“The U.S. has made incredible strides in reducing particulate matter concentrations across the nation,” said EPA Administrator Andrew Wheeler. “As a result of Clean Air Act programs and efforts by state, local and tribal governments, as well as technological improvements, average PM2.5 concentrations in the U.S. fell by 39 percent between 2000 and 2018 while average PM10 concentrations fell by 31 percent during the same period,” the agency’s press release explained.
The Clean Air Act requires the EPA to set both primary (health-based) and secondary (welfare-based) NAAQS for air pollutants emitted by numerous or diverse mobile or stationary sources. The primary NAAQS are typically more stringent, as they must be set at a level “requisite to protect public health” with an “adequate margin of safety.” The EPA sets both annual (long-term exposure) and 24-hour (short-term exposure) primary NAAQS for particulate matter, calibrated in micrograms per cubic meter (µg/c3). For PM2.5, the current annual and 24-hour standards are 12 µg/c3 and 35 µg/c3, respectively.
Environmental groups predictably condemned the EPA’s proposal, claiming the best available science requires the agency to tighten (lower) the standards. The proposed rule reviews numerous disease prevalence (epidemiological) studies purporting to find significant correlations between cardiopulmonary-related death rates in various population segments and PM2.5 levels below the current NAAQS. However, controlled human exposure studies, animal toxicology studies, and retrospective studies of health improvements in areas with declining PM2.5 levels do not establish a causal link between mortality risk and PM2.5 exposures below the NAAQS.
Thus, according to the EPA, the scientific evidence as a whole “does not call into question the public health protection provided by the current annual and 24-hour PM2.5 standards.” The EPA finds that “there is considerable uncertainty in the potential public health impacts of reductions in ambient PM2.5 below the concentrations achieved under the current primary standards and, therefore, that standards more stringent than the current standards are not supported.” That judgment reflects the agency’s “consideration of the uncertainties in the potential implications of recent epidemiologic studies due in part to the lack of supporting evidence from experimental studies and retrospective accountability studies conducted at PM2.5 concentrations meeting the current standards.”
For an even more skeptical assessment of the PM2.5—mortality link, see Will the Trump Fuel Economy Reform Proposal Create Deadly Air Pollution by regulatory analyst Steve Milloy.
The EPA’s Clean Air Scientific Advisory Committee (CASAC) advised the agency to retain the primary 24-hour PM2.5 standard, the primary PM10 standard, and the secondary standards. “The CASAC did not reach consensus on the primary annual PM2.5 standard, with some committee members recommending that EPA retain the current standard and other members recommending revision of that standard.” The final judgment, however, is the EPA administrator’s to make. He is not obligated to follow CASAC’s advice.