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air pollution
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Dense smog and orange haze blanket the skyline of Mexico City at sunset, with buildings in the Polanco neighborhood partially obscured by air pollution. Aerial view of a retail mall and residential apartments in Gurgaon, India, with thick smog obscuring the skyline due to severe air pollution. A London police officer wearing a protective mask stands in a street shrouded in thick smog, with vehicles and figures barely visible in the background, during the Great Smog of 1952.
New York City skyline tinted orange from wildfire smoke.
Diagram explaining air quality index (AQI) levels with a color-coded bar from green (good) to maroon (hazardous), describing health risks as air quality moves from healthy to unhealthy.
The video thumbnail image shows a person's hand holding small fragments of colorful plastic.
Automobile emissions Two pie charts compare sources of sulfur dioxide and nitrogen oxides emissions contributing to acid rain. The first chart shows electric utilities produce 66% of SO2, other sources 29%, and vehicles 5%. The second chart shows vehicles produce 58% of NOx, other sources 24%, and electric utilities 18%. Thick brown layer of smog obscures the Los Angeles skyline, with city buildings and neighborhoods visible beneath a blue sky. Bhopal, India: pesticide plant
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Dense smog and orange haze blanket the skyline of Mexico City at sunset, with buildings in the Polanco neighborhood partially obscured by air pollution.
air pollution summary
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Written by
Jerry A. Nathanson
Professor of Engineering, Union County College, Cranford, New Jersey. Author of Basic Environmental Technology: Water Supply, Waste Disposal, and Pollution Control.
Jerry A. Nathanson
Fact-checked by
Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
Britannica Editors
Last updated
•History
Diagram illustrating the greenhouse effect, showing sunlight reaching Earth, some visible light absorbed by the surface, some reflected, and infrared radiation absorbed and reradiated by the atmosphere.
Greenhouse effect on EarthThe greenhouse effect on Earth. Some incoming sunlight is reflected by Earth's atmosphere and surface, but most is absorbed by the surface, which is warmed. Infrared (IR) radiation is then emitted from the surface. Some IR radiation escapes to space, but some is absorbed by the atmosphere's greenhouse gases (especially water vapor, carbon dioxide, and methane) and reradiated in all directions, some to space and some back toward the surface, where it further warms the surface and the lower atmosphere.

Global warming is recognized by almost all atmospheric scientists as a significant environmental problem caused by an increase in levels of certain trace gases in the atmosphere since the beginning of the Industrial Revolution in the mid-18th century. These gases, collectively called greenhouse gases, include carbon dioxide, organic chemicals called chlorofluorocarbons (CFCs), methane, nitrous oxide, ozone, and many others. Carbon dioxide, although not the most potent of the greenhouse gases, is the most important because of the huge volume emitted into the air by the combustion of fossil fuels (e.g., gasoline, oil, coal).

News •

Dirty air actually breaks down the smell of flowers and stops bees and moths from finding their food • Nov. 6, 2026, 2:22 PM ET (Earth.com)
Restaurant cooking is becoming a surprisingly major source of air pollution • Nov. 4, 2026, 8:32 AM ET (Earth.com)
Wildfire Smoke Increasing At Schools And Childcare Centers Nationwide • Nov. 3, 2026, 9:40 AM ET (U.S. News & World Report)
Why Indy officials are warning some people to stay inside tonight • Oct. 28, 2026, 5:59 PM ET (Indianapolis Star)
Why more than half the world has likely breathed unhealthy air this year • Oct. 26, 2026, 1:00 PM ET (Washington Post)

Carbon dioxide is considered a normal component of the atmosphere, and before the Industrial Revolution, the annual global average levels of this gas were about 280 parts per million (ppm). By 2025 the annual global average of carbon dioxide had reached 426 ppm, and it continues to increase at a rate of almost 3 ppm per year. Many scientists think that carbon dioxide should be regulated as a pollutant—a position taken by the EPA in 2009 in a ruling that such regulations could be promulgated. International cooperation and agreements, such as the 2015 Paris Agreement, are necessary to reduce carbon dioxide emissions worldwide.