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Air pollution is the introduction of chemicals, particulate matter, or biological materials into the atmosphere, causing harm or discomfort to humans or other living organisms, or damaging ecosystems. Air pollution can cause health problems including, but not limited to, infections, behavioral changes, cancer, organ failure, and premature death. These health effects are not equally distributed across the U.S population; there are demographic disparities by race, ethnicity, socioeconomic status, and education. Air pollution can derive from natural sources (like wildfires), or anthropogenic sources. Anthropogenic air pollution has affected the United States since the beginning of the Industrial Revolution.

Air pollution is frequently measured in terms of the Air Quality Index, that provides a standardized numerical value for the six criteria pollutants. For example, an AQI of 100 roughly corresponds to a concentration of a pollutant at the National Ambient Air Quality Standard (NAAQS).

According to a 2009 report, around "60 percent of Americans live in areas where air pollution has reached unhealthy levels that can make people sick." Analyzing data from 2016–2018, the American Lung Association found major declines in air quality, including increases in ground-level ozone.

In 2016, a study found that levels of nitrogen oxides had plummeted over the previous decade, due to better regulations, economic shifts, and technological innovations. NASA reported a 32% decrease of nitrogen dioxide in New York City and a 42% decrease in Atlanta between the periods of 2005–2007 and 2009–2011.

Regulation
In the 1950s, 1970s, and 1990s, the United States Congress enacted a series of Clean Air Acts which significantly strengthened regulation of air pollution. Individual U.S. states, some European nations and eventually the European Union followed these initiatives. The Clean Air Act sets numerical limits on the concentrations of a basic group of air pollutants and provide reporting and enforcement mechanisms. The United States Environmental Protection Agency (EPA) is the federal agency responsible for creating and enforcing regulations that implement these laws.

The effects of these laws have been very positive. In the United States between 1970 and 2006, citizens enjoyed the following reductions in annual pollution emissions:


 * carbon monoxide emissions fell from 197 million tons to 89 million tons;
 * nitrogen oxide emissions fell from 27 million tons to 19 million tons;
 * sulfur dioxide emissions fell from 31 million tons to 15 million tons;
 * particulate emissions fell by 80%; and
 * lead emissions fell by more than 98%.

A 2020 paper published by researchers at MIT found that about half of air pollution and half of the resulting deaths are caused by emissions from outside a given state's boundaries, typically from prevailing winds moving west to east. Regulation of air pollution is split between federal, state, and local governments.

Since 1999, the EPA has used the air quality index (AQI) to communicate air pollution risk to the public, on a scale from 0 to 500, with six levels from Good to Hazardous. (The previous version was the Pollutant Standards Index (PSI), which did not incorporate PM2.5 and ozone standards.)

The actual standards for pollutant levels defining "attainment" and "non-attainment" areas for compliance purposes for six major pollutants are the National Ambient Air Quality Standards. These are required by law to be reviewed every five years, as new scientific information becomes available on the health and property impacts of pollution. These reviews typically cause political controversy as tighter requirements can have economic consequences for automobile manufacturers and companies that emit pollutants. State and local governments are responsible for enacting and enforcing regulations that achieve the federal standards, by limiting emissions from local sources.

Standards for emissions from motor vehicles are set exclusively by the federal government and the state of California under a long-standing EPA waiver. (California's regulations pre-date the national law, though its standards are still updated regularly.) This task is delegated to the California Air Resources Board; other states are allowed to adopt the California standard or the federal government's but not set their own standards. States also can, and do adopt policies which affect automobile emissions, such as vehicle inspection regimes and encouraging the use of public transportation in general or on high-pollution days (as with the Spare the Air program in the San Francisco Bay Area). Corporate average fuel economy standards are also set by the federal government. Though they have a significant impact on air pollution, they were originally created in response to the 1973–74 Arab Oil Embargo, and are administered by the National Highway Traffic Safety Administration, not the EPA.

The United States has a cap-and-trade program for two of the major pollutants, sulphur dioxide and nitrogen oxides. The Acid Rain Program, as it is known, applies to power plants that use fossil fuels, and was required by the Clean Air Act of 1990.

A much longer list of chemicals for which the EPA requires the maximum achievable reduction are covered by the National Emissions Standards for Hazardous Air Pollutants.