Air Pollution

Contamination of the atmosphere by gaseous, liquid, or solid substances that can endanger the health and welfare of humans or other living things or can attack materials, reduce visibility, or produce undesirable odors. Some air pollution is caused by natural sources, such as wildfires, volcanic eruptions, or radon gas emitted from the earth. Radon is a prominent cause of pollution of indoor air; other significant sources of indoor pollution include tobacco smoke and fumes from the combustion of various fuels, as well as asbestos fibers from old insulation and chemicals from furnishings, rugs, and cleaning materials. This article is concerned primarily with outdoor air pollution caused by human activities.

Each year industrially developed countries generate billions of tons of pollutants. Many come from directly identifiable sources; sulfur dioxide, for example, typically is produced by industrial facilities such as electric power plants burning high-sulfur coal or oil. Others are formed through the action of sunlight on previously emitted reactive materials (called precursors). For example, ozone, a dangerous pollutant in smog, is produced by the interaction of hydrocarbons and nitrogen oxides under the influence of sunlight. According to the American Lung Association's report State of the Air 2006, almost half the U.S. population lives in areas with unhealthful ozone levels. On the other hand, ozone in the upper atmosphere provides protection against the sun's ultraviolet rays. The discovery of evidence, beginning in the 1970s, that air pollutants such as chlorofluorocarbons (CFCs) were destroying the ozone layer led to moves to phase out these materials.

Meteorology and Health Effects

Pollutant concentrations are reduced by atmospheric mixing, which depends on such weather conditions as temperature, wind speed, amount of sunlight, and the movement of high and low pressure systems and their interaction with the local topography, for example, mountains and valleys. Normally, temperature decreases with altitude. But when a colder layer of air settles under a warm layer, producing a temperature, or thermal, inversion, atmospheric mixing is retarded and pollutants may accumulate near the ground. Inversions can become sustained under a stationary weather system coupled with low wind speeds.

Periods of poor atmospheric mixing of only a few days, and sometimes only a few hours, can lead to heavy concentrations of hazardous materials in high-pollution areas and, under severe conditions, can result in illness and even death. An inversion in Donora, Pa., in 1948 caused respiratory illness in over 6000 persons and led to the death of 20--the worst known air pollution disaster in U.S. history. Severe pollution in London took 3500 to 4000 lives in 1952 and another 700 in 1962. Release of methyl isocyanate into the air during a temperature inversion at Bhopal, India, in December 1984 caused at least 3300 deaths and more than 20,000 illnesses. The effects of long-term exposure to low concentrations are not well defined; however, those most at risk are the very young, the elderly, smokers, workers whose jobs expose them to toxic materials, and persons with heart or lung disease. Air pollution can also injure livestock and crops.

Often, the first noticeable effects of pollution are aesthetic and may not be dangerous. These include visibility reduction due to tiny particles suspended in air, or bad odors, such as the rotten egg smell produced by hydrogen sulfide emitted from pulp and paper mills.

The U.S. Environmental Protection Agency (EPA) sets limits, called National Ambient Air Quality Standards, for certain major pollutants regarded as harmful to health and the environment. The standards, which apply to outdoor air, are of two kinds: primary and secondary. Primary standards are intended to protect public health, including groups that tend to be especially sensitive to pollution, such as asthmatics, children, and the aged. Secondary standards focus on safeguarding public welfare, offering protection against such problems as decreased visibility and damage to animals, crops, vegetation, and buildings. As of the end of 2006, the EPA maintained National Ambient Air Quality Standards for six so-called "criteria" pollutants: carbon monoxide, sulfur dioxide, particulate matter, lead, nitrogen oxides, and ozone. These pollutants and the corresponding primary standards are described in the accompanying table. The standard is usually given in terms of atmospheric concentrations (micrograms or milligrams of pollutants per cubic meter of air) or in terms of parts per million, that is, number of pollutant molecules per million air molecules.

