 set point levels , blood circulation is controlled in specific areas of the body through the sympathetic nervous system which dilates the cutaneous vascular bed , and hence increases skin blood flow and invokes the sweating mechanism if necessary .
this provides greater potential for heat to flow from the body and hence maintain body temperature . because the heart can not supply blood to all of the body 's organ systems , the autonomic nervous and endocrine system control allocation of blood to competing organs . during exercise
there is an initial sympathetic vasoconstriction , so that blood may flow to active muscles .
if heat is required to be dissipated there is an increased cutaneous blood flow . during continuous work , in the heat
sweat glands are stimulated by cholinergic sympathetic nerves and secrete sweat onto the surface of the skin .
sweat rates of 1 l per hour are common and for each litre evaporated , 675 w of heat are lost per hour ( 2 ) . however , large sweat losses reduce body water content and hence thermoregulatory effectiveness . during sweating ,
salt is lost at about 4 g per litre ( 1 g per litre in acclimatised persons ) . as a normal diet provides 814 g of salt per day ,
salt tablets can irritate the stomach and heavier use of salt at meals is preferred , but salt supplementation will normally not be required ( 2 ) .
potassium is also lost in sweat and a high salt intake may increase potassium loss . in most cases ,
however , potassium will be replaced by a normal diet ( especially fruits and vegetables ) .
the overall physiological response for continued heat storage is therefore vasodilatation to increase skin temperature and then sweating leading to profuse sweating ( including ineffective dripping of some sweat losing insignificant heat but important water ) . as 
core temperature continues to rise and the skin is completely wet , hidromeiosis ( a reduction in sweating ) may occur due to swelling and blocking of sweat glands in the wet humid conditions ( 3 ) .
the decrease in sweating promotes a further , often rapid , increase in  core temperature to beyond 3839c , where collapse may occur to above 41c ( rectal temperature ) and heat stroke may occur .
there will be mental confusion , behavioural changes , failure in central nervous thermoregulation and sweating , and death with eventual denaturing of body protein .
national institute for occupational safety and health ( niosh ) ( 2 ) consider age , gender , body fat , drugs ( including alcohol ) and other non - thermal disorders as important individual factors .
there is a large individual variability in the mechanisms of response , which are not fully understood .
physical fitness , however , has been shown to be of great importance . the mnemonic , 
this is : sensible ( i.e. appropriate behaviour ; reduce exercise or work activity ) ; hydrated ; acclimatised ; fit ; thin : and sober ( including avoidance of alcohol and other drugs ) .
leithead and lind ( 4 ) conclude that heat disorders occur for one or more of three reasons : the existence of factors such as dehydration or lack of acclimatisation;the lack of proper appreciation of the dangers of heat , either on the part of the supervising authority or of the individuals at risk ; andaccidental or unforeseen circumstances leading to exposure to very high heat stress.they conclude that many heat - related deaths can be attributed to neglect and lack of consideration and that even when disorders do occur much can be done if all the requirements for the correct and prompt remedial treatment are available . in climates such as those found in singapore , military personnel
are exposed to hot , humid conditions and must carry out essential tasks in protective clothing .
although heat stroke will occur , severe consequences have been avoided by organisational methods including extensive training of personnel and an efficient back - up system to transport casualties rapidly to hospital .
the existence of factors such as dehydration or lack of acclimatisation ; the lack of proper appreciation of the dangers of heat , either on the part of the supervising authority or of the individuals at risk ; and accidental or unforeseen circumstances leading to exposure to very high heat stress .
causes of heat stroke ( from ref . ( 5 ) ) classification , medical aspects and prevention of heat illness ( from ( 6 ) ) there are a number of other complaints related to heat exposure . for example , in mildly sunburnt skin , sweat can be trapped and accumulate under the dead surface layer and cause discomfort as well as reducing evaporative efficiency . in industries where chemicals and particulates are present in the air
the interaction between chemical substances in the air and a sweating person has yet to be fully explored but can be significant .
pollution and poor air quality may be an additional hazard along with heat and in combination may cause and exacerbate respiratory disorders .
protective clothing and equipment may promote sweating and will reduce the ability to evaporate sweat to cool down .
