The regulatory, public health, and scientific contexts are set by the comparatively low concentrations of PM now measured in most of the United States. At these concentrations, the effects found in observational studies are small in comparison to those found in the past, particularly effects observed during dramatic air pollution episodes, such as the 1952 London Fog. The current risks are also small in comparison with those associated with some causes of the same outcomes, for example, active smoking and risk for ischemic heart disease events. Consequently, epidemiologists face the challenge of detecting a relatively weak signal of effect and of ensuring that the effects found do not reflect factors other than air pollution, for example, meteorological conditions and personal use of tobacco products.

The pollutant concentrations of interest also have implications for the design of toxicological experiments and interpretation of their findings. Although it is feasible for epidemiologists to investigate large populations to detect effects, toxicologists typically use exposure levels that are far higher than those experienced by the population to detect measurable effects. Exposures far higher than those experienced by the population generally might be needed to induce measurable effects. Consequently, there might be uncertainty as to the relevance of biological findings in model systems. Mechanisms operative at high concentrations might not be applicable at lower concentrations.

Completing the Particulate Matter Emissions Inventory and Particulate Matter Air Quality Models Necessary for NAAQS Implementation and Informing Health Research

Although the committee recognizes that its objective is to provide independent guidance for planning and monitoring a long-term PM research program, the committee has long acknowledged that this research program should also provide the tools necessary for the implementation of current and possible future PM NAAQSs. In particular, the committee views improved emissions characterization and air-quality model testing and development as critical for rapidly-approaching deadlines for state implementation plans (SIPs). As shown in Table 2-1 of Chapter 2, the states will be developing and submitting their implementation plans over the next 5 years. The committee’s second and third reports offered an agenda for research related to these topics. As described in more detail below, EPA should develop a comprehensive prioritized plan for systematically translat-

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