Because some air pollutants persist in the atmosphere and are carried long distances by winds, air pollution transcends local, regional, and continental boundaries, and it also may have an effect on global climate and weather. For example, acid rain has gained worldwide attention since the 1970s as a regional and even continental problem. Acid rain occurs when sulfur dioxide and nitrogen oxides from the burning of fossil fuels combine with water vapor in the atmosphere, forming sulfuric acid and nitric acid mists. The resulting acidic precipitation is damaging to water, forest, and soil resources. It has caused the disappearance of fish from many lakes in the Adirondack Mountains of North America, the widespread death of forests in the mountains of Europe, and damage to tree growth in the United States and Canada. Acid rain can also corrode building materials and be hazardous to human health. These problems are not contained by political boundaries. Emissions from the burning of fossil fuels in the middle sections of the United States and Canada are precipitated as acid rain in the eastern regions of those countries, and acid rain in Norway comes largely from industrial areas in Great Britain and continental Europe. The international scope of the problem has led to the signing of international agreements on the limitation of sulfur and nitrogen oxide emissions.

Another global problem caused by air pollution is ozone depletion in the stratosphere. At ground level (i.e., in the troposphere), ozone is a pollutant, but at altitudes above 12 km (7 miles) it plays a crucial role in absorbing and thereby blocking ultraviolet radiation (UV) from the Sun before it reaches the ground. Exposure to UV radiation has been linked to skin cancer and other health problems. In 1985 it was discovered that a large “ozone hole,” an ozone-depleted region, is present every year between August and November over the continent of Antarctica. The size of this hole is increased by the presence in the atmosphere of chlorofluorocarbons (CFCs) emanated from aerosol spray cans, refrigerators, industrial solvents, and other sources and transported to Antarctica by atmospheric circulation. It had already been demonstrated in the mid-1970s that CFCs posed a threat to the global ozonosphere, and in 1978 the use of CFCs as propellants in aerosol cans was banned in the United States. Their use was subsequently restricted in several other countries. In 1987 representatives from more than 45 countries signed the Montreal Protocol, agreeing to place severe limitations on the production of CFCs. The efficacy of this legislation and global efforts can be seen in the ozone layer’s continued recovery; in 2019 scientists recorded the smallest ozone hole above Antarctica since 1982.

One of the most significant effects of air pollution is on climate change, particularly global warming. As a result of the growing worldwide consumption of fossil fuels, carbon dioxide levels in the atmosphere have increased steadily since 1900, and the rate of increase is accelerating. It has been estimated that if the levels of carbon dioxide and other greenhouse gases are not reduced, average global air temperatures may rise 4 °C (7.2 °F) by the end of the 21st century. Such a warming trend would accelerate the melting of the polar ice caps, rising of the sea level, and flooding of the coastal areas of the world. Changes in precipitation patterns caused by global warming are already having adverse effects on agriculture and wild ecosystems, and higher temperatures and humidity are increasing the incidence of disease in humans and animals in some parts of the world. Implementation of international agreements on reducing greenhouse gases is required to protect global air quality and to mitigate the effects of global warming.

Air pollution prevention and control

Most air contaminants originate from combustion processes, and the best way to protect air quality is to reduce the emission of these pollutants by changing to cleaner fuels and processes. Pollutants not eliminated in this way must be collected or trapped by appropriate air-cleaning devices as they are generated and before they can escape into the atmosphere. A variety of technologies have been developed to reduce emissions of air pollutants from vehicles, power plants, and industrial facilities. In gas- and diesel-powered motor vehicles, a number of emission control systems help limit the discharge of noxious gases from the engine exhaust, crankcase, and fuel tank. Catalytic converters, for example, reduce emissions of carbon monoxide, nitrogen oxides, and unburned hydrocarbons by promoting chemical reactions that convert these pollutants into less harmful substances.

For point sources of air pollution, such as manufacturing plants or industrial smokestacks, airborne particles can be removed from a polluted airstream by a variety of physical processes. Common types of equipment for collecting fine particulates include cyclones, scrubbers, electrostatic precipitators, and baghouse filters. Once collected, particulates adhere to each other, forming agglomerates that can readily be removed from the equipment and disposed of, usually in a landfill. Wet scrubbers are usually applied in the control of flammable or explosive dusts or mists from such sources as industrial and chemical processing facilities and hazardous waste incinerators; they can handle hot airstreams and sticky particles. Electrostatic precipitators and fabric-filter baghouses are often used at power plants.

Gaseous criteria pollutants, as well as volatile organic compounds (VOCs) and other gaseous air toxics, are controlled by means of three basic techniques: absorption, adsorption, and incineration (or combustion). These techniques can be employed singly or in combination. They are effective against the major greenhouse gases as well. See also air pollution control.

Indoor air pollution

Health risks related to indoor air pollution have become an issue of concern because people generally spend most of their time indoors at home and at work. The problem has been exacerbated by well-meaning efforts to lower air-exchange rates in buildings in order to conserve energy; these efforts unfortunately allow contaminants to accumulate indoors. Indoor air pollutants include various combustion products from stoves, kerosene space heaters, and fireplaces, as well as volatile organic compounds (VOCs) from household products (e.g., paints, cleaning agents, and pesticides). Formaldehyde off-gassing from building products (especially particleboard and plywood) and from dry-cleaned textiles can accumulate in indoor air. Bacteria, viruses, molds, animal dander, dust mites, and pollen are biological contaminants that can cause disease and other health problems, especially if they build up in and are spread by central heating or cooling systems. Environmental tobacco smoke, also called secondhand smoke, is an indoor air pollutant in many homes, despite widespread knowledge about the harmful effects of smoking. Secondhand smoke contains many carcinogenic compounds as well as strong irritants. In some geographic regions naturally occurring radon, a radioactive gas, can seep from the ground into buildings and accumulate to harmful levels. Exposure to all indoor air pollutants can be reduced by appropriate building construction and maintenance methods, limitations on pollutant sources, and provision of adequate ventilation.