Asserting that there is no clear scientific evidence that more stringent standards would produce additional human health and environmental benefits, the EPA is proposing to retain without change the 2012 National Ambient Air Quality Standards (NAAQS) for particulate matter (PM).

The Clean Air Act (CAA) requires the Agency to set two types of outdoor air quality standards: primary standards to protect public health with an adequate margin of safety and secondary or welfare-based standards to protect the public against adverse environmental effects. The CAA also requires that the EPA review the adequacy of the primary NAAQS every 5 years and amend the standards to ensure protection of human health. The 5-year interval typically produces new scientific research that the Agency must incorporate into its review.

Policy assessment and CASAC advice

The proposed decision to retain the PM NAAQS was made by the EPA administrator based on three avenues of input: an Integrated Science Assessment prepared by the Agency’s Center for Public Health and Environmental Assessment, which summarizes the science related to the health and welfare effects of PM; a Policy Assessment (PA) that presents the EPA staff’s analysis of the scientific basis for policy options for consideration by senior EPA management; and “consensus advice” from the EPA’s Clean Air Scientific Advisory Committee (CASAC).

According to the EPA, work performed by these entities indicate to the administrator that there are critical uncertainties in much of the science being cited by those who support tightening the NAAQS. Accordingly, the EPA believes this science cannot reasonably support more stringent standards at this time. For example, the Agency notes that the mortality risk from exposure to fine particulates (PM2.5) “cannot be attributed to one factor, but instead a combination of factors including, but not limited to, PM composition and sources as well as community characteristics that could influence exposures.” The Agency also notes that there has not been sufficient research into the impact of other air pollutants, or co-pollutants that act in conjunction with PM, to advance the belief that additional limitations on PM will produce the desired benefits. 

Primary standards

The EPA’s proposal covers the following primary standards.

• PM2.5 (fine particles). This standard applies to particles smaller than 2.5 microns. The Agency sets both an annual and a 24-hour standard for PM2.5. The proposal would retain the current annual standard of 12.0 micrograms of PM2.5 per cubic meter (µg/m3). An area would meet the standard if the 3-year average of its annual average PM2.5 concentration is less than or equal to the level of the standard. The proposal would also retain the existing 24-hour standard of 35 µg/m3. An area would meet the 24-hour standard if the 98th percentile of the yearly distribution of 24-hour PM2.5 concentrations, averaged over 3 years, is less than or equal to 35 µg/m3.
• The Agency notes a lack of studies reporting public health improvements attributable to reductions in ambient PM2.5 in locations meeting the current standards.
• PM10 (coarse particles). The PM10 NAAQS addresses thoracic coarse particles, which have a mean aerodynamic diameter greater than 2.5 millimeters (mm) and less than or equal to 10 mm. These particles are also referred to as PM10-2.5. The EPA is proposing to retain the existing 24-hour primary standard for PM10 at 150 µg/m3. An area meets the 24-hour PM10 standard if it does not exceed the 150 µg/m3 level more than once per year on average over a 3-year period.

The EPA states that the proposed retention of the existing PM10 standard was based on limitations in estimates of ambient PM10-2.5 concentrations used in epidemiologic studies, the limited evaluation of co-pollutant models to address the potential for confounding, and the limited number of experimental studies supporting biologically plausible pathways for PM10-2.5-related effects.

Secondary standards

The EPA’s current secondary standards for particle pollution are identical to the primary standards for PM2.5 and PM10, except for the annual PM2.5 standard, which has a level of 15.0 µg/m3.

The current review focuses on the prior administration’s consideration of climate, materials, and visibility effects. That administration found there was insufficient information available to base a national ambient air quality standard on climate impacts associated with ambient air concentrations of PM.

Material effects include dry and wet deposition of PM leading to both physical damage and loss of aesthetic qualities through soiling. The EPA’s 2011 PA concluded that no available evidence called into question the adequacy of the existing secondary PM standards to protect against material effects and soiling. The prior administration also concluded that the existing secondary standard was requisite to protect public welfare with regard to visual air quality.

The current proposal states:

• The scientific information available in the current review remains insufficient to quantify, with confidence, the impacts of ambient PM on climate in the United States.
• The scientific information available remains insufficient to quantify, with confidence, the public welfare impacts of ambient PM on materials; also, there is insufficient information to support a distinct NAAQS standard based on materials impacts.
• Updated scientific evidence and technical information support the adequacy of the current secondary PM2.5 and PM10 standards to protect against PM-related visibility impairment.
• Overall, says the Agency, “newly available evidence and updated analyses do not call into question the adequacy of the current secondary PM standards, and [the administrator] proposes to retain those standards without revision in this review.”