Air Pollution, contamination of the atmosphere by gaseous, liquid, or solid wastes or by-products that can endanger human health and the health and welfare of plants and animals, or can attack materials, reduce visibility, or produce undesirable odors. Among air pollutants emitted by natural sources, only the radioactive gas radon is recognized as a major health threat. A byproduct of the radioactive decay of uranium minerals in certain kinds of rock, radon seeps into the basements of homes built on these rocks.
According to recent estimates by the U. S. government, 20 percent of the homes in the U. S. harbor radon concentrations that are high enough to pose a risk of lung cancer. Each year industrially developed countries generate billions of tons of pollutants. The most prevalent and widely dispersed air pollutants are described in the accompanying table. The level is usually given in terms of atmospheric concentrations (micrograms of pollutants per cubic meter of air) or, for gases, in terms of parts per million, that is, number of pollutant molecules per million air molecules.
Many come from directly identifiable sources; sulfur dioxide, for example, comes from electric power plants burning 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. Ozone has also caused serious crop damage. On the other hand, the discovery in the 1980s that air pollutants such as fluorocarbons are causing a loss of ozone from the earth’s protective ozone layer has caused the phasing out of these materials.
Pollutant concentrates are reduced by atmospheric mixing, which depends on such weather conditions as temperature, wind speed, 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 high-pressure system coupled with low wind speeds. Periods of only three days of poor atmospheric mixing can lead to high concentrations of hazardous materials in high-pollution areas and, under severe conditions, can result in injury and even death. An inversion over Donora, Pennsylvania, in 1948 caused respiratory illness in over 6000 persons and led to the death of 20. 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 caused the disaster at Bhopal, India, in December 1984, with 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. Other adverse effects of air pollution are potential injury to livestock and crops. Often, the first noticeable effects of pollution are aesthetic and may not necessarily 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 emanating from pulp and paper mills. Sources and Control The combustion of coal, oil, and gasoline accounts for much of the airborne pollutants. More than 80 percent of the sulfur dioxide, 50 percent of the nitrogen oxides, and 30 to 40 percent of the particulate matter emitted to the atmosphere in the U. S. are produced by fossil-fuel-fired electric power plants, industrial boilers, and residential furnaces.
Eighty percent of the carbon monoxide and 40 percent of the nitrogen oxides and hydrocarbons come from burning gasoline and diesel fuels in cars and trucks. Other major pollution sources include iron and steel mills; zinc, lead, and copper smelters; municipal incinerators; petroleum refineries; cement plants; and nitric and sulfuric acid plants. Potential pollutants may exist in the materials entering a chemical or combustion process (such as lead in gasoline), or they 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, or altering the process so that the pollutant is not formed or occurs only at very low levels. 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 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. are causing acid rain in New York State, New England, and eastern Canada. The pH level, or relative acidity, of many freshwater lakes in that region has been altered so dramatically by this rain that entire fish populations have been destroyed. Similar effects have been observed in Europe. Sulfur dioxide emissions and the subsequent formation of sulfuric acid can also be responsible for the attack on limestone and marble at large distances from the source.
The worldwide increase in the burning of coal and oil since the late 1940s has led to ever increasing concentrations of carbon dioxide. The resulting “greenhouse effect”, which allows solar energy to enter the atmosphere but reduces the reemission of infrared radiation from the earth, could conceivably lead to a warming trend that might affect the global climate and lead to a partial melting of the polar ice caps. Possibly an increase in cloud cover or absorption of excess carbon dioxide by the oceans would check the greenhouse effect before it reached the stage of polar melting.
Nevertheless, research reports released in the U. S. in the 1980s indicate that the greenhouse effect is definitely under way and that the nations of the world should be taking immediate steps to deal with it. Government Action In the U. S. , the Clean Air Act of 1967 as amended in 1970, 1977, and 1990 is the legal basis for air-pollution control throughout the U. S. The Environmental Protection Agency (EPA) has primary responsibility for carrying out the requirements of the act, which specifies that air-quality standards be established for hazardous substances. These standards are in the form of concentration levels that are believed to be low enough to protect public health.
Source emission standards are also specified to limit 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 air toxins as major air pollution problems. On the international scene, 49 countries agreed in March 1985 on a United Nations convention to protect the ozone layer.
This “Montral Protocol,” which was renegotiated in 1990, calls for the phaseout of certain chlorocarbons and fluorocarbons by the year 2000 and provides aid to developing countries in making this transition. Smog, mixture of solid and liquid fog and smoke particles formed when humidity is high and the air so calm that smoke and fumes accumulate near their source. