Air pollution is made up of many kinds of gases, droplets and particles that reduce the quality of the air. Air can be polluted in both the city and the country.

In the city, cars, buses and airplanes, as well as industry and construction may cause air pollution.  In the country, dust from tractors plowing fields, trucks and cars driving on dirt or gravel roads, rock quarries and smoke from wood and crop fires may cause air pollution.

Ground-level ozone is the major part of air pollution in most cities. Ground-level ozone is created when engine and fuel gases already released into the air interact when sunlight hits them. Ozone levels increase in cities when the air is still, the sun is bright and the temperature is warm. Ground-level ozone should not be confused with the "good" ozone that is miles up in the atmosphere and that protects us from the sun's harmful radiation.