The U.S. National Ambient Air Quality Standards (NAAQS) are rules set by the Environmental Protection Agency (EPA) to limit the amount of six harmful pollutants in the air. These pollutants cause smog, acid rain, and other health problems. The EPA created these standards under the Clean Air Act, and they apply to outdoor air across the United States.

The six pollutants for which limits are set are ozone (O₃), tiny particles in the air (PM₂.₅ and PM₁₀), lead (Pb), carbon monoxide (CO), sulfur oxides (SOₓ), and nitrogen oxides (NOₓ). These pollutants are usually released from many sources, such as factories, mines, vehicles, power plants, and farms. In many cases, they come from burning fuels like coal, oil, or gas, or from industrial processes.

The National Emissions Standards for Hazardous Air Pollutants include many other harmful chemicals and require the highest level of pollution reduction that the EPA determines is possible.

A 2025 study found that NAAQS has helped save more than a trillion dollars in health and environmental benefits since 1997. If stricter emissions rules were used, these benefits could almost triple.

Background

The six criteria air pollutants are the first group of pollutants that the United States Environmental Protection Agency (EPA) identified as needing national standards. The Clean Air Act requires the EPA to set U.S. National Ambient Air Quality Standards (NAAQS) for these six pollutants. The NAAQS are based on health and the EPA sets two types of standards: primary and secondary. Primary standards are designed to protect the health of sensitive groups, such as people with asthma, children, and older adults. Secondary standards focus on protecting the environment by addressing issues like reduced visibility, harm to crops and plants, damage to buildings, and effects on animals.

The EPA created the NAAQS according to Sections 108 and 109 of the U.S. Clean Air Act, which was most recently updated in 1990. These sections require the EPA to (1) list common air pollutants that could be expected to harm public health or well-being; (2) develop air quality criteria that summarize the latest scientific information about how these pollutants affect health and the environment; (3) set primary NAAQS to protect human health with enough safety and secondary NAAQS to prevent harm to the environment, such as effects on plants, ecosystems, visibility, climate, and manmade materials; and (5) regularly review and update, as needed, the criteria and NAAQS for each listed pollutant or group of pollutants.

Descriptions

• Ground-level ozone (O₃): Ozone found near the Earth's surface, called tropospheric ozone, is regulated by the National Ambient Air Quality Standards (NAAQS) under the Clean Air Act. Scientists first discovered ozone could harm grapes in the 1950s. In 1971, the U.S. Environmental Protection Agency (EPA) created standards for "oxidants," which included ozone. These standards aimed to reduce harm to agriculture and other areas. Like lead, ozone standards must be reviewed regularly to update health and plant-related findings. This led to the creation of an EPA criteria document. Further studies in 1979 and 1997 required major changes to the pollution standards.

• Atmospheric particulate matter PM₁₀, coarse particles: These are particles between 2.5 micrometers (μm) and 10 μm in size (though current standards include all particles 10 μm or smaller). PM₂.₅ refers to fine particles 2.5 μm or smaller. Particulate Matter (PM) was included in the 1996 EPA criteria document. In April 2001, the EPA released a second draft of the Air Quality Criteria for PM, which addressed new research and updated standards since the first draft. A third draft was published in May 2002, and PM requirements were revised again. After releasing a fourth version, the final document was issued in October 2004.

• Lead (Pb): In the mid-1970s, lead was listed as a criteria air pollutant requiring NAAQS regulation. In 1977, the EPA published a document detailing Air Quality Criteria for lead, based on scientific studies from that time. Using this report, the EPA set a "1.5 μg/m³ maximum quarterly calendar average" NAAQS for lead in 1978. The Clean Air Act requires regular reviews of NAAQS, and new data after 1977 led to updates. An addendum to the document was published in 1986, followed by a supplement in 1990. In 1990, the EPA prepared a Lead Staff Paper based on these updates and other research. The paper suggested revising the lead NAAQS further, but the EPA chose not to change the standards. Instead, it focused on reducing lead exposure from non-air sources, such as contaminated soil and water, which posed greater health risks.

