• Uncertainty about the degree to which epidemiologic studies can identify the effects of individual pollutants, such as ozone, that are present in mixtures of pollutants in the ambient air.

  • Uncertainty about the degree to which current study designs and datasets enable understanding of risks resulting from exposures at very low pollutant concentrations and whether there is any threshold for effects.

  • Uncertainty, despite high-quality studies of long-term effects on lung growth in children (CalEPA 2006), about long-term exposure to ozone as a risk factor for chronic illness and premature death.

On the basis of the available evidence, EPA in its most recent criteria document (EPA 2006a, p. 8-77) concluded that “the overall evidence supports a causal relationship between acute ambient ozone exposures and increased respiratory morbidity outcomes resulting in increased ED (emergency-department) visits and respiratory hospitalizations during the warm season.” EPA’s most recent staff paper (EPA 2007a, pp. 5-92 and 5-93), summarizing staff recommendations on the NAAQS for the administrator) further concludes that there is clear and convincing evidence of causality for lung function decrements in healthy children under moderate exertion for 8-hr average ozone exposures. We also judge that there is strong evidence for a causal relationship between respiratory symptoms in asthmatic children and ozone exposures and between hospital admissions for respiratory causes and ambient ozone exposures. There is greater uncertainty and somewhat less confidence about the relationship between ozone and non-accidental and cardiorespiratory mortality, although the Criteria Document’s overall evaluation is that it is highly suggestive that this relationship exists. The strengths and weaknesses of the recent literature on mortality and ozone are explored in greater detail in Chapter 4.

Recent reviews of the evidence by the World Health Organization in its establishment of world air-quality guidelines (WHO 2006) and by the California Air Resources Board in its establishment of state air-quality standards (CalEPA 2007) have reached similar conclusions although the latter reviews have given somewhat greater causal weight to the associations with premature mortality.

Evolution of the Science of the Health Effects of Ozone and Particulate Matter

A broad and deep literature on the health effects of exposure to ozone has developed over the last 35 y and has resulted in a series of actions to set and revise the NAAQS and to reduce emissions of precursors. Similarly, over the last 15 y, much scientific and regulatory activity has been generated by intense interest in the effects of exposure to particulate matter (PM), especially PM with a diameter equal to or less than 2.5 μm (PM2.5) (EPA 2007f). There are useful parallels and differences between the literature on ozone and that on PM2.5, and



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