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The Environmental Protection Agency (EPA) uses a variety of methods to determine whether an area is polluted. The EPA's Air Quality Index (AQI) is a tool for communicating about outdoor air quality and health. The AQI includes six color-coded categories, each corresponding to a range of index values. The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 or below represents good air quality, while an AQI over 300 represents hazardous air quality. The EPA also works with states and tribes to collect data from air quality monitors and models to determine whether an area meets National Ambient Air Quality Standards (NAAQS). The Clean Air Act requires the EPA to set NAAQS for six commonly found air pollutants, and the EPA issues federal emissions standards for new motor vehicles and non-road engines to help states meet these standards.
| Characteristics | Values |
|---|---|
| Air Quality | The EPA uses the Air Quality Index (AQI) to communicate about outdoor air quality and health. The AQI includes six color-coded categories, each corresponding to a range of index values. |
| AQI Values | An AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality. |
| National Ambient Air Quality Standards (NAAQS) | The EPA sets NAAQS for six commonly found air pollutants known as criteria air pollutants. |
| Criteria Air Pollutants | Examples include sulfur dioxide, nitrogen dioxide, and lead. |
| Clean Air Act | The Clean Air Act requires states to adopt implementation plans to meet NAAQS and reduce air pollution. |
| Hazardous Air Pollutants | Examples include benzene, perchloroethylene, and methylene chloride. |
| High-Risk Areas | Urban areas, industrial facilities, and regions with high transportation emissions are more prone to elevated levels of air pollution. |
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What You'll Learn
The Clean Air Act
One of the key goals of the Clean Air Act is to establish National Ambient Air Quality Standards (NAAQS) to protect public health and welfare. The EPA sets these standards for six commonly found air pollutants, also known as criteria air pollutants: ground-level ozone, carbon monoxide, particulate matter, lead, sulfur dioxide, and nitrogen dioxide. These pollutants are known to have adverse effects on human health, including respiratory issues and neurological problems in children.
The Act also includes programs to tackle specific issues such as acid rain, ozone layer depletion, and climate change. The Acid Rain Program (ARP), for example, is a cap-and-trade emissions program aimed at reducing sulfur dioxide and nitrogen oxide emissions from power plants, which are the primary contributors to acid rain.
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National Ambient Air Quality Standards (NAAQS)
The Clean Air Act requires the US Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six principal pollutants, also known as "criteria pollutants", that are common in outdoor air and harmful to public health and the environment. These pollutants come from numerous and diverse sources. The two types of NAAQS are primary standards and secondary standards.
Primary standards are designed to protect public health with an adequate margin of safety, including the health of "sensitive" populations such as asthmatics, children, and the elderly. Adverse health effects related to air pollution exposure include hospitalizations, emergency department visits, premature death due to worsening of chronic heart and lung diseases, increased symptoms of respiratory irritation, increased asthma symptoms, and increased asthma medication usage, among others.
Secondary standards are designed to protect the public welfare from adverse effects, including those related to effects on soils, water, crops, vegetation, man-made (anthropogenic) materials, animals, wildlife, weather, visibility, and climate; damage to property; transportation hazards; economic values, and personal comfort and well-being. Air pollution can reduce agricultural crop yields, damage forests, ornamental and native plants, and create haze that reduces visibility.
The NAAQS are selected by the US EPA Administrator at the conclusion of a public process that takes about five years. The process starts with a comprehensive review of the relevant scientific literature, which is summarized in a document called the Integrated Science Assessment (ISA). Based on the ISA, US EPA staff perform a risk and exposure assessment, which is summarized in the Risk and Exposure Assessment (REA) document. The third document, the Policy Assessment (PA), integrates the findings and conclusions of the ISA and REA into a policy context and provides lines of reasoning that could be used to support the retention or revision of the existing NAAQS, as well as alternative standards. Each of these three documents is released for public comment and public peer review by the Clean Air Scientific Advisory Committee (CASAC), a subcommittee of the US EPA. Members of CASAC are appointed by the US EPA Administrator for their expertise in one or more of the subject areas covered in the ISA. The committee’s role is to peer review the NAAQS documents, ensure that they reflect the thinking of the scientific community, and advise the Administrator on the technical and scientific aspects of standard setting. Each document goes through two to three drafts before CASAC deems it final. Once all three documents are final, they are given to the US EPA Administrator to use in selecting a proposed NAAQS, which is released through the Federal Register for public comment. Following the close of the comment period, the Administrator considers the comments received, makes changes to the proposed NAAQS if necessary, and publishes the final NAAQS in the Federal Register. Periodically, the standards are reviewed and sometimes may be revised, establishing new standards.
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Air Quality Index (AQI)
The Air Quality Index (AQI) is a standardised system for reporting daily air quality across different regions. It is based on the measurement of six criteria air pollutants: particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO). These pollutants are monitored by air quality sensors and models, which provide data on their concentrations in the air.
The AQI is calculated using a specific formula that considers the concentration of each pollutant and the time of exposure, known as the dose of the air pollutant. The potency of each pollutant varies, and different functions are used to convert pollutant concentrations into AQI values. These values are then grouped into ranges, with each range assigned a specific colour code, descriptor, and public health advisory. For example, during rush hour traffic or when there is an upwind forest fire, the AQI may increase due to higher levels of air emissions and the lack of dilution of air pollutants.
