Under the Clean Air Act, EPA establishes air quality standards to protect public health, including the health of "sensitive" populations such as people with asthma, children, and older adults. EPA also sets limits to protect public welfare.This includes protecting ecosystems, including plants and animals, from harm, as well as protecting against decreased visibility and damage to crops, vegetation, and buildings. View more information regarding the national standards for these pollutants.

EPA has set national air quality standards for six common air pollutants (also called the criteria pollutants):

• nitrogen dioxide (NO2),
• ozone (O3),
• sulfur dioxide (SO2),
• particulate matter (PM),
• carbon monoxide (CO), and
• lead (Pb)

Four of these pollutants (CO, Pb, NO2, and SO2) are emitted directly from a variety of sources.  Ozone is not directly emitted, but is formed when oxides of nitrogen (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight. PM can be emitted, or it can be formed when emissions of NOx, sulfur oxides (SOx), ammonia, organic compounds and other gases react in the atmosphere.

To achieve these standards, EPA has in place regulatory and voluntary programs to reduce the amount of air pollutants emitted from a wide range of emission sources. To keep track of these emissions, the EPA maintains the National Emission Inventory (NEI), the national database of air pollutant emissions information. EPA conducts a comprehensive emissions inventory every three years. EPA also modifies the NEI at least annually to correct and augment its inventory data. Developing and updating the inventory is time-consuming, and EPA is working to reduce the amount of time it takes. While the data provided in this site are several years old, they do reflect the most recent inventory. For additional technical information about the NEI, visit the National Emission Inventory website.

Emission Data Source(1 pg, 14 K) : The attached document groups emissions into 8 major source sector and provides more detail about which sources are included in each sector. 

Air Emissions by Pollutant:

• Carbon Monoxide
• Lead (Pb)
• Ground-level Ozone Precursor: Nitrogen Oxides (NOx)
• Ground-level Ozone Precursor: Volatile Organic Compounds (VOCs)
• Particulate Matter (PM)
• Sulfur Dioxide (SO2)

Where You Live - View state, county and facility-level emission summaries for the six common air pollutants.

National Multi-pollutant Emissions Comparison - Percent contribution from source sectors across common pollutants.  Comparision is based on the most recent emissions inventory.

Air Quality Trends - EPA also tracks the levels of these pollutants in the ambient air.  View current and historical air quality trends.

National Air Toxics Assessments (NATA) - EPA is also working with other Federal, State, local and tribal air quality agencies to achieve reductions in emissions of toxic air pollutants. NATA is EPA's ongoing comprehensive evaluation of air toxics in the U.S.