For over 50 y, detailed scientific investigations have examined nearly every aspect of ozone and other photochemical oxidants, and this robust body of research has served as the basis of each succeeding NAAQS determination by EPA. The research has taken several forms:
Atmospheric chemistry to understand the sources of ozone precursors and the formation of ozone and other photochemical oxidants.
Exposure research to characterize pollutant concentrations and human activity patterns that lead to contact with the pollutants over specific periods. Toxicologic research in vitro test systems (such as various human cells and other biologic media) and in vivo experiments (involving a wide array of animal species and animal models of human disease).
Epidemiologic research, including panel, cohort, and broader population studies of the acute or chronic effects of ozone exposure on morbidity and mortality in children and adults.
Controlled human exposure (or “clinical”) studies of varied healthy and compromised adult volunteers exposed to ozone or ozone combined with other gases.
Research findings have contributed to the understanding necessary to establish a NAAQS. In particular, the human studies—epidemiologic and clinical studies—have contributed several key pieces of standard-setting information, including information on the effects of real-world exposure of human beings to contaminants, the smallest exposure that can be demonstrated to have effects in humans, and the degree to which effects are found in sensitive populations.
Overall, extensive toxicologic, epidemiologic, and clinical research on the effects of exposure to ozone has yielded strong evidence of effects on respiratory end points at or near current regulatory levels, and in a variety of populations. However, clinical studies have shown substantial variability in individual responses; even sensitive populations, such as asthmatics, exhibit a range of responses and nonresponse (EPA 2006a). Although an extensive literature—in toxicology, epidemiology, and controlled human exposure studies—has reported consistent evidence of such effects, there are still uncertainties in our understanding, for example:
Uncertainty about the relationship between outdoor ozone concentrations and personal exposure, especially of persons who spend most of their time indoors or use air conditioning during periods of peak ozone concentrations.