The AQI is an essential tool for understanding the impact of air quality on human health and is used to make decisions to protect public health and the environment. It provides a standardised way to communicate air quality information to the public, with each range indicating the level of health risk associated with exposure to the monitored pollutants. For instance, the Air Quality Health Index (AQHI) used in Canada and Hong Kong provides a number from 1 to 10+ to indicate the health risk, with higher numbers representing a greater risk to human health.
The AQI is also used by government agencies and environmental organisations to track air pollution levels and develop strategies to improve air quality. For example, the United States Environmental Protection Agency (EPA) uses the AQI to set National Ambient Air Quality Standards (NAAQS) and work with states and local agencies to implement measures to improve air quality, such as revising emissions standards for motor vehicles and industrial equipment.
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Hazardous air pollutants
The EPA uses hazardous air pollutant emissions and ambient monitoring data to develop regulatory programs that limit emissions from stationary sources. Reducing emissions of urban air toxics has been a priority for the EPA since the 1990 Clean Air Act Amendments. The EPA helps states meet standards for common pollutants by issuing federal emissions standards for new motor vehicles and non-road engines, as well as national emissions standards for categories of new industrial equipment, such as power plants, industrial boilers, and cement manufacturing.
In 2024, the Biden administration passed a rule tied to the Clean Air Act to limit seven of the most hazardous air pollutants emitted by heavy industries. This rule closed a loophole that required "major" sources of these pollutants to reduce their emissions by the maximum achievable amount, known as the "Once in, Always In" policy. Industrial facilities classified as "major" sources of toxic air pollution, such as chemical plants and oil refineries, must maintain strict pollution controls. However, in 2025, the Senate voted to overturn this rule, marking the first time Congress has moved to weaken the Clean Air Act in its 55-year history.
Despite overall emissions of air toxics declining since 1990, substantial quantities of toxic pollutants continue to be released into the air. Elevated risks of hazardous air pollution occur in urban areas, near industrial facilities, and in regions with high transportation emissions. To address this, the EPA works with states and other organizations to determine the frequency and locations of unhealthy peaks of pollutants like sulfur dioxide and nitrogen dioxide, which have multiple adverse respiratory effects and are associated with increased hospital admissions.
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State implementation plans
The Environmental Protection Agency (EPA) works in collaboration with state, local, and tribal air agencies to monitor and improve air quality across the United States. A key component of this effort is the implementation of the National Ambient Air Quality Standards (NAAQS), which are set by the EPA to protect public health and welfare. The EPA has established standards for six common air pollutants, also known as criteria air pollutants, which include ozone, particulate matter, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are known to have detrimental effects on human health and the environment, and their levels are regularly monitored by the EPA and partner agencies.
The contents of SIPs can vary depending on the specific air quality issues faced by each state or tribe, but they generally include control strategies and regulations to reduce emissions of the criteria air pollutants. Some common measures found in SIPs include vehicle emissions inspection and maintenance programs, regulations on industrial sources of pollution, and volatile organic compound (VOC) and nitrogen oxide (NOx) control measures. Additionally, SIPs may include provisions for monitoring and reporting air quality data, as well as enforcement actions for non-compliance. The EPA reviews and approves SIPs to ensure they meet all the necessary requirements, and once approved, the plans become federally enforceable.
An integral part of the SIP process is public participation and involvement. States and tribes are required to involve the public in the development and implementation of their plans, providing opportunities for input and feedback. This ensures that the SIPs are responsive to the needs and concerns of the communities they aim to protect. The public is also involved in the EPA's review process, as the Agency seeks public comment on proposed SIP revisions and holds hearings to gather input before making final decisions.
After a SIP is approved, the state or tribe is responsible for implementing and enforcing it. The EPA provides technical and financial assistance to support these efforts. However, if a state or tribe fails to attain or maintain the NAAQS, the EPA has the authority to step in and implement a Federal Implementation Plan (FIP). FIPs are typically more stringent and prescriptive than SIPs and are implemented when a state or tribe is unable or unwilling to develop a plan that ensures compliance with the air quality standards.
Overall, SIPs play a crucial role in the EPA's efforts to ensure clean air for all Americans. Through the development and implementation of these plans, states and tribes take ownership of improving and maintaining air quality, tailoring their approaches to the unique needs and characteristics of their regions. This collaborative process, involving state, local, tribal, and federal partners, along with public participation, helps ensure that effective measures are in place to protect public health and the environment from the harmful effects of air pollution.
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Frequently asked questions
The EPA determines if an area is polluted by using the Air Quality Index (AQI), which is an index for reporting air quality. The AQI includes six color-coded categories, each corresponding to a range of index values. The higher the AQI value, the greater the level of air pollution and the greater the health concern.
The six common air pollutants are particle pollution, ground-level ozone pollution, sulfur dioxide, nitrogen dioxide, lead, and benzene.
An AQI value of 50 or below represents good air quality.
If an area does not meet the national air quality standards, the EPA helps states to meet the standards by issuing federal emissions standards for new motor vehicles and non-road engines, and providing technical and policy guidance for state implementation plans.
