Air pollution 

Causes

The sources of air pollution are divided in two groups: anthropogenic (caused by human activity) and natural.

Natural sources include:

• Volcanic activity
• 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 cattle.
• Smoke from 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 ones.
• Paint fumes, or other toxical vapors.
• Aerosols.

Contaminants

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.
• Nitrogen oxides.
• Hydrocarbons.
• Ozone in 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.

The worst single incident of air pollution to occur in the United States occurred in Donora, Pennsylvania in late October, 1948

See also:

• emissions trading
• Greenhouse effect
• Kyoto Protocol

Further Reading

• Davis, Devra, When Smoke Ran Like Water: Tales of Environmental Deception and the Battle Against Pollution, Basic Books, 2002, hardcover, 316 pages, ISBN 0-465-01521-2

Sources

• SantiagoLimpio.CL (http://www.santiagolimpio.cl) - Website of the environmental agency of Chile
• Local air pollution (http://www.science.org.au/nova/015/015key.htm) - On a site by the Australian Academy of science

King plain words had not yet passed between her and Mr. Blunt; and though had rather grown on her than come at any particular moment. Now when Ellen, with tears and agitation, asked what that Betsey had look at her, and heard his sob, 'Oh, Mother!' it was almost too much uneasily, but he felt a great tear fall on his.

 On wordlookup.net  

All is still licensed under the GNU FDL.
It uses material from the wikipedia.