 95th percentile for the community for 1 may through 30 september. heat waves were characterized by their intensity, duration, and timing in season. within each community, we estimated mortality risk during each heat wave compared with non heat wave days, controlling for potential confounders. we combined individual heat wave effect estimates using bayesian hierarchical modeling to generate overall effects at the community, regional, and national levels. we estimated how heat wave mortality effects were modified by heat wave characteristics (intensity, duration, timing in season). nationally, mortality increased 3.74% [95% posterior interval (pi), 2.29 5.22%] during heat waves compared with non heat wave days. heat wave mortality risk increased 2.49% for every 1°f increase in heat wave intensity and 0.38% for every 1 day increase in heat wave duration. mortality increased 5.04% (95% pi, 3.06 7.06%) during the first heat wave of the summer versus 2.65% (95% pi, 1.14 4.18%) during later heat waves, compared with non heat wave days. heat wave mortality impacts and effect modification by heat wave characteristics were more pronounced in the northeast and midwest compared with the south. we found higher mortality risk from heat waves that were more intense or longer, or those occurring earlier in summer. these findings have implications for decision makers and researchers estimating health effects from climate change. keywords: climate change, extreme temperature events, heat waves, human health, mortality, temperature mortality relationships heat waves can have large impacts on human health; mortality occasionally more than doubles during heat waves (e.g., oechsli and buechley 1970; whitman et al. 1997). interest in related health effects increased after major heat waves (i.e., chicago, il, 1995; europe, 2003) (le tertre et al. 2006; semenza et al. 1996) and with projections that climate change may increase the frequency, duration, and intensity of heat waves (meehl and tebaldi 2004). understanding how heat waves affect health is key to preparing communities for heat waves and to estimating the health impacts of climate change. most studies of mortality and heat estimated health effects during specific heat waves (e.g., kaiser et al. 2007; weisskopf et al. 2002) or applied time series or case crossover methods to estimate the effects of single days of heat. few studies combined approaches to consider the effects of both heat waves and single days of high temperature (anderson and bell 2009; hajat et al. 2006); they showed additional health effects from prolonged heat beyond the sum of anticipated effects associated with single hot days. these studies, however, did not distinguish between effects of individual heat waves, but rather estimated health responses assuming that all heat waves of a specified definition have the same impact on health. the study of heat waves faces several challenges. heat waves are usually defined as extended periods of extreme heat, although no consistent definition exists regarding the temperature threshold, temperature metric, and number of days used to define heat waves. for example, studies have used thresholds of mean temperature (hajat et al. 2006), apparent temperature (smoyer 1998), or combinations of thresholds of apparent and minimum temperature (robinson 2001; weisskopf et al. 2002). use of various heat wave definitions results in different time periods being classified as heat waves, hindering comparison and synthesis of results across studies. further, heat waves differ in their intensity (degree of heat) and duration. although most studies use measures of intensity and duration to define a heat wave, few have investigated how these heat wave characteristics affect the mortality impact. early studies of one or a few heat waves proposed that differences between the health effects of different heat waves might relate to a heat wave’s intensity, duration, or timing in the summer, although these studies did not investigate effect modification (e.g., ellis et al. 1975; schuman 1972). research of heat waves in new york, new york (marmor 1975); madrid, spain (diaz et al. 2002); and st. louis, missouri (smoyer 1998) found that mortality effects of heat waves decreased as summer progressed. studies estimating other elements of the temperature mortality relationship also found differences in effects with timing in season. effects of single days of high temperature were larger earlier in the summer in several u.s. and european cities (e.g., baccini et al. 2008; hajat et al. 2002; kalkstein and smoyer 1993). a study in philadelphia, pennsylvania, using a synoptic approach found that oppressive air masses had greater effects when they occurred earlier in the summer (kalkstein et al. 1996). of this research, only the single city studies considered heat waves (i.e., extended periods of high temperature) (diaz et al. 2002; marmor 1975; smoyer 1998). other studies provided