this will often increase the risk for the person the clothing is protecting against and dangerous levels can occur at what would normally be considered to be moderate temperatures .
acclimatisation refers to the increased ability of a person to reduce heat strain due to experience of exposure to heat .
behavioural acclimatisation is most effective and is mainly gained in the context of the heat exposure . if we go to a hot climate , we soon learn to drink appropriately and keep out of the sun .
physiological acclimatisation involves adjustments to our ability to thermoregulate and allow a greater capacity for and more efficient vasodilatation and in particular , sweating .
when people become exposed to heat they increase their ability to sweat ( 7 ) .
it is possible to gain physiological acclimatisation by exposing people for a few hours per day to high levels of heat ( e.g. in a laboratory  called acclimation ) and this is often used as a technique to reduce strain when people are about to travel to a hot climate . in short ,
if a person is used to being exposed to heat then he / she will adjust physiologically to some extent and learn how to behave to tolerate the heat .
in extremely hot tropical environments this  behavioural change will include a reduction of physical activity workload , and an increase of frequency and length of rest breaks .
if they are not used to the heat and when the weather suddenly becomes hot , then they will be more vulnerable to heat strain .
there is no reason why a person should suffer ill health in a heat wave as the principles of reducing heat strain are well understood .
physiological responses to heat have generally been studied on fit young males and under conditions that could be regarded as very hot and towards the extremes of weather conditions . for less fit and vulnerable people , physiological responses such as increased heart rate , prolonged
sweating and respiratory responses ( related to air quality or dryness ) will pose a threat .
increased body temperature will eventually be a threat to health , but it may not be the primary cause of illness and death in a heat wave .
we set up a management system to ensure that people are not exposed to unacceptable heat stress .
heat stress is determined by the interaction of air temperature , radiant temperature , air velocity and humidity .
it is also greatly influenced by the clothes a person is wearing and the activity they are performing .
a full discussion of management programme for heat waves is provided in ( 8) . of particular importance will be the availability of household water and electrical power . moving people away from hot homes to cooler areas ( e.g. public libraries or modern shopping centres ) will reduce heat strain but may not be practicable .
the use of water provides an effective and often practical method of keeping cool in the heat .
immersing hands and arms in cool water is a practical way for vulnerable people to avoid health problems in the heat .
for those with a bathtub , running a normal warm bath ( e.g. 38c water temperature ) to half full and getting in , then running in cool water to fill the bath allows a non - stressful method of keeping cool .
the initial warm water will promote vasodilatation and will not cause thermal shock , which will occur with cool water and raise blood pressure .
cool bath will greatly enhance heat loss . spraying water on the face or exposed skin and enhancing evaporative cooling with fans , for example ,
will reduce heat loss even when the air temperature is above desired internal body temperature . the direction and strength of fans , however , has to be controlled as evaporation of mucous from the airways and moisture from the eyes can cause irritation and respiratory problems .
the use of air conditioning is very effective in reducing air temperature and hence thermal strain .
it requires electrical power , however , and if electric grid systems can not meet capacity then lack of power can cause a critical problem .
it is not exhaustive and the reader is referred to ( 8) for a complete discussion .
conditions for thermal comfort are well described by fanger ( 9 ) and were later adopted worldwide in iso 7730 ( 10 ) .
they are , that for a person to be in thermal comfort , he / she must be in heat balance ( maintaining internal body temperature ) and that skin temperatures and sweat rates must be within comfort limits . a method is derived from those three premises that allows the prediction ( from air temperature , radiant temperature , air velocity , humidity , clothing and activity ) of thermal comfort conditions and of any thermal discomfort . the predicted mean vote ( pmv ) index predicts the mean rating of a large group of people exposed to thermal conditions ( six factors above ) on the scale : + 3 hot ; + 2 warm ; + 1 slightly warm ; 0 neutral ; 1 slightly cool ; 2 cool ; 3 cold . for a given pmv value ,
parsons ( 1 ) and hodder and parsons ( 11 ) extend the scale from 7 to 11 points by adding very hot / cold and extremely hot / cold on either end .
they then provide a simple rule of thumb method for taking account of the direct sun on a person .
for every 200 wm of solar radiation ( from 0 wm totally cloudy to 600 blue sky to 1,000 absolute maximum ) , the pmv ( now pmvsolar ) is increased by one scale value .
