The US Environmental Protection Agency (EPA) tightened the annual health-based national ambient air quality standard for fine particulate matter (PM_2.5) from a level of 12 micrograms per cubic meter to 9 micrograms per cubic meter.

Along with strengthening the primary annual PM_2.5 standard, EPA is modifying the PM_2.5 monitoring network design criteria to include a factor that accounts for proximity of populations at increased risk of PM_2.5-related health effects to sources of air pollution.

Based on 2020-2022 air monitoring data, the dark green areas on the map indicate counties that do not meet the annual PM_2.5 standard of 9 ug/m³. Courtesy US EPA.

In June 2021, EPA announced it would reconsider the December 2020 decision to retain the 2012 standards because the available scientific evidence and technical information indicated that the standards may not be adequate to protect public health and welfare. EPA considered the available science and technical information, as well as the recommendations of the independent advisors comprising the Clean Air Scientific Advisory Committee and CASAC PM expert panel when making the decision on whether to strengthen the PM standards.

Based on the scientific evidence, technical information, recommendations from CASAC, and public comments on the 2023 proposed standards, EPA has set two primary standards for PM_2.5, which work together to protect public health: the annual standard, which EPA has revised, and a 24-hour standard, which the agency retained.

EPA also retained the current primary 24-hour standard for PM10, which provides protection against coarse particles. EPA is also not changing the secondary (welfare-based) standards for fine particles and coarse particles at this time.