Air pollution comes from a wide range of sources, which are categorized into mobile, stationary, area and natural sources. Mobile sources include vehicles, trains and planes. Stationary sources include industries and oil refineries. Examples of area sources are cities and agricultural areas, while natural sources include volcanoes and wildfires.
Air pollutants can either be visible or invisible. Fumes from aerosol sprays, varnish, paint and hair spray are other sources of air pollution. Air pollution can also come from military sources, such rocketry, germ warfare and nuclear weapons. Dust and smoke are common air pollutants. Volcanic activity causes air pollution because it emits ash particles, sulfur dioxide and chlorine. Sulfur dioxide is also contained in smog and it causes acid rain.
Automobiles and other mobile sources contribute to more than 50 percent of air pollution within the United States, reveals National Park Service. Deposition of waste in landfills generates methane, which displaces oxygen. Methane is a greenhouse gas that is also emitted by livestock and other natural sources, such as swamps. Carbon dioxide is another greenhouse gas which contributes most to global warming. The health risks associated with air pollution include lung cancer, respiratory infections, stroke, asthma and heart disease.
Densely populated areas, such as cities, have the highest level of air pollution. Personal initiatives, such as driving less, contribute to the reduction of air pollution. Government policies, such as increasing tax on gasoline, also contribute towards reducing greenhouse gas emissions.