to estimate how efforts to attain the ozone NAAQS will affect ozone exposure and health.

The committee notes that EPA has apparently not considered the use of two or more averaging times jointly. Does adding metric C to a model that already contains metric A, or metrics A and B, improve the fit of the model? Such analyses should be fairly easy to conduct and might show that 1-hour, 8-hour, or 24-hour averaging times are roughly equivalent and that any one of these can substitute for the others. Conversely, one might find that one or two of these metrics add little when the third is already in the model. Or, it is possible that some combination of two or three of these is a better measure for analysis than any one alone. Measurement error is likely to blur the actual contributions of each metric and the relationships among them, but that is not relevant when the goal is to find the best empirical fit to the data.

Regulators need to consider that control strategies may affect 24-h average concentrations quite differently from how they affect shorter-term exposures and that peak short-term concentrations on lower-ozone days will respond differently, and often in the opposite direction, from the same measure of ozone on a high-ozone day.

Finding: Human-exposure simulation models, such as APEX, present an approach to estimating ozone exposure different from reliance solely on observations. However, such models introduce their own uncertainties, and they need to be further evaluated and their uncertainties characterized.

Recommendation: EPA should conduct more detailed evaluations of APEX and other models used to improve characterization of ozone exposure at the population level by taking human activity into account. Detailed evaluation will require further human-exposure studies, which will need improved instrumentation and approaches for monitoring personal exposures. The extent to which the human diaries represent the population under study, as used by APEX, warrants further explication, and more extensive and more up-to-date diary data should be collected, in particular for children and minority groups. Those data, when tied to data on pollution concentrations and ozone-alert days, can be used to assess the type and degree of changes in behavior to avoid ozone exposure. The cost of such changes in behavior associated with increased pollution currently is not addressed in RIAs prepared by EPA.

Finding: Epidemiologic studies of ozone health effects are limited, in part, by the reduced availability of ozone data in winter.

Recommendation: EPA and states should extend operation of ozone monitoring into winter and report the measurements. The size of the winter program should be sufficient to enhance researchers’ ability to examine (1) seasonal differences in risk, (2) how these seasonal risk differences vary spatially between communities with warmer and cooler winters, and (3) ozone-mortality relationships at lower ozone concentrations. In recognition that ozone is a regional pollutant, winter measurements need not be collected in all the summer locations but, when they are collected, should be collected at the frequency of summer measurements.



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