• Carbon monoxide (CO): The EPA established the first NAAQS for carbon monoxide in 1971. The primary standard was set at 9 ppm averaged over 8 hours and 35 ppm over 1 hour. Most CO in the air comes from vehicles. The EPA reviewed CO-related scientific studies in 1979, 1984, 1991, and 1994. After the 1984 review, the EPA removed the secondary standard for CO due to limited evidence of environmental harm. In 2011, the EPA concluded the current CO standards were sufficient and proposed keeping them. The EPA is improving monitoring by placing CO monitors in strategic locations near large cities. By 2015, monitors were required in areas with populations over 2.5 million, and by 2017 in areas with populations over 1 million. CO monitors are also being paired with nitrogen dioxide (NO₂) monitors in cities with populations over 1 million. As of May 2011, there were about 328 CO monitors nationwide. The EPA allows regional administrators to handle special cases for additional monitoring. National CO levels have dropped 82% since 1980, and all U.S. areas are now in compliance with CO standards.

• Sulfur oxides (SOₓ): Sulfur oxides are a group of reactive gases, with sulfur dioxide (SO₂) being the most important indicator. The EPA set primary and secondary standards for SOₓ in 1971. Primary standards were 140 ppb averaged over 24 hours and 30 ppb annually. The secondary standard was 500 ppb averaged over 3 hours, not to be exceeded more than once a year. A 1996 review considered new standards for short-term SO₂ peaks affecting sensitive groups, but the EPA kept existing standards. In 2010, the EPA replaced dual primary standards with a new 1-hour standard of 75 ppb. On March 20, 2012, the EPA confirmed the existing NAAQS remained in place. Only three monitoring sites in Hawaii Volcanoes National Park exceeded SO₂ standards between 2007–2008, but Hawaii requested exemptions due to volcanic activity. National SO₂ levels have decreased 83% since 1980, with annual averages between 1–6 ppb. All areas are currently in compliance with SO₂ standards.

• Nitrogen oxides (NOₓ): The EPA set primary and secondary standards for nitrogen oxides in 1971. These include nitric oxide (NO), nitrous oxide (N₂O), and nitrogen dioxide (NO₂), with NO₂ used as the main indicator due to its role in forming ground-level ozone. In 1971, the NAAQS for NO₂ was set at an annual average of 0.053 ppm. The EPA reviewed this standard in 1985 and 1996, finding it sufficient. A 2010 review introduced a new 1-hour primary standard of 100 ppb, while the annual average remained at 0.053 ppm. A new 1-hour secondary standard of 100 ppb was also proposed, marking the first time the EPA separated environmental and health impacts for this pollutant. In 2010, the EPA strengthened monitoring by increasing the number of systems near cities and highways. On March 20, 2012, the EPA confirmed existing NAAQS remained in place. National NOₓ levels have dropped 52% since 1980, with NO₂ averaging 10–20 ppb annually. New vehicle regulations are expected to further reduce NO₂ levels. All U.S. areas are currently in compliance with NOₓ standards.

Standards

The standards are found in 40 CFR 50. Primary standards are created to protect human health, with enough safety to include groups like children, older adults, and people with breathing problems. Secondary standards help protect public well-being, including things like property damage, transportation issues, economic losses, and personal comfort from harmful effects of pollution. A place that meets a standard is called an "attainment area" for that standard, and a place that does not meet it is called a "non-attainment area."

Standards must "accurately reflect the latest scientific knowledge" and are reviewed every five years by the Clean Air Scientific Advisory Committee (CASAC), which has "seven members chosen by the EPA administrator."

The EPA has set National Ambient Air Quality Standards (NAAQS) for six major pollutants, which are also known as criteria air pollutants. These include:

• ^a Starting June 15, 2005, the 1-hour ozone standard no longer applies to areas using the 8-hour ozone standard (which covers most of the United States, except parts of 10 states).
• Source: USEPA

Detection methods

The EPA National Exposure Research Laboratory can name a measurement device that uses proven technology as a Federal Reference Method (FRM). This means the device has been tested and checked using a standard process to ensure it can accurately monitor if air quality meets national standards. Devices that use new technology can be named a Federal Equivalent Method (FEM). FEMs use different ways to collect and test samples compared to FRMs, but they must still provide the same level of accuracy when determining if air quality standards are met. Approved new methods are officially shared in the Federal Register. A full list of FRMs and FEMs is available.

Air quality control region

An air quality control region is an area set up by the national government where groups of people live in the same place and face the same air pollution issue.