Blue Skies Bluer? Puzzling implications of a possible supra-linear relationship between PM exposure and mortality

摘要

Background and Aims: Recent studies raise the possibility that some health endpoints follow supra-linear concentration-response functions (CRFs). Here, we examine implications of supra-linearity for the design of economically efficient policies to reduce health effects associated with PM2.5 exposures. Methods: We compare two classes of recently published CRFs for cardiopulmonary mortality associated with PM-2.5: (1) a conventional log-linear model, and (2) a supra-linear log-log model. Each CRF is used to model the concentration dependence of the marginal health benefit associated with a fixed 1 μg m-3 reduction in ambient PM-2.5. Results: For a log-linear CRF, the marginal health benefit of a 1 μg m-3 reduction in ambient concentrations has weak dependence on the initial level. In contrast, for the supra-linear (log-log) CRF, the marginal health benefit of a 1 μg m-3 improvement is greatest at low concentrations and drops sharply for more polluted conditions. Surprisingly, for a supra-linear CRF, an efficiency-oriented solution might prioritize marginal pollutant improvements in locales with low annual-average concentrations. This unexpected feature of supra-linear CRFs goes against a key assumption in environmental economics that the marginal benefit of pollution control declines with increasing abatement. Conclusions: Several new challenges would arise for science and policy if supra-linear CRFs were demonstrated to exist for PM2.5. To achieve a given level of population health, more stringent controls on ambient PM2.5- levels may be required than previously assumed. Furthermore, individual policies cannot be understood in isolation; reducing emissions at two sources may have a greater benefit than the addition of the benefits of cleaning either source individually. An economically "optimal" pollution abatement strategy might de-emphasize control in the most polluted communities, raising important concerns for environmental justice.