evidence of the importance of heat wave duration and intensity. one international study showed that duration sometimes modified heat wave mortality effects (kalkstein and smoyer 1993). studies of madrid (diaz et al. 2002) and st. louis (smoyer 1998) found greater mortality effects for longer heat waves. the duration and intensity of oppressive air masses sometimes modified their mortality effects (kalkstein and greene 1996; kalkstein et al. 1996). two studies found that average mortality effects increase when heat wave definitions are limited to longer or more intense heat waves (anderson and bell 2009; hajat et al. 2006). in this study, we estimated the mortality effects of heat waves across the united states (43 communities) for the period 1987 2005 and determined how these effects changed when heat waves were more intense, longer, or earlier in the summer. although several studies have suggested that heat wave characteristics affect mortality risk, to our knowledge this is the first multicity study of prolonged periods of high temperature (heat waves) in the united states to investigate the impacts of heat wave timing and the largest to examine effect modification by heat wave duration and intensity. this research is also one of the largest studies to date of heat wave effects in the united states. we combined episodic and time series approaches to estimate the risk of nonaccidental mortality during each individual heat wave compared with risk on non heat wave days. then we estimated whether heat wave intensity, duration, or timing in season explained variability in effect estimates. we examined regional differences in heat wave effects and in the modification of effects by heat wave characteristics. daily nonaccidental mortality data for 108 u.s. urban communities (1987 2005) were obtained from an extended version of the national morbidity, mortality, and air pollution study data set (originally 1987 2000) (anderson and bell 2009; internet based health and air pollution surveillance system 2005). communities were defined as contiguous counties of a metropolitan area (bell et al. 2004). we omitted data for noncontinental communities, those with population < 200,000 (dominici et al. 2006; peng et al. 2008), and those without weather data for ≥ 99.5% of study days, leaving 59 communities. weather data came from the national climatic data center (2011). daily relative humidity was calculated from dew point temperature (bosen 1958) and used to calculate daily apparent temperature, a metric incorporating air temperature and relative humidity to better approximate the physical experience of heat (robinson 2001). dew point temperature is correlated with air temperature and was adjusted for daily mean temperature to limit collinearity in the model (after adjustment, average community correlation between daily dew point temperature and maximum temperature = 0.12) (bell et al. 2004). we limited analysis to the warm season (1 may 30 september). a heat wave consists of consecutive days with temperatures above a threshold temperature that can either be physiologically based (absolute threshold) or location based (relative threshold) (robinson 2001). we used a relative threshold based on the community’s own long term weather, to allow for regional acclimatization to temperatures normal for a community. we identified heat waves as ≥ 2 consecutive days with daily mean temperature (tmean) higher than the community’s 95th percentile warm season tmean (for 1987 2005), a definition similar to those used previously (anderson and bell 2009; hajat et al. 2006). sixteen of the 59 communities had very mild summers (90th percentile summertime mean apparent temperature < 80°f). under our heat wave definition, the threshold temperature for a heat wave was low in these communities (e.g., 67.3°f in san francisco, ca). even though such temperatures are rare for these communities, we did not categorize days with these low temperatures as heat waves. therefore, we excluded these communities with mild climates. our final data set included 43 communities [see supplemental material, table 1 (doi:10.1289/ehp.1002313)]. we first identified all heat waves in each community for 1987 2005 using the community specific heat wave definition. we characterized each heat wave by its intensity, duration, and timing in season. heat wave intensity measured average tmean during the heat wave [other metrics for heat wave intensity were considered in sensitivity analysis; see supplemental material, table 2 (doi:10.1289/ehp.1002313)]. heat wave duration measured the heat wave’s length in days. we characterized the timing of the heat wave in the summer in two ways. first, timing in season measured the day in the season when the heat wave started (with 1 may = 0, 2 may = 1, etc.). second, first in season identified whether the heat wave was the first heat wave of its year. supplemental material, table 3 (doi:10.1289/ehp.1002313), gives an example of