Four papers on fine particulate matter (PM2.5) by Anenberg et al., Fann et al., Shin et al., and Smith contribute to a growing body of literature on estimated epidemiological associations between ambient PM2.5 concentrations and increases in health responses relative to baseline notes. This article provides context for the four articles, including a historical review of provisions of the U.S. Clean Air Act as amended in 1970, requiring the setting of National Ambient Air Quality Standards (NAAQS) for criteria pollutants such as particulate matter (PM). The substantial improvements in both air quality for PM and population health as measured by decreased mortality rates are illustrated. The most recent revision of the NAAQS for PM2.5 in 2013 by the Environmental Protection Agency distinguished between (1) uncertainties in characterizing PM2.5 as having a causal association with various health endpoints, and as all‐cause mortality, and (2) uncertainties in concentration––excess health response relationships at low ambient PM2.5 concentrations below the majority of annual concentrations studied in the United States in the past. In future reviews, and potential revisions, of the NAAQS for PM2.5, it will be even more important to distinguish between uncertainties in (1) characterizing the causal associations between ambient PM2.5 concentrations and specific health outcomes, such as all‐source mortality, irrespective of the concentrations, (2) characterizing the potency of major constituents of PM2.5, and (3) uncertainties in the association between ambient PM2.5 concentrations and specific health outcomes at various ambient PM2.5 concentrations. The latter uncertainties are of special concern as ambient PM2.5 concentrations and health morbidity and mortality rates approach background or baseline rates.
