The US Environmental Protection Agency (EPA) is proposing to retain, without changes, the National Ambient Air Quality Standards (NAAQS) for particulate matter (PM) including both fine particles (PM_2.5) and coarse particles (PM₁₀).

The Clean Air Act requires the EPA to set NAAQS for criteria pollutants—currently PM, ground-level ozone (O₃), carbon monoxide (CO), lead (Pb), sulfur dioxide (SO₂) and nitrogen dioxide (NO₂) The law requires the EPA to periodically review the relevant scientific information and the standards and revise them, if appropriate, to ensure that the standards provide the requisite protection for public health and welfare.

EPA sets both an annual and a 24-hour standard for fine particles (PM_2.5). These standards work together to protect the public from harmful health effects from both long- and short-term fine particle exposures.

• The annual fine particle standard is designed to protect against health effects associated with both long- and short- term exposure to PM_2.5. The current annual standard has been in place since 2012. EPA is proposing to retain the current annual standard, with its level of 12.0 μg/m³. An area would meet the standard if the three-year average of its annual average PM_2.5 concentration is less than or equal to the level of the standard.

• The 24-hour primary standard is designed to provide supplemental health protection against short-term fine particle exposures, particularly in areas with high peak PM_2.5 concentrations. The current 24-hour standard was issued in 2006. EPA is proposing to retain the existing 24-hour standard, with its level of 35 μg/m³. An area would meet the 24-hour standard if the 98^th percentile of the yearly distribution of 24-hour PM_2.5 concentrations, averaged over three years, is less than or equal to 35 μg/m³.

• EPA is proposing to retain the existing 24-hour primary standard for coarse particles (PM₁₀), with its level of 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 three-year period. The existing coarse particle standard has been in place since 1987.

EPA’s current secondary standards for particle pollution are identical to the primary standards for PM_2.5 and PM₁₀, except for the annual PM_2.5 standard which has a level of 15.0 μg/m³. EPA is proposing that the current secondary standards are adequate to protect against PM-related visibility impairment, climate effects, and effects on materials.

Average PM_2.5 concentrations in the US fell by 39% between 2000 and 2018 while average PM₁₀ concentrations fell by 31% during the same period.