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The dramatic air pollution episodes that occurred in the early part of the twentieth century in Belgium’s Meuse Valley, Donora, Pennsylvania, and London, England, are not likely to occur in high-income countries today. These episodes were caused by the large-scale burning of coal in the presence of “ideal” meteorologic conditions—atmospheric inversion leading to a stagnant air mass. A clearly evident excess mortality was observed during and after these episodes. Current air quality standards in North America generally avoid the development of episodes of this magnitude today. Yet, in some Asian countries, where sulfur-containing fuels are burned without adequate air quality regulations, air pollution levels may be attained similar to those that occurred during the historic episodes listed above. Furthermore, catastrophic wildfires associated with climate change have begun to create pollution crises that could rival those historic events. In addition to air pollution crises, certain outdoor air pollutants, such as ozone and respirable particles, regularly reach levels that may cause chronic health effects. Thus, clinicians should be aware of factors contributing to high-level air pollution episodes around the globe, as well as the health effects that can be attributed to chronic, lower-level exposures.


The Clean Air Act (CAA) was passed by the U.S. Congress in 1970 and last amended in 1990. It is the principal US standard addressing outdoor air quality. It requires the U.S. Environmental Protection Agency (EPA) to list those pollutants for which there is sufficient scientific evidence documenting the risk to public health from unregulated exposure. To achieve this, the EPA periodically reviews a large body of scientific research dealing with the adverse health effects of the so-called criteria pollutants. These reviews do not involve a cost-benefit analysis. The subsequently produced integrated science assessment documents are used in the development of a National Ambient Air Quality Standard (NAAQS) for each of the criteria pollutants. Table 50–1 lists the six criteria air pollutants, their NAAQSs, and their principal adverse health effects.

Table 50–1.Criteria air pollutants (U.S. National Ambient Air Quality Standards).

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