Air pollution is physical or chemical changes brought about by natural processes or human activities that result in air quality degradation.
Primary pollutants are released directly into the air in a harmful form. Secondary pollutants are created or converted to a hazardous form after they enter the atmosphere, usually by photochemical reactions. Fugitive emissions come from non-point sources such as soil erosion, mining, or building deconstruction.
The Clean Air Act of 1970 designated seven major "criteria" pollutants (sulfur dioxide, carbon monoxide, particulates, volatile hydrocarbons, nitrogen oxides, photochemical oxidants, and lead) because these were regarded as the greatest threat to human health. Subsequently, some 660 other hazardous air pollutants have been added to the regulatory list.
More than 200 million Americans live in areas where the risk of death from air pollution effects is greater than the acceptable level of 1 in 1 million. Indoor air is a major part of this health risk, with pollution levels often greater than those outdoors.
Globally, some 2.5 billion people (mostly women and children) are exposed to hazardous levels of smoke from poorly ventilated heating and cooking fires. Altogether, air pollution probably contributes to more deaths per year than any single infectious disease.
Aerosols and air toxins can be carried long distances by wind currents. Dust from Chinese deserts, for example, often falls out on the American West. Through a process of sequential evaporation and precipitation, hazardous air pollutants are accumulating in the Arctic and Antarctic, where they concentrate through food chains to reach dangerous levels in both humans and top predators like polar bears and whales.
Chloroflurocarbons (CFCs) and other long-lasting chlorine-containing compounds migrate into the stratosphere where they destroy the ozone layer that protects us from harmful ultraviolet solar radiation. The Montreal Protocol, which called for a phase-out of these chemicals, is one of the best examples of international cooperation to fight global air pollution.
Sulfur and nitrogen oxides react in the air to form sulfuric and nitric acids, which fall to earth as acid rain, snow, or dry precipitation. These acids pollute surface waters, kill aquatic organisms, harm vegetation, destroy building materials, and reduce visibility. Pollution controls have significantly reduced these emissions, but more remains to be done.
Clean Air Acts passed in most developed countries and in many developing nations are among the central tools for environmental protection. They have played a major role in dramatic air pollution reduction in many nations. A highly controversial provision of the U.S. Clean Air Act is the "new source review," which requires that modern pollution-control equipment be installed when old power plants or factories are expanded or upgraded.
Market-based approaches to pollution control have been proposed as efficient alternatives to government mandates for specific equipment requirements or emission limits. Determining the best way to regulate air quality remains a controversial question.
Air quality has improved dramatically over the past 30 years in most developed countries. Air pollution remains a grave issue in many poorer countries, especially in the megacities of the developing world and the former Soviet Union.