this heat wave classification. for each community, we estimated the increase in nonaccidental mortality risk during each heat wave compared with non heat wave days, controlling for daily temperature. several studies found that mortality risk increases on individual days of heat (e.g., anderson and bell 2009; baccini et al. 2008). hajat et al. (2006) discussed the concept of an added heat wave effect and evaluated whether heat wave days affected mortality risk differently than nonconsecutive individual days of high temperatures. this effect has been used to quantify the effects of single notable heat waves [e.g., the 2003 french heat wave (le tertre et al. 2006), the 1995 chicago heat wave (kaiser et al. 2007)] and to quantify effects of all heat waves over a study period (hajat et al. 2006; rocklov and forsberg 2008). we estimated this added heat wave effect for each heat wave using community specific generalized linear models. we controlled for daily maximum temperature to separate effects of heat waves from effects of single days of hot temperature; we also controlled for day of the week and adjusted dew point temperature and time trends to account for seasonal and long term changes in mortality patterns in a community. we used a nonordered categorical factor to identify heat waves. this variable took a different value for each heat wave. this approach is similar to methods used in earlier studies to compare mortality risk on heat wave days with non heat wave days (anderson and bell 2009; hajat et al. 2006); however, our model extends the earlier method by allowing risk of mortality to differ by heat wave. because we estimate a separate effect for each heat wave, we can investigate effect modification by heat wave characteristics (e.g., duration). the model can be stated as where μtc = expected mortality rate for community c on day t; δ0 = model intercept; ac = vector of regression coefficients for heat waves for community c (one per heat wave); hwt = 0 if day t is a non heat wave day, 1 if day t is the first day of any heat wave, 2 if day t is the second or later day in the first heat wave in the community, 3 if day t is the second or later day in the second heat wave in the community, and so forth; yc = vector of regression coefficients for day of the week for community c; dowt = categorical variable for day of the week; ns(timet) = natural cubic spline of time, with 3 degrees of freedom (df) per warm season (1 may 30 september); ns(ttc) = natural cubic spline of maximum temperature for community c for day t (df = 3 with knots at quantiles); and (dtc) = natural cubic spline of adjusted dew point temperature for community c on day t (df = 2). equation 1 estimates a separate mortality effect for each heat wave in a community. we used a community specific bayesian hierarchical model to generate an overall heat wave effect for each community by combining effects of individual heat waves within that community, incorporating the estimates’ variance (everson and morris 2000; kass and wasserman 1996): where β̂h = estimated effect of heat wave h on mortality, βh = true effect of heat wave h on mortality, v̂h = statistical variance of β̂h, μ = true average heat wave effect across all heat waves, τ2 = between heat wave variance of the true effect, and n = number of heat waves. this model was fit separately for each community. this method is often used to combine effect estimates across communities in air pollution and temperature studies (anderson and bell 2009; bell and dominici 2008). national and regional heat wave effects were estimated using similar multistage hierarchical bayesian models. results were generated separately for three u.s. regions, northeast, midwest, and south, based on regions used previously (anderson and bell 2009; barnett 2007). insufficient numbers of communities precluded estimates for other regions. within each community, we estimated the relationship between each heat wave characteristic (i.e., intensity, duration, or timing in season) and heat wave effects using a hierarchical bayesian model, with the heat wave characteristic as an independent variable: where xjh = heat wave characteristic j (intensity, duration, or timing) for heat wave h, x̄j = mean characteristic j across all heat waves, α0 = average ln(relative rate) when xjh = x̄j, α1,j = change in ln(relative rate) for unit increase in xjh − x̄j, and τ2 = variance of heat wave effects. we repeated this model separately for each heat wave characteristic and community. use of different heat wave definitions can change the days identified as heat waves [see supplemental material, figure 1 (doi:10.1289/ehp.1002313)]. as a sensitivity analysis, we recalculated main results using different heat wave definitions. similarly, we investigated effect modification using a variety of metrics for heat wave intensity. some heat waves have had alarming health effects. for example, > 3,000 excess deaths occurred in france during a 