Blue Skies Bluer?

The largest U.S. environmental health risk is cardiopulmonary mortality from ambient PM2.5. The concentration–response (C–R) for ambient PM2.5 in the U.S. is generally assumed to be linear: from any initial baseline, a given concentration reduction would yield the same improvement in health risk. Recent evidence points to the perplexing possibility that the PM2.5 C–R for cardiopulmonary mortality and some other major endpoints might be supralinear: a given concentration reduction would yield greater improvements in health risk as the initial baseline becomes cleaner. We explore the implications of supralinearity for air policy, emphasizing U.S. conditions. If C–R is supralinear, an economically efficient PM2.5 target may be substantially more stringent than under current standards. Also, if a goal of air policy is to achieve the greatest health improvement per unit of PM2.5 reduction, the optimal policy might call for greater emission reductions in already-clean localesmaking “blue skies bluer”which may be at odds with environmental equity goals. Regardless of whether the C–R is linear or supralinear, the health benefits of attaining U.S. PM2.5 levels well below the current standard would be large. For the supralinear C–R considered here, attaining the current U.S. EPA standard, 12 μg m–3, would avert only ∼17% (if C–R is linear: ∼ 25%) of the total annual cardiopulmonary mortality attributable to PM2.5.