Although the CAA enables EPA to consider the full range of sources of a particular set of emissions in considering residual risk, in practice, EPA has focused primarily on the emissions from the source categories that were the subject of MACT. To better address the full range of pollutant exposures in all settings (especially in hot spots and in areas surrounding major stationary sources), EPA should attempt to include other major sources of the same chemicals as much as possible, so that the contribution of the MACT-regulated source is assessed in the context of its contribution to broader exposures. That assessment might take the form of creating hot-spot scenarios for estimating risk, drawn from actual locations of some of the regulated stationary sources. Targeted monitoring in areas in which relevant industrial activity is heavily concentrated could be useful in this attempt.
Enhance protection of ecosystems and other aspects of public welfare.
The CAA was established to protect both human health and welfare, and in one key aspect, the setting of NAAQS, the CAA mandates the establishment of both primary standards to protect public health and secondary standards to protect welfare (including sensitive ecosystems, forests, crops, materials, historical monuments, visibility, and other resources). Indeed, ecosystems provide invaluable services, such as the supply of high-quality water, soils that support the structure and function of ecosystems, forest and crop production, diverse aquatic habitat, and maintenance of fisheries. A loss or limitation of these services as a result of air pollution can therefore have significant consequences on the economy and quality of life.
However, programs and actions undertaken thus far in response to the CAA have largely focused on the protection of human health, neglecting efforts to protect environmental quality with secondary standards or to take actions to address air pollution impacts on ecosystems and crops.
The current practice of using the primary standard to serve as the secondary standard for most criteria pollutants does not appear to be sufficiently protective of sensitive crops and unmanaged ecosystems (see Chapter 2), although in one case EPA did recommend a separate secondary standard that was never implemented (EPA 1996b).
Concentration-based standards are inappropriate for some resources at risk, such as soils, groundwater, forests, surface water, and coastal eco-