Causes of air pollution
Air pollution is caused by harmful man-made and natural substances being introduced into the Earth's atmosphere. Air pollution deaths run into the millions every year but according to reports by the World Health Organisation, more deaths are linked to indoor sources of air pollution than to outdoor sources.
Air pollution types
Air pollution can be caused by liquid, gaseous or solid substances. These substances have harmful impacts on the natural world - including humans. Air pollution can be classified into indoor and outdoor sub-types.
Outdoor air pollution is a huge environmental health issue affecting the populations in developing and developed countries across the globe. Outdoor air pollution in both urban and rural areas was estimated - by the World Health Organisation - to cause 3 million premature deaths worldwide per year in 2012; this death rate is caused by exposure to small airbourne particles 10 microns or less in diameter (1 micron = 1/1000th of a millimeter).
Indoor air pollution from smoke is a major health risk for around 3 billion people who cook and warm their dwellings with carbon-based fuels and coal. In 2012, around 4.3 million premature deaths were linked to domestic air pollution. Indoor air pollution disproportionately impacts low-middle-income countries.
Primary and secondary pollutants
Pollutants can be described as either primary or secondary. Primary pollutants usually derive from a distinct process. For example, ash emissions from a volcanic eruption.
Secondary pollutants are those which typically form as a result of primary pollutants interacting or reacting with other substances.
Man-made causes of air pollution
Fixed sources include power plant smoke stacks, factories and manufacturing facilities, incinerators of waste along with furnaces and other types of fossil fuel-burning heating devices.
In poor and developing countries, traditional biomass burning is the main generator of airbourne pollutants; traditional biomass includes crop waste, dung and wood.
Mobile sources of air pollution include automotive vehicles on land, sea and air.
Controlled burn practices in agriculture and forest management also make a significant contribution - 'controlled' or 'prescribed' burning is a technique used in forest management, land management and farming. Fire is a natural part of both forest and grassland ecology but it is also used in a 'controlled' way to stimulate the germination of some desirable forest trees, thus renewing the forest.
Waste deposited in landfills can generate methane. Methane is highly flammable and can form highly flammable mixtures with air. Methane can also act to displace oxygen in an enclosed space through a process known as asphyxiation.
Military weaponary such as nuclear weapons, toxic gases, germ warfare and rockets are all known contributors to air pollution.
Fumes from paint, varnish, aerosol sprays and other solvents.
Natural causes of air pollution
Dust from natural sources, usually large areas of land with little or no vegetation. The world's major deserts are key contributors.
Methane, emitted by the digestion of food by animals, for example cattle.
Radon gas from radioactive decay within the Earth's crust. It is considered to be a health hazard. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as the basement and it is the second most frequent cause of lung cancer, after cigarette smoking.
Smoke and carbon monoxide from wildfires.
Vegetation can release large amounts of Volatile organic compounds (VOCs) into the atmosphere. These VOCs react with primary man-made pollutants especially carbon, nitrogen and sulphur-based compounds — to produce hazes of secondary pollutants typically following a seasonal pattern. Poplar, oak and willow are some examples of vegetation that contribute to VOC production. The VOC production from these species can have a massive impact on local ozone levels.
Volcanic activity, which produces sulphur, chlorine, and ash particulates emits millions of tons of material into the air each year. The US Geological Survey estimate that between 180-440 million tons of carbon dioxide is released through volcanic activity each year. See also the aerosol pollution pages.