Environmental Federalism when Numbers Matter More than Size

Abstract

C. Urban Density and Air Quality Ambient levels of criteria pollutants vary dramatically across the country, but the highest levels occur disproportionately in large urban areas. (171) Figure 6 displays the counties in nonattainment for ozone and small particulate matter ([PM.sub.2.5]), which are the criteria pollutants that affect the largest number of people and are associated with the greatest cumulative health risks. The relatively small number of areas in nonattainment obscures the large size of the affected population--123 million people, or about 40 percent of the U.S. population, for ozone and 91 million, or about 30 percent of the U.S. population, for [PM.sub.2.5]. The aggregate population in nonattainment areas for all six criteria pollutants is almost 150 million, which represented about half of the U.S. population in 2005. [FIGURE 6 OMITTED] The strong correlation between high ambient levels of criteria pollutants and urbanization explains the divergence between population and geographic area--almost every nonattainment area, aside from California's Central Valley, encompasses a major metropolitan center. The cities in California, Texas, and the northeastern states, which are home to many of the largest cities in the country, stand out as areas of poor air quality for criteria pollutants. A quantitative measure of this geographic pattern is provided in Figure 7, which displays the populations of nonattainment areas by state. This grouping of the data also underscores the extent to which elevated levels of criteria pollutants are concentrated in a few states. Ambient levels of air toxics, and the cancer risks associated with them, likewise vary across the country. Figure 8 displays the populations, at the census-tract level, (172) exposed to different levels of cancer risks from NATA Toxics in 2005. While about 90 percent of the U.S. population was exposed to excess cancer risks of 20 to 80 per million, the distribution has a long tail that extends above 200 per million. In this tail of the distribution, 16 million people were exposed to excess cancer risks above 100 per million. [FIGURE 7 OMITTED] Most of the people who are exposed to these elevated cancer risks live in urban areas. In the ten largest cities, which were home to about 27 percent of the U.S. population in 2005, the average excess cancer risk from all outdoor sources of air toxics was 68 per million (by contrast, the cancer risks from industrial sources in these cities averaged about 2 per million). More importantly, the most severe risks from air pollution are concentrated in the largest cities: 88 percent of the 16 million people subjected to cancer risks above 100 per million--a level EPA deems clearly unacceptable--live in the ten largest metropolitan areas. (173) [FIGURE 8 OMITTED] At the other end of the distribution, only about 40 thousand people lived in census tracts where cumulative excess cancer risks from NATA Toxics were below 10 per million. (174) Moreover, these estimates represent a lower bound on cancer risks, as they are based on a subset of all air pollutants. This result reveals that cancer risks from air toxics exceed EPA's target risk of one per million by tenfold even in very remote areas of the country. (175) An important implication of this finding is that background risks from air toxics, whether from natural or distant anthropogenic sources, are responsible for a baseline excess cancer risk of roughly 10 per million that cannot be influenced by local regulatory efforts. [FIGURE 9 OMITTED] Figure 9 displays census-tract-level data on the excess cancer risks from air toxics. (176) While elevated cancer risks are closely associated with urban areas, the impacts from industrial sources and inter-jurisdictional transport of air toxics is evident in the rural areas of the southeastern states. However, unlike the largest metropolitan areas, the cancer risks in virtually all of these areas still fall within the range of 40 to 70 per million, making them comparable to the national average of 50 per million. …