What Makes Outdoor Air Unhealthy

Whether it comes from your car's exhaust pipe, dirty smokestacks or a forest fire, air pollution can be deadly. Learn about outdoor air pollutants and the sources of pollution and how they can be cleaned up so everyone can breathe safer, healthier air.

Common Outdoor Air Pollutants

  • Ozone
    Ozone (also called smog) is currently one of the least-well-controlled pollutants in the United States. It is also one of the most dangerous—and it's invisible.
  • Particulate Matter
    Particulate matter, or particle pollution, refers to a mix of very tiny solid and liquid particles that are in the air we breathe. It is dangerous enough to shorten your life.
  • Nitrogen Dioxide
    Nitrogen dioxide is a gas that comes from burning fuels. Live near a busy highway? You may be breathing a lot of nitrogen dioxide.
  • Sulfur Dioxide
    Sulfur dioxide forms when sulfur-containing fuel such as coal, oil or diesel is burned. Live near a power plant or a port? You may have more sulfur dioxide in your air. 
  • Carbon Monoxide
    Carbon monoxide is another gas that forms from burning fuels.  High levels often show up near roadways.
  • Toxic Air Pollutants
    Nearly 200 other pollutants emitted into the air can cause diseases, including lung cancer, or cause other harmful effects.

Sources of Outdoor Air Pollution

  • Electric Utilities
    Electric utilities burning coal, natural gas, oil and biomass produce air pollutants that harm lung health.
  • Transportation
    Vehicles and equipment used to move people, goods and fuel from one place to another can pollute the air and harm human health.
  • Residential Sources
    Heating, cooling and powering our homes all produce emissions that can harm health, including in our own neighborhoods.
  • Commercial and Industrial
    Harmful emissions come from heating, cooling and powering businesses and industrial operations, as well as from manufacturing processes.
  • Emergencies and Natural Disasters
    Wildfires, flooding, hurricanes and other natural events can create unhealthy air, especially for people with lung disease. And the cleanup often adds to the challenge.
  • Climate Change
    Climate change poses a grave health threat to everyone, with serious risks to lung health.

Page last updated: March 6, 2020

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