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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 the world 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 preclude the development of episodes of this magnitude today. However, certain environmental air pollutants, such as ozone and respirable particles, do reach levels that may cause acute and chronic respiratory effects. Furthermore, in some eastern European and Asian countries, where sulfur-containing fuels are burned without adequate air quality regulations, air pollution levels may be attained similar to those that were associated with excess mortality.


The Clean Air Act (CAA) was passed by the U.S. Congress in 1970 and last amended in 1990. It is the principal federal standard addressing outdoor air quality. It requires the 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 pollutants. The subsequently produced documents are used in the development of a National Ambient Air Quality Standard (NAAQS) for each of the so-called criteria pollutants. Table 46–1 lists the six criteria air pollutants, their NAAQSs, and their principal adverse health effects.

Table 46–1.Criteria air pollutants.

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