EPA sets National Ambient Air Quality Standards (NAAQS) for six common and harmful pollutants (also called criteria pollutants): carbon monoxide, lead, particulate matter (PM), ozone, nitrogen dioxide, and sulfur dioxide. The Clean Air Act (CAA) directs EPA to set the standards based solely on public health and welfare protection. EPA does not consider the cost when setting or revising a standard. The NAAQS program is the cornerstone of EPA’s work to protect public health and the environment. EPA estimates that between 1980 and 2021, total emissions of these six pollutants dropped by 71% while the economy grew by 182%.
The Clean Air Act (CAA) requires that EPA review the NAAQS every five years to ensure their adequacy. For more information on changes to the NAAQS scientific review and standard setting process, visit our NAAQS Review Regulatory Tracker page.
NAAQS Currently Under Review
Ozone: We are tracking EPA’s review and approval process for the 2020 ozone NAAQS. Ground-level ozone is emitted from vehicles, power plants, and other industrial sources. It reacts with sunlight and other pollutants to form smog, which can aggravate asthma and cause other respiratory issues, especially in children who are playing outdoors and people with existing lung problems.
Particulate Matter: We also track EPA’s review and approval of the 2020 PM NAAQS. PM varies in size but can be small enough to enter the lungs and even the bloodstream when inhaled, and is linked to asthma and other respiratory impacts, cardiovascular disease, cancer, and premature death. It can be emitted by many sources such as vehicles, industrial operations (including power plants), construction sites, and fires. EPA regulates PM2.5 and PM10 (2.5 and 10 micrometers in size).
State Implementation of the NAAQS
Once EPA establishes a new NAAQS, states must develop State Implementation Plans (SIPs) to ensure it meets the standard within the deadlines set under the Act. If EPA finds that a SIP is inadequate, it must issue a Federal Implementation Plan (FIP) to ensure the state can reach attainment with the NAAQS. Learn more about implementation of the 2015 Ozone NAAQS.
Obligation for States to Address Transport of Criteria Pollutants Across State Lines
The CAA’s “Good Neighbor” provision requires upwind states’ SIPs to ensure that the air pollution they create does not affect downwind states’ ability to meet, or maintain the achievement of, the applicable NAAQS. A downwind state can file a CAA section 126 petition asking EPA to regulate pollution from sources in another state when that pollution is impairing its air quality.
To address upwind contributions to downwind states’ nonattainment of the 2008 ozone standard, EPA established the Cross-State Air Pollution Rule (CSAPR). In 2023, EPA finalized the Good Neighbor Plan FIP to address air transport related to the 2015 ozone standard.
Additionally, section 176A of the CAA authorizes EPA to establish a transport region of states that contribute significantly to a NAAQS violation. Section 184 established such a region for ozone — the Ozone Transport Region (OTR) — for the northeast and mid-Atlantic and requires certain control requirements to address interstate ozone pollution transport within the OTR.