2003 heat wave (le tertre et al. 2006). previous researchers hypothesized that a heat wave might have a larger effect if it is more intense or longer or occurs at the beginning of summer (ellis et al. 1975; schuman 1972). we investigated whether these characteristics could explain the catastrophic health effects of certain heat waves. we defined a catastrophic heat wave as one associated with a mortality effect estimate at least three times larger than that of any other heat wave in the same community during our study period. only a few communities in our study experienced a catastrophic heat wave during our study period. (however, this does not mean that the other communities cannot or will not experience similarly devastating heat waves.) for each community that experienced a catastrophic heat wave, we created a multivariate bayesian hierarchical model for heat wave effects using 18 years of data (all years except the year of the catastrophic heat wave) with terms for heat wave intensity, duration, and timing in season. we then estimated the mortality effect for the catastrophic heat wave based on this model and compared results with the actual mortality effect. supplemental material, figure 2 (doi:10.1289/ehp.1002313), maps the communities included in this study. under our heat wave definition (≥ 2 consecutive days with daily mean temperature, tmean, higher than the community’s 95th percentile summertime tmean), communities in the study experienced on average 1.9 heat waves/year, with little variation by region (table 1). heat waves varied across regions in intensity, duration, and timing in season (figure 1). longer and more intense heat waves were more common in the u.s. south, although heat wave characteristics showed wide ranges within each community (figure 1, table 1). most identified heat waves were 2 3 days; heat waves > 7 days were rare in the north and midwest, and heat waves > 10 days were rare in any community (figure 1b). early or late heat waves (i.e., in may or september) were rare (figure 1c). in most communities, more intense heat waves were also likely to be longer and earlier in the season, although the three heat wave characteristics were not highly correlated [see supplemental material, table 4 (doi:10.1289/ehp.1002313)]. summaries of heat wave characteristics (1987 2005). for each characteristic shown, community specific averages were calculated and then averaged nationally or by region. heat wave characteristics in 43 individual communities (1987 2005). each horizontal line represents one community, and the length of each line indicates the range of each heat wave characteristic in that community. (a) intensity of each individual heat wave within a community (tick marks within blue bars). (b) number of heat waves according to duration (shading) by community. by definition, all heat waves lasted ≥ 2 days. (c) heat wave timing in season by community (tick marks, individual heat waves). we found, on average, higher risk of mortality during heat waves than during non heat wave periods, with variation in health effects of different heat waves (table 2). effect estimates were much lower in the south than in the northeast or midwest (table 2), although the threshold temperatures used to define heat waves (95th percentile of warm season daily tmean) were higher in the south [see supplemental material, figure 2 (doi:10.1289/ehp.1002313)]. some southern cities (e.g., charlotte, nc; dallas/fort worth, tx; oklahoma city, ok; tulsa, ok) showed no mortality increase during heat waves. summary of heat wave mortality effects (1987 2005). the heat wave effect is the increase in nonaccidental mortality risk for heat wave days compared with non heat wave days, controlling for daily temperature (the added heat wave effect described by hajat et al. 2006). the health effects of individual heat waves were associated with heat wave intensity, duration, and timing in many of the communities, with large heterogeneity across communities. figure 2 shows community specific plots of heat wave effects versus heat wave characteristics for the 10 most populous communities and maps of the overall association in each community. in general, all three heat wave characteristics were more strongly associated with heat wave effects in the northeast and midwest than in the south (figure 2, table 3). a 1°f increase in average tmean during a heat wave was associated with a 4.39% increase in the relative risk of mortality during that heat wave in the northeast and a 3.22% increase in the midwest. the association between heat wave duration and mortality effects was largest in the northeast, where mortality risk during a heat wave was on average 2.50% higher for every extra day a heat wave lasted. heat wave characteristics and heat wave effects in 43 communities (1987 2005): associations between heat wave effects (percent mortality increase during the heat wave vs. non heat wave days) and average tmean during each