One of the shortcomings in the available literature is that most of the reported studies of environmental justice have characterized disparities only in terms of proximity to potential sources of exposure or, in a few cases, to measured exposures and have not taken the next step of trying to characterize or quantify either exposure or potential differences in health status or disease incidence between these populations and the general population (Bullard, 1990; Glickman et al., 1995; Greenburg, 1993; United Church of Christ Commission for Racial Justice, 1987; U.S. General Accounting Office, 1983; Zimmerman, 1993). Few studies have looked directly at whether local, residential exposure to environmental agents is associated with an increased incidence of disease (Sexton et al., 1993; Wagener et al., 1993).
Part of this shortcoming may be due to the fact that, except for some hazards such as lead (see Characterizing Exposure above), little is known about the physiologic or biological mechanisms by which the health hazards cause disease. Although the toxicologies of ozone and other air pollutants have been extensively studied, the mechanisms by which such gases and particles injure the lungs are diverse and not yet fully understood. The goal of fully understanding the effects of any air pollutant has been described as a "daunting task" (Brooks et al., 1995).
A large proportion of what is known about disease processes has come from research in the occupational health field. Examples include lung diseases due to dusts (e.g., silica, coal, cotton), the systemic and lung toxicities of metals (e.g., lead, mercury, cadmium, beryllium, and hard metals) and many carcinogens (e.g., asbestos, hexavalent chromates, nickel, vinyl chloride, polycyclic aromatic hydrocarbons, radon, and bischloromethyl ether), and the neurotoxicities of a variety of pesticides and solvents.
The lack of knowledge about the specific disease process does not, however, preclude the use of epidemiologic methods to explore possible associations between hazards and disease. The air pollutants listed in Table 2-2 have long been associated with clinically significant adverse health effects, including decreased respiratory function, respiratory infections, the exacerbation of asthma, chronic obstructive pulmonary disease, congestive heart failure, and increased mortality (Brooks et al, 1995). A recent National Research Council report (1998) describes how epidemiologic research has established consistent associations between exposure to outdoor concentrations of small particulate matter (particles smaller than 2.5 micrometers [0.006 inches] in diameter) and the adverse health effects described above. Although the biological basis for such associations is largely unknown and there is limited scientific information about the specific types of particles that cause these health effects, the results of these epidemiologic studies have been relied upon by EPA in setting national ambient air quality standards (National Research Council, 1998). Thus, at least for some air pollutants and lead poisoning, there is strong evidence to support the connection between disproportionate exposure and disproportionate health outcome.