The sources of air pollution are divided in two groups: anthropogenic (caused by human activity) and natural.
Natural sources include:
- Dust from natural sources, usually large areas of land with little or no vegetation
- Gases, such as methane, emitted by the digestion of animals, usually
- Smokefrom wildfires.
Anthropogenic sources are mostly related to burning different kinds of fuel. They include:
- Dust and chemicals from farming, especially of erodible land, see Dust Bowl
- Industrial activity in general.
- Vehicles with internal-combustion engines.
- Stoves and incinerators, specially coal
- Paint fumes, or other toxical vapors.
Contaminants of air can be divided in particles and gases.
Particles are classified by their sizes. A usual division is in PM10 and PM2.5. PM10 are particles whose size is less than 10 microns (0.01 mm); they are dangerous to humans because they can be breathed and reach the lungs. PM2.5 are particles whose size is less than 2.5 microns (0.0025 mm), and they are even more dangerous because they can pass the alveoli and reach the blood.
Important pollutant gases include:
- Carbon monoxide.
- Ozonein the lower part of the atmosphere (in the upper part, it helps to reduce the amount of ultraviolet radiation from the sun that reaches earth).
- Sulfur dioxide, which causes acid rain.