of reductions in ambient ozone concentrations because of uncertainty about the appropriate interpretation of these results.

Studies published since 1990 have yielded evidence in support of a relationship between short-term exposure and premature death. They include city-specific time-series studies, many of which have been included in recent meta-analyses (Bell et al. 2004, 2005; Ito et al. 2005; Levy et al. 2005). However, interpretation of the evidence is complicated by ozone’s occurrence in mixtures of other pollutants whose concentrations fluctuate in a similar manner and can be influenced by short-term meteorologic changes. It is important to understand whether it is ozone or a copollutant that is causing health effects, because steps to control the wrong agent will be both expensive and ineffective. Interpretation of the health studies results is also complicated by uncertainties that result from relying on ozone measurements from outdoor monitoring sites to estimate exposures of people who spend most of their time indoors.

If changes in risks of premature mortality are attributed to changes in ambient ozone concentrations in regulatory benefits assessments, EPA also needs an estimate of the monetary value of such changes in mortality risk. In previous sensitivity analyses of mortality risk reduction benefits from ozone reductions, EPA used the same value of statistical life (VSL) as that used for analysis of other mortality risks, such as those associated with particulate matter with a diameter less than or equal to 2.5 microns (PM2.5). Available VSL estimates are drawn largely from studies of working-age adults with average remaining life expectancies. There are many questions about the applicability of these estimates to mortality risks associated with ozone, which may fall disproportionately to an older population with shorter remaining life expectancy and more frail health status.

CHARGE TO THE COMMITTEE

In light of the recent evidence on ozone mortality risk and questions about its implications for benefit analysis, EPA asked the National Research Council for scientific advice on how the ozone-mortality research findings could be used in the context of health-benefit analyses associated with regulatory assessments. In response, the National Research Council established the Committee on Mortality Risk Reduction Benefits from Decreasing Tropospheric Ozone Exposure (see Appendix A).

The Statement of Task to the committee (see Box 1-1) includes evaluation of the scientific and technical bases of approaches used by EPA for estimating reductions in mortality risk and associated benefits of health-based ozone standards over time. The committee was to assess methods for estimating reductions in premature death due to diminished short-term exposure to ozone, increases in life expectancy, economic valuation of the increased life expectancy, and associated uncertainties and their general implications for decision-making. The



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