Air pollution is the contamination of air with gases or particles that can cause damage to the health of humans and other animals and plants and also to materials and structures. It can be the result of both natural processes and human activities. Natural sources of air pollution include volcanos, wildfires, dust storms, and some types of vegetation on warmer days.
The largest source of air pollution from human activities is the burning of fossil fuels, especially for transportation and electric power generation. Also important are chemicals released from a wide variety of industrial processes. Among the many air pollutant substances are carbon monoxide, sulfur dioxide, nitrous oxides, methane, ozone, ammonia, carbon dioxide, chlorofluorocarbons, mercury, lead, and biological molecules.
Smog, which is typically created from vehicular and industrial emissions, is perhaps the most common and obvious type of air pollution, because it can be highly visible and now occurs throughout much of the world. However, unlike smog, much air pollution is invisible, but is no less harmful.
In addition to outdoor air pollution, indoor air pollution has also been becoming an increasingly severe problem. Among the causes are toxic gasses released by cooking with wood, charcoal, natural gas, etc., toxic gasses emitted by household furnishings such as furniture and carpets made from synthetic materials, and the trend towards hermetically sealing houses and other buildings in attempt to minimize heat loss. Air pollution from outdoor and indoor and outdoor sources together is one of the leading causes of premature human death.