heat wave (a), heat wave duration (b), and timing in season of the heat wave (c). maps show relative risks of nonaccidental mortality during heat wave days compared with non heat wave days (controlling for daily temperature) and heat wave characteristics. on the map, the color of each community’s circle reflects the magnitude of effect modification; the size reflects the statistical variance, with larger circles indicating more precise estimates. graph insets plot the value of each heat wave characteristic versus its nonaccidental mortality effect [estimated as an added heat wave effect (hajat et al. 2006)] for the 10 most populous communities. for these graphs, each circle represents an individual heat wave, and the circle’s color reflects the variance of the mortality effect. the july 1995 chicago heat wave and august 1988 minneapolis/st. paul heat wave are omitted as they are outside the range of the figures. scales on x axes differ among the graphs for individual cities. percent increase in relative risk of mortality during a heat wave per unit increase in heat wave characteristic (1987 2005). data are percent increase in the relative risk of nonaccidental mortality on heat wave days compared with non heat wave days, controlling for daily temperature, for a unit increase in the heat wave characteristic. p < 0.05; p < 0.01. we divided heat waves into two groups: those that were the first to occur in a year (first in season) and those that were preceded by another heat wave in their year (not first in season) for a given year and community. on average, we categorized 40% of all heat waves as first in season. first in season heat waves generally had higher effects than did later heat waves, although effects were similar in the midwest (table 4). the importance of timing in the season on heat wave effects was also evident when we considered the influence of day in season (i.e., which day of the season the heat wave started) on heat wave effects, particularly in the northeast (figure 2c, table 3). average mortality effects of the first heat wave in a summer versus later heat waves (1987 2005). the heat wave effect is the increase in nonaccidental mortality risk for heat wave days compared with non heat wave days, controlling for daily temperature [the added heat wave effect described by hajat et al. (2006)]. we reestimated mortality risk during heat waves compared with non heat wave days using different heat wave definitions [see supplemental material, tables 2 and 5 (doi:10.1289/ehp.1002313)]. point estimates changed but trends were similar under all definitions considered: average heat wave effects were positive and significant, more intense and longer heat waves were positively associated with heat wave effects, and the first heat waves in the summer almost always had a higher average effect than later heat waves. heat wave intensity was significantly associated with heat wave effects for almost all heat wave definitions [see supplemental material, table 5 (doi:10.1289/ehp.1002313)]. regional trends were largely consistent regardless of heat wave definition, with higher, more significant heat wave effects in the northeast than in other areas and a stronger association between heat wave intensity and heat wave effects in the northeast (data not shown). we tested effect modification by heat wave intensity using several different metrics for intensity [see supplemental material, tables 2 and 7 (doi:10.1289/ehp.1002313)]. for each temperature metric, we considered the effect of the peak value (the highest value of that metric during a single day of the heat wave) and the average value across all days of the heat wave. metrics of heat wave intensity were not all strongly correlated across communities [see supplemental material, table 6 (doi:10.1289/ehp.1002313)], although intensity measures based on the peak value of a particular temperature metric and the average value of the same temperature metric were highly correlated (0.87 0.93). supplemental material, table 7 (doi:10.1289/ehp.1002313), shows the percent increase in mortality risk for heat wave days compared with non heat wave days for a 1°f increase in heat wave intensity, under different heat wave intensity definitions. more intense (i.e., higher temperature) heat waves were associated with higher mortality risk under all definitions of heat wave intensity. in a separate analysis, we analyzed effects of particularly extreme heat waves. we identified two catastrophic heat waves; both involved the same weather system: 12 16 july 1995 in chicago and 13 15 july 1995 in milwaukee, wisconsin. both heat waves were the most intense in their community over the study period (average tmean = 87.2°f for chicago, 87.7°f for milwaukee) [see supplemental material, figure 3 (doi:10.1289/ehp.1002313)]. they were not extreme in duration or timing in the summer (both were in july, and they lasted 5 and 3 days, respectively). mortality risks during these heat