Five Decades of Particulate Air Pollution Health Effects Research and the Focus on PM2.5

Our national ambient air quality standards for fine particles, PM_2.5, and the current research programs into particulate aerosols are the result of five decades of health effects research to understand the health effects of particulate air pollution. In 1971, the EPA identified total suspended particulates (TSP) as a criteria pollutant and set the first national ambient air quality standards to protect the public health. There was little observational data to support this health standard. Moreover, there was a recognition that the dosimetry and toxicity of suspended particles likely depended on particle size. In 1974, we undertook the Harvard Six Cities Study to measure the health effects of respirable suspended particles. The EPA provided the study with automatic dichotomous particle samplers, which provided daily samples of particles in two size ranges (<2.5 μm and 2.5 to 10 μm). In 1987, EPA promulgated a new standard for inhalable particles less than 10 μm (PM₁₀) based on analyses of deposition in the respiratory tract. But the epidemiologic analyses of the Six Cities Study pointed to the smaller particles (PM_2.5) rather than PM₁₀ as having stronger associations with health effects among people in the community. However, there was not experimental data showing physiologic or toxicologic effects of PM_2.5 in controlled laboratory conditions. Still, in 1997 the EPA promulgated a new ambient air quality standard for PM_2.5 based on observational data from the Six Cities and other epidemiologic studies. This action was highly controversial, and led to an independent review and validation of the Six Cities data and results, and a call by the Nation Academy of Sciences for increased experimental research into the mechanisms of these observed health effects. Two decades later these original epidemiologic studies of health effects of PM_2.5 have been replicated and validated. The efforts to improve assessment of exposure to PM_2.5 have led to stronger epidemiologic associations in studies around the world. An estimate 4.2 million premature deaths across the world were attributed to exposure to PM_2.5 in 2015. Even in communities meeting the current PM_2.5 air quality standards, there is evidence that further reductions in PM_2.5 levels would lead to improvements in health. Research continues to identify the specific physical or chemical characteristic of PM_2.5 responsible for these health effects. Nevertheless, PM_2.5, the mass of particles less than 2.5 μm aerodynamic diameter is the benchmark to which other aerosol metrics must be compared in evaluating health impacts.