Environmental Protection Agency Proposes to Strengthen National Ambient Air Quality Standard

The U.S. Environmental Protection Agency on Jan. 6 announced a proposal that would strengthen a key national ambient air quality standard for fine particle emissions, also known as PM2.5.

EPA will specifically take comment on strengthening the primary annual PM2.5 standard from a level of 12 micrograms per cubic meter to a level between 9 and 10 micrograms per cubic meter. The agency is also taking comment on the full range (between 8 and 11 micrograms per cubic meter) included in the Clean Air Scientific Advisory Committee’s latest report.


On December 18, 2020, EPA reviewed and retained the primary and secondary PM NAAQS without revision, by final rule

After President Biden took office in 2021, EPA decided to reconsider the 2020 final rule based on available scientific evidence and the Policy Assessment. The EPA also reconstituted the membership of the Clean Air Scientific Advisory Committee, which provides independent advice to EPA on the technical bases for National Ambient Air Quality Standards. That committee ultimately recommended tightening the annual and daily PM 2.5 NAAQS. 

All members of the committee agreed that the current annual PM 2.5 value of 12 micrograms per cubic meter should be lowered.  

An EPA policy statement concluded that the daily PM 2.5 standard should not be reduced.  EPA reasoned that “we reach the conclusion that, in conjunction with a lower annual standard level intended to increase protection against average short- and long-term PM2.5 exposures across the U.S., the evidence does not support the need for additional protection against short-term exposures to peak PM 2.5 concentrations.” 

EPA sent the Proposed Rule to the Office of Management and Budget on August 16, 2022.  OMB conducted a series of meetings before completing its review on December 27, 2022. 

The proposed rule supports lowering the PM2.5 primary annual standard, while taking comment on reducing other PM standards. 

Primary NAAQS standards focus on public health protections against the health effects caused by exposures to PM 10 and PM 2.5, while secondary NAAQS standards concentrate on visibility, climate, and materials effects. 

The implications of the proposed rule can be evaluated by reviewing the areas of the country that cannot attain a lower standard of 9.0-10.0 ug/m3.  EPA developed a map and table of vulnerable areas. 

Using 2019-2021 Air monitoring data, EPA identified 50 counties that would not meet the standard of 10.0 micrograms/cubic meter and 62 counties that would not meet 9.0 micrograms/cubic meter. 

The table identifies the counties that have monitors and provides the design values for each. A future nonattainment area might include a county not on this list (i.e., without an ambient monitor) that is part of a metropolitan statistical area that has a monitor out of attainment.  Overall, western states (California, Oregon and Washington) would have a large number of nonattainment areas, while select, populated urban areas in the east would be in nonattainment.

After the final rule, EPA will move toward the implementation process of designating areas as attaining/not attaining the new standard. States must then develop state implementation plans to address areas in nonattainment by incorporating air pollution control strategies to reduce PM. 

Under the Clean Air Act, EPA is required to set the NAAQS for the six criteria air pollutants, carbon monoxide, lead, nitrogen dioxide, ozone, particle pollution and sulfur dioxide.

There are two types of NAAQS, the primary standards that provide public health protection and the secondary standards that provide public welfare protection.

NAAQS are periodically reviewed and updated if EPA determines that there is a need for more stringent standard to protect the public health or welfare. The PM NAAQS were last reviewed and updated in 2012.

In June 30, 2020, APPA and the National Rural Electric Cooperative Association submitted joint comments in response to EPA’s 2020 proposal to retain the primary 24-hour PM2.5 standard, the primary PM10 standard, and the secondary standards.