waves, compared with non heat wave days (using equation 1), were 133.9% [95% posterior interval (pi), 116.9 152.2%] for chicago and 93.0% (95% pi, 53.1 143.3%) for milwaukee. we investigated whether the mortality risk of these two events could be explained by their intensity, duration, and timing in season. we fitted multivariate community specific models separately for chicago and milwaukee, excluding heat waves during the year of the catastrophic heat wave (1995). we then used these two models to estimate the expected mortality risk from the two catastrophic heat waves and found an increase in mortality risk of 18.9% for the chicago catastrophic heat wave and 10.3% for milwaukee, much smaller than the mortality risks observed. estimated overall heat wave effects were similar to those calculated in an earlier study of u.s. heat waves that included more communities over a shorter time period (1987 2000); however, that previous study did not investigate variation in effects of individual heat waves within a community or explore how heat wave characteristics modify mortality risk (anderson and bell 2009). we found that, within a community, heat wave mortality effects are influenced by the heat wave’s intensity, duration, and timing in the season. this effect modification probably results from physiological responses to heat and/or behavior modification. when ambient temperature is high, the human body responds via thermoregulation: blood vessels dilate near the skin to transfer heat from the body’s core to the skin, and then sweat transfers heat from the skin by evaporation (havenith 2005). even when body temperature remains normal, thermoregulation strains the cardiovascular system (havenith 2005). the higher the temperature or the longer the heat wave, the more work required of the cardiovascular system to maintain normal temperature; therefore, more intense or longer heat waves are likely to have greater health effects. heat wave timing in the season might influence mortality risk from a) deaths of particularly susceptible individuals during heat waves or single hot days earlier in the summer, leaving a smaller pool of susceptible individuals later in the summer (mortality displacement), or b) acclimatization of individuals to heat over the course of a summer. evidence of both effects exists: studies considering health effects of both single days of heat (baccini et al. 2008; braga et al. 2001; hajat et al. 2005) and of specific heat waves (kaiser et al. 2007; le tertre et al. 2006) found evidence of mortality displacement, whereas experimental studies found that humans acclimatize to heat, usually within several weeks of exposure (bouchama and knochel 2002). we found that heat waves earlier in summer had higher mortality risk than did later heat waves and that the first heat wave of the summer had a higher impact than did later heat waves. additional research is needed to determine whether the lessening impact of heat waves over the summer is due to mortality displacement, biological adaptation (e.g., persons becoming physically accustomed to higher temperatures), and/or behavior modification [e.g., increased use of air conditioning (ac), staying indoors]. additional research could also investigate if the timing of heat waves has a nonlinear relationship with mortality effects. the characteristics of heat waves considered here (intensity, duration, and timing in season) could not fully explain variation in heat wave effects; for example, often heat waves of the same intensity or length affected mortality very differently within a community. other possible sources of variation in heat wave mortality risk within a community include changes over time in communities’ responses to heat waves (e.g., implementation of heat wave warning systems) and changes in public perception of heat waves (e.g., willingness to take protective measures) (kalkstein et al. 1996; kovats and ebi 2006). power outages sometimes occur during heat waves (e.g., bridger et al. 1976; luber and mcgeehin 2008), and these could contribute to particularly high impacts due to increased heat exposure without ac and greater difficulty leaving upper level apartments in buildings with elevators. often studies cite the differences between the effects of two heat waves as evidence of improvements in community response (e.g., palecki et al. 2001; weisskopf et al. 2002). this study shows a large variation in heat wave effects for a variety of reasons, suggesting that studies analyzing the efficacy of response measures should not rely on differences between only two heat waves or ignore differences in heat wave characteristics. heterogeneity between communities in both heat wave effects and effect modification by heat wave characteristics might result from a) physical acclimatization of residents of warmer communities, b) different levels of exposure in different communities (e.g., ac use, housing structure, clothing type), c) different