Exposure to high doses of many pollutants can be fatal, an outcome particularly apt to occur in indoor settings that allow pollutants to accumulate. Carbon monoxide, once it enters the bloodstream via the lungs, can bind to hemoglobin, resulting in decreased oxygen reaching the body's cells; people with cardiovascular problems are particularly at risk, but even healthy people may experience impaired mental alertness and vision. Even brief exposure to sulfur dioxide may lead to narrowing of the airways (bronchoconstriction), causing wheezing and shortness of breath; especially susceptible are persons with asthma who engage in outdoor physical activity. Long-term exposure to sulfur dioxide can result in respiratory disease and exacerbate existing cardiovascular disease. Particulate matter typically consists of a mixture of solids and liquid droplets in the air; particles that are small enough can get into the lungs and give rise to or worsen respiratory and other illnesses, sometimes even with just brief exposure. Lead exposure may damage organs, including the brain and nervous system, and can lead to osteoporosis or reproductive disorders. Nitrogen oxides may trigger respiratory symptoms; and though U.S. levels of these oxides, in particular of nitrogen dioxide, have generally been low enough by national EPA air quality standards, oxides continue raise concern because they contribute to the formation of ozone, particle pollution, and acid rain. Inhaling ozone can lead to a variety of respiratory problems and aggravate existing conditions, such as asthma; and frequent exposure over a period of time can cause permanent scarring of lung tissue and impairment of lung function.

A number of countries issue periodic assessments of the quality of outdoor air. In the U.S., this rating, compiled by the EPA on a daily basis for localities around the country, is called the Air Quality Index. Intended to reflect the health effects that may occur in a few hours or days after breathing polluted air, it is calculated for ground-level ozone and particulate matter, as well as for carbon monoxide, nitrogen dioxide, and sulfur dioxide. The Air Quality Index values for those pollutants of most concern in a given area are commonly made available via newspapers, broadcast media, and Internet. The index runs from 0 to 500. Values up to 50 indicate good or satisfactory air quality with minimal risk; values from 51 to 100 are classed as moderate, representing acceptable air quality, although some people unusually sensitive to the pollutant involved may suffer effects. A figure from 101 to 150 means that members of sensitive groups may experience health effects. Values from 151 to 200 are considered unhealthy, with health affects beginning to show up in the general population. The next level, from 201 to 300, is regarded as very unhealthy, involving more serious effects among the general public. Values above 300 are classed as hazardous, signaling an emergency situation.

Sources and Control

The combustion of coal, oil, and gasoline accounts for much of air pollution. more than 80 percent of the sulfur dioxide and roughly 40 percent of the nitrogen oxides released into the atmosphere in the U.S. come from fossil-fuel-fired electric power plants, industrial boilers, and residential furnaces. Nearly 80 percent of the carbon monoxide, more than 50 percent of the nitrogen oxides, and a somewhat smaller proportion of the hydrocarbons come from burning gasoline and diesel fuels in motor vehicles. Other major pollution sources include iron and steel mills; coke ovens; zinc, lead, and copper smelters; municipal incinerators; petroleum refineries; cement plants; large solvent users; and nitric and sulfuric acid plants.

Potential pollutants may exist in the materials entering a chemical or combustion process, or may be produced as a result of the process itself. Carbon monoxide, for example, is a typical product of internal-combustion engines. Methods for controlling air pollution include removing the hazardous material before it is used, removing the pollutant after it is formed, altering the process so that the pollutant is not formed or occurs only at very low levels (such as by replacing a high-pollution fuel with a cleaner one), and reducing use of the process. Automobile pollutants can be controlled by burning the gasoline as completely as possible, by recirculating fumes from fuel tank, carburetor, and crankcase, and by changing the engine exhaust to harmless substances in catalytic converters . Industrially emitted particulates may be trapped in cyclones, electrostatic precipitators, and filters. Pollutant gases can be collected in liquids or on solids, or incinerated into harmless substances.

Large-Scale Effects

The tall smokestacks traditionally used by industries and utilities do not remove pollutants but simply boost them higher into the atmosphere, thereby reducing their concentration at the site. These pollutants may then be transported over large distances and produce adverse effects in areas far from the site of the original emission. Sulfur dioxide and nitrogen oxide emissions from the central and eastern U.S., for example, have been found responsible for acid rain in New York State, New England, and eastern Canada. The pH, level, or relative acidity, of some freshwater lakes and streams in that region has been altered so dramatically by acid precipitation that entire fish populations have been destroyed. Similar effects have been observed in Europe and elsewhere. Sulfur dioxide emissions and the subsequent formation of sulfuric acid have also contributed to the degradation of limestone and marble at large distances from the source. Efforts at controlling emissions in North America and Europe, however, have shown that acidification can be slowed and in some cases even reversed.

