Air Pollution

What is Air Pollution?

Air pollution occurs when harmful substances are found at an unhealthy level in the air. Pollutants from anthropogenic (human caused) sources can be categorized as either primary or secondary pollutants.

Primary pollutants are put directly into the air by humans. Review the 5 main categories of primary pollutants below.

Click on each of the Primary Pollutants to learn more.

Carbon Monoxide

produced by incomplete burning of fossil fuels, interferes with body’s ability to carry oxygen


Nitrogen Oxide

formed when fuels burn at temperatures over 538 degrees C, contributes to respiratory infections, lung disease, and possibly cancer


Sulfur Dioxide

formed by chemical reactions between sulfur and oxygen, contributes to acid rain


Volatile Organic Compounds

formed by organic chemicals that vaporize, contribute to formation of smog and can cause serious health problems like cancer


Particulate Matter

tiny particles of liquid or solid matter (ex. smoke), can cause respiratory problems and cancer

ozone is a primary component of smog Secondary pollutants are formed when primary pollutants come in contact with other primary pollutants or with naturally occurring substances and a chemical reaction occurs. An example of a secondary pollutant is ozone (O­3).

Ozone is a primary component of smog. This molecule is formed when nitrogen oxide and volatile organic compounds are combined and warmed by the sun.

Air pollution can also be categorized by its form as either gaseous or particulate.

Examples of gaseous pollutants include hydrogen sulfide (H2S), carbon monoxide (CO), and volatile organic compounds (VOC). Particulate pollutants can vary in size from large to small solid forms. Examples of particulate pollutants include dust, asbestos, and nitrates.

Clean Air Act

The United States Clean Air Act was designed to protect the public from the damage to health and welfare due to air pollution.

Congress outlined the basic structure of this act in 1970 with major revisions occurring in 1977 and 1990. The Clean Air Act empowered the Environmental Protection Agency (EPA) to set national ambient air quality standards for common pollutants based on current studies in science.

Today the EPA has standards set for the six most commonly occurring air pollutants in the United States:

  • Ground-level Ozone
  • Particulate Matter
  • Carbon Monoxide
  • Lead
  • Nitrogen Oxide
  • Sulfur Dioxide

Under this act, states are required to adopt and enforce plans that help maintain optimal air quality. This includes emissions that might travel across state lines.

Other provisions seek to reduce pollution created by motor vehicles as well as newly established industrial plants. Congress built in general authorities in the Clean Air Act to address problems that may be identified over time such as climate change caused by greenhouse gases.