Estimating Mortality Risk Reduction and Economic Benefits from Controlling Ozone Air Pollution (2008) / Chapter Skim
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Because human exposure to ozone in the lower atmosphere at the increased concentrations that result from precursor emissions can cause respiratory problems and other health effects, ozone is one of the six criteria pollutants regulated by the U.S. Environmental Protection Agency (EPA)
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The committee was also asked to assess methods for estimating how much a reduction in short-term exposure to ozone would reduce premature deaths, to assess methods for estimating associated increases in life expectancy, and to assess methods for estimating the monetary value of the reduced risk of premature death and increased life expectancy in the context of health-benefits analysis. The charge to this National Research Council committee focused on benefit analysis; it did not include considering how evidence is used to set the ozone NAAQS.3 OVERALL CONCLUSIONS AND RECOMMENDATIONS The committee concludes from its review of the health-based evidence that short-term exposure to ambient ozone is likely to contribute to premature deaths.
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The health-benefits estimates should be ac companied by a broad array of analyses of uncertainty but should give little or no weight to the assumption that there is no causal asso ciation between estimated reductions in premature mortality and re duced ozone exposure. Because older persons appear to be at higher risk of health-related effects from ozone pollution, it is appropriate to consider whether the willingness to pay for mortality risk reductions should and could reflect the number of years of life by which life would be extended by reductions in ozone.
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From page 6... ...
The choice of averaging period to characterize short-term ozone exposure in linking ambient ozone concentration with mortality risk can have a large effect on estimates of benefits of emission-control programs. For example, under some conditions, efforts to lower emissions of oxides of nitrogen could reduce the daily peak ozone concentration but raise daily average concentrations (see Chapter 3)
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Although ambient ozone is one of the best-characterized pollutants in the United States, ozone monitors are usually operated only during the so-called ozone season -- the warmer period of the year, which varies from city to city.
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Ozone is a regional pollut ant, so winter measurements need not be collected at all the summer locations; but if measurements are collected in winter, they should be collected with the same frequency as summer measurements. Frailty and Ozone Mortality Benefits assessors seek information on whether the mortality risk associated with acute ozone exposure is attributable to short-term displacement (in this case, advancement)
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From page 9... ...
Because the association is based on epidemiologic studies that can only approximate exposure on the basis of ambient monitoring data, the assumption of linearity should be viewed with caution. At low ozone concentrations, the question arises whether the association is linear or more accurately characterized as having a "threshold" -- a concentration of ozone below which exposure poses no risk of death.
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From page 10... ...
The weak current evidence from cohort studies of an association of premature mortality with chronic exposure to ozone suggests that risks may be larger than those observed in acute effects studies alone. The standard approach to investigating effects of cumulative ozone exposure on life expectancy is the cohort study, in which large numbers of subjects are followed for several years.
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From page 11... ...
Characterizing Mortality Risk by Using Studies of Acute and Chronic Exposure Ozone-mortality risk is often expressed as the expected number of deaths attributable to ozone air pollution or lives saved by reducing ozone pollution by some amount. However, reductions in ozone exposure are expected to increase life expectancy and decrease age-adjusted annual death rates (for example, number of deaths per 100,000 of population)
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From page 12... ...
The committee concludes that the empirical evidence is insufficient to support a specific quantitative adjustment of WTP estimates to account for differences in remaining life expectancy, but it does not reject the general concept that such adjustments may be appropriate. It is plausible that people with shorter remaining life expectancy would be willing to devote less of their resources to reducing their mortality risk than those with longer remaining life expectancy.
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The Value of a Statistical Life Year Given that the committee recommends the development of models for estimating life years saved (in addition to estimating changes in annual death rates and reductions in premature deaths) , is it feasible to assign a monetary value directly to changes in life expectancy?
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From page 14... ...
However, the effect of shorter life expectancy on older people's WTP may be offset to some extent by a higher WTP for a reduction in risk because of their poorer health status or their higher baseline risk compared with those of the general population. Results in the empirical literature are not consistent, but several studies suggest that WTP to reduce mortality risk is constant or declines slightly with age.
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FUTURE REGULATORY-IMPACT ANALYSES INVOLVING OZONE MORTALITY Because short-term exposure to ambient ozone is likely to contribute to premature deaths, future regulatory-impact analyses (RIAs) concerning ozonecontrol measures should include the benefits of reduced mortality risk.
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From page 16... ...
Distributed-lag models over several days appear better than single-day models at capturing the acute and subacute mortality effects of ozone exposure and should be part of future benefits assessments to the extent that they are supported in the literature. Future RIAs should incorporate research results on the mortality effects of chronic ozone exposure and research that addresses key uncertainties related to potential confounding factors, exposure measures, and susceptibility as appropriate.
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