community level responses to extreme heat (e.g., heat wave warning systems), d) different demographics (e.g., population in high risk categories such as the elderly or less healthy), or e) different geographical, meteorological, or pollution factors within communities that might confound or modify temperature mortality relationships. we observed general regional patterns in community level heat wave effects that are consistent with other studies of heat waves or high temperature, which report highest effects in the u.s. northeast and midwest (e.g., anderson and bell 2009; braga et al. 2001). some researchers found that temperature effects were related to ac (e.g., braga et al. 2001; o’neill et al. 2005), although an early study noted similar geographic patterns before widespread ac prevalence (gover 1938). in contrast, patterns are reversed in europe, where communities with warmer climates generally had larger heat effects (baccini et al. 2008). heterogeneity in effect modification by heat wave characteristics followed similar regional patterns. heat wave intensity, duration, and timing in the summer had smaller effects on mortality in the south than elsewhere. differences between the south and other parts of the country in exposure to outdoor air temperature (e.g., because of ac use) might explain why even very long and intense heat waves failed to affect mortality in some communities. different studies have used a variety of definitions to select what days constitute “heat waves.” additionally, a variety of theories exist concerning which temperature metric should be used to best capture the heat wave periods most dangerous to human health. some researchers propose that high minimum temperatures might be particularly important, theorizing that the body is more affected with no relief from cool nighttime temperatures (schwartz 2005). others suggest the use of apparent temperature, because the body may be more stressed if humidity limits cooling by sweat evaporation (robinson 2001). this question is complex, and the appropriate metric may vary by individual, depending on health status and exposure. for example, minimum temperature might be important for someone without ac but a less valuable metric for someone living in a home with central ac. we performed sensitivity analysis under a variety of heat wave definitions (intensity, duration, metric) to aid interpretation and meaningful comparison with other studies. we did not control for air pollution in our model, because earlier research showed that temperature effects in the united states were robust to air pollution (anderson and bell 2009). however, air pollution and temperature might act synergistically during heat waves; future research could investigate this question. several days early in july 1995 qualified as a heat wave in many of the midwest and northeast communities in our study; mortality effects were particularly extreme in chicago and milwaukee. the july 1995 heat wave was unusual in this data set in terms of the magnitude of mortality effects, although it was not particularly long or early in the season. we found that the extremity of this heat wave’s effects could partially be explained by its intensity, although comparison is difficult because the heat wave is outside the range of other heat waves in terms of intensity and mortality. although we found heat wave mortality risk to be affected by intensity, duration, and timing in season, these factors could not fully explain catastrophic heat wave effects. some variation may relate to changes from summer to summer in the number of people in a community who are very susceptible to heat. for example, one study found that heat related mortality can be affected by rates of influenza related mortality the preceding winter (rocklov et al. 2009). other factors, possibly social, may also play a role. such factors, including changes in community and public health responses to heat, are beyond the scope of this study and could be explored in future research. the finding that catastrophic heat wave effects cannot be adequately explained by heat wave intensity, duration, and timing in the summer highlights the difficulty of predicting catastrophic heat waves, either with heat warning systems or with long term climate change projections. a meteorologic study showed that heat waves of similar intensity to the july 1995 heat wave are very rare in chicago, although not unprecedented in the 20th century (kunkel et al. 1996). earlier studies of individual heat waves in the united states used different methods to estimate excess mortality but showed that mortality effects as high as those observed in chicago and milwaukee in july 1995 have occurred. estimated approximate mortality increases during other extreme heat wave periods, compared with non heat wave periods, for los angeles, california, are 109% in late september 1939 and 112% in early september 1955 (oechsli and buechley 1970); for