Taming the Fine Particulate–Mortality Curve

Estimating the population-level mortality burden attributable to exposure to outdoor fine particulate matter PM2.5 requires characterizing both the magnitude and shape of the relative risk function that mathematically models how exposure affects response. The relationship can be derived using cohort studies where the association between PM2.5 exposure and mortality is directly observed. As policy issues of interest can involve exposures that exceed by several-fold those observed in cohort studies, how the association is extrapolated to these high concentrations is a major source of uncertainty. To address this issue, we suggest the extrapolation criterium that the estimated proportion of deaths attributed to exposure above the observed cohort exposure range is no more than that below the range. This criterion implies that the relative risk function must be bounded from above with marginal changes in risk, rapidly declining with increasing concentration. We illustrated the approach with the use of meta-data from 44 cohorts conducted in North America, Western Europe, Asia, and China, examining the association between long-term exposure to outdoor PM2.5 and non-accidental mortality.