USA air quality index (AQI) and PM2.5 air pollution

The United States has greatly improved its air quality since implementing the Clean Air Act in the late-twentieth century; however, it still faces challenges in ensuring all its citizens may breathe clean air. According to the WHO’s 2016 air quality database, 38 American cities exceeded the WHO’s recommended safe limit for annual mean PM2.5 concentration of 10μg/m3, with a highest measurement of 18μg/m3 recorded in Visalia-Porterville, California. Hanford-Corcoran, Portelo, Fresno (all California) and Elkhart-Goshen, IN are among the cities that followed. 23 cities were recorded as exceeding the WHO’s safe limit for annual mean PM10 concentration. Six out of the ten most polluted cities on this database are located in California, which has been experiencing prolonged issues with drought.

Conversely, the American Lung Association’s annual “State of the Air” report of America’s most polluted cities, based on particle and ozone pollution, ranked Bakersfield as the US’s most polluted city for particle pollution. Los Angeles’ Long Island Beach topped their list for levels of ozone.

Ozone and particle pollution are the two main pollutants of concern in the US, and also the most closely measured. The nation’s largest contributor to air pollution is transportation, which contributed over half of the country’s carbon monoxide and nitrogen oxide emissions, and almost a quarter of emitted hydrocarbons during 2013. A growing concern for North America’s air pollution situation is prolonged droughts and increasingly hot and dry conditions in response to climate change, laying a dangerous foundation for increased forest fires. 2015 saw a record amount of forests fall victim to fires, generating huge amounts of soot and particle pollution.

The Clean Air Act remains the country’s primary legislation aiming to control air pollution, since being implemented in 1963, with numerous amendments since. The Act targets six “criteria pollutants”: particle pollution, ozone, sulphur dioxide, nitrogen dioxide, carbon monoxide and lead. It addresses multiple areas in order to achieve cleaner air: imposing standards on motor vehicles and industrial plants; minimising conditions for acid rain; regional haze; and chemical emissions that deplete the ozone layer.