The worldwide concentration of carbon dioxide in the atmosphere has risen since the Industrial Revolution. The period since the 1940s has seen a particularly high rate of increase as a result of a surge in the burning of coal, oil, and natural gas. Available data indicate that the increased carbon dioxide, along with certain other gases, is producing a "greenhouse effect," which allows solar energy to enter the atmosphere but reduces the reemission into space of infrared radiation from the earth. The greenhouse effect is expected to lead to a warming trend that, if prolonged, could alter the global climate and lead to a partial melting of the polar ice caps. The global climate is an extraordinarily complex system, and it is conceivable that a development such as an increase in cloud cover, the absorption of excess carbon dioxide by the oceans, or a decrease in solar radiation could halt the warming before it reached the stage of large-scale polar melting. Nevertheless, evidence has accumulated since at least the 1980s that the greenhouse effect is definitely under way, prompting many scientists to call for international action to deal with it.

Government Action

In the U.S., the Clean Air Act of 1963 as amended in 1966, 1967, 1970, 1977, and 1990 is the legal basis for air-pollution control throughout the U.S. The EPA has primary responsibility for carrying out the requirements of the act, which mandates the establishment of the National Ambient Air Quality Standards. The law sets limits on the discharge of pollutants into the air so that air-quality standards will be achieved. The act was also designed to prevent significant deterioration of air quality in areas where the air is currently cleaner than the standards require. The amendments of 1990 identified ozone, carbon monoxide, particulate matter, acid rain, and scores of "air toxics" (hazardous pollutants not dealt with elsewhere in the act, such as carcinogens, mutagens, and reproductive toxins) as major air pollution problems.

Internationally, a trailblazing attempt at limiting and preventing air pollution was the Convention on Long-Range Transboundary Air Pollution, the first legally binding multinational pact to confront air pollution on a broad regional basis. Adopted in 1979 within the framework of the United Nations Economic Commission for Europe (whose membership includes such countries as Canada and the U.S.), it took effect in 1983. As of 2006 it had been extended by eight protocols, although none of them had been signed by all of the approximately 50 parties to the convention. The protocols deal with long-term financing of monitoring and evaluation efforts; reduction of sulfur emissions; control of nitrogen oxides; control of emissions of volatile organic compounds (hydrocarbons); reduction of emissions of heavy metals (cadmium, lead, and mercury); curtailment of persistent organic pollutants (mainly pesticides, along with a couple of industrial chemicals and their by-products/contaminants); and abatement of acidification, eutrophication, and ground-level ozone.

A series of international agreements concluded in association with the United Nations have focused on the ozone layer of the atmosphere. The Vienna Convention for the Protection of the Ozone Layer, signed by 28 parties in 1985, acknowledged the importance of safeguarding the ozone layer. Specific commitments came later, in the Montréal Protocol on Substances that Deplete the Ozone Layer, adopted in 1987. The original number of signatories was 46. As of 2006, the number of parties to the protocol, which had by then been amended several times, exceeded 190. The Montréal Protocol called for a general phaseout of the production of chlorofluorocarbon (CFCs) and certain other compounds thought to contribute to ozone depletion. It also provided aid to developing countries in making this transition.

A concerted international effort to address the problem of global warming due to human-caused, or anthropogenic, emissions began with the United Nations Framework Convention on Climate Change, which was opened for signature at a 1992 "Earth Summit" held in Rio de Janeiro, Brazil, and went into force in 1994. As of the end of 2006, the number of parties to the convention totaled some 190. The convention's declared objective was "stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system." Specific emissions reduction commitments for certain gases (carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride) were expressed in a protocol adopted at a 1997 conference in Kyoto, Japan. These included an overall reduction in developed countries' greenhouse gas emissions of 5 percent below 1990 levels between the years 2008 and 2012. The European Union was to curb emissions by 8 percent, the U.S. by 7 percent, and Japan by 6 percent, but mandatory reductions were not imposed on developing countries. Although the U.S. signed the Kyoto Protocol, it subsequently declined to ratify the pact, objecting to the lack of limits on developing countries' emissions and to the harm the protocol's constraints were expected to cause the U.S. economy. For the pact to go into effect, it had to be ratified by developed countries accounting for at least 55 percent of total industrialized-nation greenhouse gas emissions. As a result of the rejection of the accord by the U.S., the world's leading emitter of greenhouse gases, this threshold was not reached until late 2004. The protocol finally went into force in February 2005, and as of late 2006 more than 160 nations had become parties to it. Among those that had not ratified the agreement, and thus were not bound by its terms, were the U.S. and Australia.       

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