Air pollution is a significant cause of morbidity and mortality worldwide, particularly among at-risk individuals with preexisting pulmonary or cardiac disease. The life-giving air that we breathe is also often laden with many potential causes of disease. Airborne microorganisms have long been major causes of morbidity and death. More widespread are the chemical and particulate pollutants found in the air, both in so-called “developed” and “underdeveloped” countries. Specific hazards have been recognized for both outdoor and indoor air.
The ambient air in industrialized nations is contaminated with an unsavory mixture of gaseous and particulate pollutants, more so in cities and in proximity to heavy industry. In the United States, the Environmental Protection Agency (EPA) monitors and sets allowable upper limits for six pollutants: sulfur dioxide, CO, ozone, nitrogen dioxide, lead, and particulate matter. Together, some of these agents produce the well-known smog that sometimes stifles major cities such as Cairo, Los Angeles, Houston, Mexico City, and São Paulo. It may seem that air pollution is a modern phenomenon. This is not the case; Seneca wrote in AD 61 that he felt an alteration of his disposition as soon as he left the “pestilential vapors, soot, and heavy air of Rome.” The first environmental-control law was proclaimed by Edward I in 1306 and was straightforward in its simplicity: “Whoever should be found guilty of burning coal shall suffer the loss of his head.” What has changed in modern times is the nature and sources of air pollutants, and the types of regulations that control their emission. It could be argued that modern man has lost his head to drown himself in pollution!
The lungs bear the brunt of the adverse consequences of air pollution, but air pollutants affect many organ systems (as with the effects of lead poisoning and CO, discussed later). More detailed discussion of pollutant-caused lung diseases is found in Chapter 13. Here we consider the major health effects of ozone, sulfur dioxide, particulates, and CO (
Table 8.1
Table 8.1 Health Effects of Outdoor Air Pollutants
DatafromHealthEffectsofOutdoorAirPollution,Part2.CommitteeoftheEnvironmentalandOccupationalHealthAssemblyoftheAmericanThoracicSociety,Am J Respir Crit Care Med 153:477,1996.
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Ozone is one of the most pervasive air pollutants, with levels in many cities exceeding EPA standards. It is a gas formed by sunlight-driven reactions involving nitrogen oxides, which are released mostly by automobile exhaust. Together with oxides and fine particulate matter, ozone forms the familiar smog (from the words smoke and fog). Its toxicity stems from its participation in chemical reactions that generate free radicals, which injure the lining cells of the respiratory tract and the alveoli. Low levels of ozone may be tolerated by healthy persons but are detrimental to lung function, especially in those with asthma or emphysema, and when present along with particulate pollution. Unfortunately, pollutants rarely occur singly but combine to create a veritable “witches’ brew.”
Sulfur dioxide, particles, and acid aerosols are emitted by coaland oil-fired power plants and industrial processes burning these fuels. Of these, particles appear to be the main cause of morbidity and death. Particles less than 10 µm in diameter are particularly harmful, because when inhaled they are carried by the airstream all the way to the alveoli. Here, they are phagocytosed by macrophages and neutrophils, causing the release of mediators (possibly by activating inflammasomes, Chapter 2) and inciting an inflammatory reaction. By contrast, larger particles are removed in the nose or are trapped by the mucociliary “escalator” and as a result are less dangerous.