Reducing ozone: how much is it worth? (Science Selections).

In recent years, ozone alerts on hot days have become common-place in the media. They warn the public in a given area to avoid prolonged outdoor activity and thus limit their exposure to ozone in outdoor air. It has become generally accepted that exposure to high concentrations of ozone can be detrimental to health, particularly in sensitive populations such as children, the elderly, and people with respiratory ailments.

This would imply, naturally, that environmental measures aimed at reducing atmospheric ozone levels would offer concomitant benefits to public health. But environmental policies and expenditures must be justified by clear rationales based upon sound scientific evidence and realistic benefit-cost analysis that acknowledges and quantifies uncertainties. In this month's issue, researchers led by Jonathan I. Levy of the Harvard Center for Risk Analysis selected a sample city--Houston, Texas--to experiment with statistical analysis approaches gleaned from previously published research [EHP 109:1215-1226]. They focused on addressing the critical issues and uncertainties involved in performing benefit-cost analyses of reduced ozone concentrations.

To evaluate health outcomes, the authors focused on four factors potentially impacted by exposure to ozone: premature mortality, chronic asthma, respiratory hospital admissions, and minor restricted activity days (those that do not result in work or school loss or bed disability). Interpretation of the relationship between ozone and each health effect is complicated by potential confounding factors, including particulate matter levels and weather. The authors surveyed relevant studies with an eve toward statistical approaches to deal with these confounders, arriving at a quantified estimation of the possible benefits to public health in Houston.

To assign a monetary value to those health benefits, they applied findings from studies that used two competing methods. The "value of statistical life" approach measures the dollar amount an individual is willing to pay to avoid what the authors describe as "a small increase" in mortality risk. The "quality-adjusted life year" approach measures the loss of life expectancy and health quality and is used in medical decision making to compare the cost-effectiveness of different medical interventions. The authors arrived at their conclusions for the Houston case study--that a reasonable estimate of the annual monetary value of reducing ozone concentrations would be between $0.70 and $40 per person per [micro]g/[m.sup.3] of annual average reduction--by assigning subjective probabilities to values derived using both methods. Because the derived values differed substantially, the uncertainty about the monetary value assigned to mortality contributes significantly to the overall uncertainty in the analysis.

The authors' survey of the existing evidence and their methods for extrapolating conclusions from that body of work showcase the need for further research, which they recommend "should focus on those health outcomes and topics for which reduced uncertainty or different values could lead to different policy options."