The US Environmental Protection Agency (EPA) announced a proposal to to revise the primary (health-based) annual PM_2.5 standard from its current level of 12.0 µg/m³ to within the range of 9.0 to 10.0 µg/m³. EPA is taking comment on the full range (between 8 and 11 10 µg/m³) included in the Clean Air Scientific Advisory Committee’s (CASAC) latest report.

EPA also proposed not to change the current:

• secondary (welfare-based) annual PM_2.5 standard;
• primary and secondary 24-hour PM_2.5 standards; and
• primary and secondary PM₁₀ standards.

Currently, EPA has primary and secondary standards for PM_2.5 (annual average standards with levels of 12.0 µg/m³ and 15.0 µg/m₃, respectively; 24-hour standards with 98^th percentile forms and levels of 35 µg/m³) and PM₁₀ (24-hour standards with one-expected exceedance forms and levels of 150 µg/m³).

The EPA is proposing to retain the primary 24-hour PM_2.5 standard of 35 µg/m³, while taking comment on revising this level to as low as 25 µg/m³.

Since EPA completed its last review of the PM NAAQS in 2012, thousands of new scientific studies have demonstrated the dangers of soot exposure. EPA estimates that if finalized, a strengthened primary annual PM_2.5 standard at a level of 9 µg/m³, the lower end of the proposed range, would prevent:

• up to 4,200 premature deaths per year;
• 270,000 lost workdays per year; and
• result in as much as $43 billion in net health benefits in 2032.

EPA is also proposing to revise other aspects related to the PM standards—such as monitoring requirements and the Air Quality Index (AQI)—that will help states and Tribal Nations meet the revised standards while making significant strides toward protecting the health of all people, including at-risk populations.

In developing this proposed rule, EPA considered the thousands of studies in the 2019 PM Integrated Science Assessment (ISA) and the Supplement to the 2019 PM ISA, both of which were made available for public comment and expert review. The studies support a causal relationship between long- and short-term exposures to PM2.5 and cardiovascular effects, respiratory effects, nervous system effects, and cancer. New epidemiologic studies support associations between exposure and adverse health effects at levels below the current annual PM_2.5 standard level.

EPA will accept public comment for 60 days after the proposal is published in the Federal Register. EPA will also conduct a virtual public hearing over several days for this proposed rulemaking, with the hearing beginning at 11:00 am Eastern Time and concluding at 7:00 pm ET each day. EPA will begin pre-registering speakers for the hearing upon publication of the announcement of the public hearings in the Federal Register. Additional information will also be made available on the National Ambient Air Quality Standards (NAAQS) for PM webpage.

In 1971, EPA expressed the PM NAAQS for TSP (total solid particulates) using an annual geometric mean. In 1987, the Agency specified annual standards for PM₁₀, and moved to an annual arithmetic mean, averaged over 3 years. In 1997, the Agency added PM_2.5 at a level of 15 µg/m³; the annual average for PM₁₀ was 50 µg/m³. In 2006, EPA vacated the annual average for PM₁₀.

EPA began regulating particle pollution in 1971. The agency has revised the standards four times since: in 1987, 1997, 2006 and 2012. After reviewing comments, EPA plans to issue final standards later this year.

The California Air Resources Board (ARB), while applauding the EPA proposal, encouraged EPA to consider further strengthening of the proposal, noting that studies demonstrate health impacts down to the level of 8 micrograms. CARB also urged EPA to consider strengthening the short-term or 24-hour standard.

CARB has previously urged EPA to consider a short-term standard for fine particulate matter as low as 25 µg.