they have to some extent resulted in different approaches to the estimation of benefits of reducing exposure and in differences in actions taken.

In general terms, the ozone literature developed first out of observations of populations exposed outdoors (for example, panel studies of summer campers) and then from what has become a large set of controlled human exposure (clinical) studies. More recently, and to some extent in parallel with the PM literature, ozone research has moved into the realm of broader population epidemiology with a growing number of hospitalization studies and other time-series studies starting in the 1990s. Cohort studies have not been designed to look specifically at ozone and premature mortality, and testing for the statistical significance of ozone-mortality relationships was not feasible due to insufficient variation in estimated long-term exposure to ozone (see Chapter 4). However several studies, especially the Southern California Children’s study, have examined longer-term morbidity effects (Tager et al. 2005; CalEPA 2006). Most cohort studies of PM have examined potential effects of ozone both as a confounder and for its independent effects, although few have found statistically significant evidence of such effects (see Chapter 4 for a review of the studies). One regulatory result of the relative absence of data on longer-term effects has been a focus on setting only a short-term NAAQS for ozone: a 1-h standard, which after nearly 30 y was replaced by an 8-h standard in 1997.

The PM2.5 literature expanded rapidly after findings in the early 1990s of the associations of PM with morbidity and mortality in single-city time-series studies followed by the publication of results of two major cohort studies—the Harvard Six Cities Study (Dockery et al. 1993) and an American Cancer Society (ACS) study (Pope et al. 1995)—that showed much larger associations with longer-term residence in areas with high concentrations of PM2.5. Those results motivated the first major revision of the PM NAAQS in a decade in 1997 with the development of both shorter-term (24-h) and longer-term (annual) standards and a host of aggressive new regulatory actions for vehicles, power plants, and other sources. The controversy over those standards resulted in the launching of a major multidisciplinary research program (over $50 million/y for nearly a decade), which has provided substantial additional information on exposure, toxicology, and epidemiology and involved the first use of clinical studies in this context (NRC 2004b). In turn, the new information led to many specific control actions related to such sources as diesel engines (EPA 2007g) and electric-power plants (EPA 2007h) and, in September 2006, an EPA decision to tighten the daily PM standard (but not the annual standard). Those actions were justified largely by cost-benefit analyses based on the substantial potential mortality benefits estimated from the ACS study and later reanalyses and extended analyses (Krewski et al. 2000; Pope et al. 2002, 2004).

The focus on PM science and regulation over the last decade has had several ramifications for ozone. The understandably enhanced policy attention to PM resulted in substantially increased funding for and scientific attention to PM research, to some extent at the expense of research on ozone and other pollutants (except as they might be confounders of PM effects). The substantial estimated

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