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April 22, 2021 marks the 51st anniversary of Earth Day, an event that began in 1970 when 20 million Americans demonstrated across the country to call for greater protection of the nation’s environment. Initiated by Senator Gaylord Nelson of Wisconsin, the event was an expression of the public’s growing concern about issues like air and water pollution. The same year also marked the creation of the Environmental Protection Agency (EPA) and President Richard Nixon signing the Clean Air Act.

What is the Clean Air Act?

The Clean Air Act is a federal law that regulates air pollutant emissions in the United States to “promote the public health and welfare” of the nation. One part of the law directs the EPA to set air pollution standards, known as National Ambient Air Quality Standards, which states, local governments, and tribes must meet in their respective jurisdictions. The agency sets standards for six common pollutants known as “criteria pollutants":

  • Particular matter (PM 2.5 and PM 10)
  • Ground-level ozone
  • Sulfur dioxide
  • Nitrogen dioxide
  • Carbon monoxide
  • Lead

These pollutants come from a number of sources, including highway vehicles, electric utilities, and industrial plants. The health effects associated with exposure range from increased risk of respiratory diseases like asthma to impaired organ development in children. The EPA periodically revises the air quality standards as new scientific evidence emerges about the health risks associated with each pollutant.

How has air quality changed since the passage of the Clean Air Act? 

EPA data shows that air quality has generally improved in the United States since 1980, although trends vary based on location. One common measurement of overall air quality is the Air Quality Index, or AQI, which captures concentrations of five out of the six criteria pollutants (excluding lead). AQI values range from 0 to 500, with higher values indicating greater levels of pollution. The scale is also divided into categories that indicate the level of health concern, with AQI values between 0 and 50 considered good, or satisfactory for human health.

In 1980, the average AQI across US counties was 59.6, a value considered in the moderate range given its effects on human health. By 2020, the value had fallen to 36.4, reaching the range considered good.

This is one way to estimate the air quality in the average place in the US. Another method uses a population-weighted average to estimate the air quality experienced by the average person. Since more densely populated areas tend to have worse air quality, the average AQI adjusted for population is higher than the non-adjusted value. In 2020, the population-weighted average AQI was 44.4, down from 78.5 in 1980.

Adjusting for population, California had the largest air quality improvement of any state: the average AQI fell from 114.2 (in the unhealthy range) to 61.1 (moderate range). The improvement was even more pronounced in the Los Angeles metro area, where the AQI fell from 174.2 to 75.6.

Despite the positive impact of the Clean Air Act, air pollution is still a problem in many areas of the US. EPA data shows that as of 2019, 82 million people lived in counties where the concentration of one or more criteria air pollutants exceeded agency standards. Ground-level ozone affected the largest population, with 74 million Americans exposed to ozone levels above the national standard.

The EPA also reports other ongoing challenges related to protecting the nation’s air, from limiting greenhouse gas emissions that trap heat in the atmosphere to preventing the release of toxic air pollutants that hurt human health.

For more data on the nation's air, land, and energy, visit the State of the Earth.

Air Quality System