st. louis, missouri, are 91% in july 1966 (bridger et al. 1976) and 159% in 9 14 july 1966 (henschel et al. 1969); and for new york, new york, are 113% in 28 august through 3 september 1948 (ellis and nelson 1978). study of heat waves’ effects on human health is important for the present day and central to estimating the impacts of climate change. heat waves are anticipated to be more common, longer, and more intense in the future (meehl and tebaldi 2004). this study is unique among heat wave studies in its national scale, long time frame, multiple heat wave definitions, and consideration of how heat wave mortality effects are associated with heat wave intensity, duration, and timing in the season. we found that mortality risk increased during prolonged periods of extreme heat, compared with the community’s usual climate, and estimated how this mortality risk was affected by heat wave intensity, duration, or timing within the year. because of the national scale, we were able to examine regional trends and observed both larger heat wave effects and greater effect modification by heat wave characteristics in certain regions, particularly the northeast; trends were less defined or in some cases inversed in some communities in the south. heat wave related mortality under a changing climate will need to address acclimatization over a long time frame (e.g., changes over years) and a shorter time frame (e.g., changes within a summer). additionally, the observed heterogeneity, both in heat wave effects and in the influence of heat wave characteristics on mortality effects between different communities, indicates the importance of developing heat wave response plans that are community specific. supplemental material is available online (doi:10.1289/ehp.1002313 via http://dx.doi.org/). this work was supported by a u.s. environmental protection agency science to achieve results graduate fellowship (91689201 0), a national science foundation graduate fellowship, the national institute for environmental health sciences (es012054 03), and the u.s. environmental protection agency through the johns hopkins particulate matter research center (rd 83241701). articles from environmental health perspectives are provided here courtesy of national institute of environmental health sciences connect with nlm national library of medicine 8600 rockville pike bethesda, md 20894
researchers have found that longer lasting heat waves can be deadlier and can pose unique health risks. by raymond zhong for tens of millions of americans this week, summer is starting with a multiday blast of fierce heat. temperatures in parts of the country are likely to be well into the 90s through the weekend. the national weather service has warned that the heat wave could be the longest one some places have experienced in decades. weather this hot creates plenty of health risks, even for people who sweat through it for just a few hours. but researchers have found that the dangers of heat can be compounded when conditions are sweltering for day after grinding day, night after sleepless night. with so much of the united states facing such hazards this week, it’s a good time to linger a bit on what we know about heat that lingers. in general, human caused global warming is making heat waves even hotter, more frequent and longer lasting. already this year, india has experienced what its top weather official described as the country’s longest hot spell on record, spanning 24 days in april and may. greece is in its second week of deadly heat, less than a year after experiencing a 16 day heat wave, the longest it has measured. for the planet as a whole, 2023 was the warmest year in human history and global temperatures have continued breaking monthly records well into 2024. when it’s very hot, the human body has to work harder to keep organs and tissues at their normal healthy temperatures. the longer your body is forced to do this, the greater the strain on your cardiovascular system, and the higher the risk of negative health effects, which, in extreme cases, can include heart and kidney failure. how much more risk? two researchers, g. brooke anderson and michelle l. bell, crunched the numbers for 43 u.s. communities between 1987 and 2005. they found that for every additional day that a heat wave dragged on, the risk of nonaccidental death rose by 0.4 percent. for every extra degree fahrenheit above normal temperatures, the risk increased by 2.5 percent. both effects were more pronounced in the northeast than in the midwest or south, the researchers found, possibly because people there were less accustomed to stifling weather.

deadly heatwaves around the world are being made more likely by the climate crisis. here, we look at how extreme heat impacts health, who is most at risk   and what can be done to help. a man is silhouetted against the sun as he bathes on a hot summer day at a village on the outskirts of ajmer, india, on 11 june 2024 amid a heatwave.