it is emphasised that this rule of thumb provides a simplistic practical method which accounts for main effects . for
a more accurate assessment , direct and diffuse radiation , depending upon turbidity and cloudiness , elevation of the sun , posture of the person and more , also play a role but involve increasingly complex analysis . to take account of the ability for people to change behaviour or adapt to the environment , parsons ( 1 ) suggested an iequiv index which allows the clothing insulation value , used in the calculation of pmv , to be adjusted to take account of the effects of any adaptive opportunities in the environment ( e.g. ability to reduce clothing , open windows , etc . ) .
the iequiv method provides a simplistic but direct method of accounting for adaptive opportunities available to people .
 causal adaptive models which relate indoor comfort temperatures to outside conditions based upon data from thermal comfort surveys ( e.g. 12 ) . the  causal model assumed in such models is that the effects of the adaptive opportunity , such as that used in the iequiv method , are captured in the behaviour of people to outside conditions , for example , a reduction in clothing when outside temperatures are high .
the iequiv method suggests that a consequence of this is to provide the suggestion that for reasonable levels of adaptive opportunity , it will not be necessary to cool offices or homes below 25c to provide thermal comfort .
this measure is related to sustainable thermal comfort discussions ( 13 ) and will also have the effect of significantly reducing electrical power requirements and hence avoiding power failure .
it is interesting to note that in japan a few years ago , the government in tokyo ran a campaign to set a lower limit for cooling offices to 28c air temperature .
the 28c limit clearly requires less energy to achieve in hot outside conditions than a limit of 25c or below , however , it is likely to cause some discomfort .
the practical point is that it is a common practice to cool rooms to around 20c ( and often causes cold discomfort ) in hot outside conditions .
energy saving by not cooling below 25c would be highly significant worldwide and it is generally agreed would still allow thermal comfort .
the most effective way to maintain human performance and productivity during a heat wave is to provide thermal comfort conditions .
this is not always possible , however , and in hot conditions productivity is likely to fall .
this is by no means certain , however , and motivation , morale and team spirit will all be significant factors .
there have been numerous studies into the effects of heat on manual dexterity and cognitive performance .
the most direct and measurable effect on productivity , however , is  time off task ( 1 ) caused by work having to be stopped or due to heat causing a distraction to people ( e.g. concentrating or finding fans or adjusting control systems or simply paying attention to the heat ) .
the relationship between the level of distraction caused by heat and the level of heat stress is not known .
further investigation is needed if the full economic costs of heat waves are to be determined . for practical purposes
the costs in terms of health and loss in productivity can be weighed against the costs of systems for ensuring the avoidance of unacceptable thermal strain during heat waves .

 much is known about human response to heat and basic principles can be used to provide guidance on effective heat management systems.with electrical power and especially water , it is possible to avoid health problems due to heat.for sedentary and light activities it is not necessary to cool rooms below 25c to provide thermal comfort.there is a need to determine the relationships between levels of heat stress and how much distraction and time off work it causes . 
 
much is known about human response to heat and basic principles can be used to provide guidance on effective heat management systems . with electrical power and especially water , it is possible to avoid health problems due to heat . for sedentary and light activities
there is a need to determine the relationships between levels of heat stress and how much distraction and time off work it causes .
the author has not received any funding or benefits from industry to conduct this study .
 backgroundthe aim of this paper is to summarise what is known about human response to heat and to use this knowledge to provide guidance on how to maintain the health , comfort and performance of people in heat waves.designthe use of power and especially water are critical in providing cooling . 
 a practical method of cooling people in a water bath is described . 
 a warm bath slowly cooled will provide effective cooling but not thermal trauma.resultit is concluded that for sedentary and light activities , it is not necessary to cool offices or homes below 25c for thermal comfort.conclusionto compare the costs due to loss of productivity caused by a heat wave , with the cost of taking action , more research is needed into the relationship between levels of heat stress and how much distraction and  time off task it causes . 
Physiological response to heat
Heat physiology and health
Acclimatisation
Maintaining health in heat waves
Maintaining comfort in heat waves
Maintaining productivity in heat waves
Conclusions
Conflict of interest and funding