(HAPs). VOCs and HAPs can also be precursors to or components of PM. In a similar manner, NOx has several significant environmental effects that warrant its control. In addition to being an O3 precursor, it has a direct impact on human health, is a precursor for acid rain and the formation of inhalable fine particles, decreases atmospheric visibility, and contributes to the eutrophication of water bodies.
The primary objective of most air quality standards in the United States is the protection of human health. Humans are exposed to air pollution outdoors and indoors, including during transit in vehicles. Indoor air pollution comprises a mixture of contaminants penetrating from outdoors and those generated indoors. Especially high exposure to air pollution can also occur in various microenvironments, referred to here as “hot spots,” which include highway toll plazas; truck stops; airport aprons; and areas adjacent to industrial facilities, busy roadways, and idling vehicles. Some studies have suggested that disproportionate exposures may be found in low-income and predominantly minority communities and have raised concerns about environmental justice (see discussion on environmental justice in Chapter 2 and references cited therein).
Many types of health effects have been attributed to air pollution, including pulmonary, cardiac, vascular, and neurological impairments—all of which can lead directly to mortality. In addition, a number of regulated air pollutants are known or probable carcinogens. Some health effects, such as an increase in asthma attacks, have been observed in conjunction with episodes of high pollution concentrations lasting 1 or 2 days. Such effects are considered acute, because they are associated with short-term exposures to a pollutant. Other health effects, particularly increased risk of cancer, are associated with long-term exposure (EPA 2002b).
The scientific techniques for assessing health impacts of air pollution include air pollutant monitoring, exposure assessment, dosimetry, toxicology, and epidemiology (NRC 1998b, 1999a). Because most of the health effects attributable to air pollutants can also be attributable to a wide variety of other risk factors, the impact of air pollution on human health can be difficult to distinguish and quantify. Determining the impact of air pollution on human health is further complicated by human exposure to a mixture of substances at various concentrations present in the air. Also, a number of subgroups within the human population at large are considered more susceptible to the effects of air pollution. They include people who have coronary disease, asthma, or chronic pulmonary diseases; the elderly; and infants. Fetuses are also possibly susceptible (Wilhelm and Ritz 2003).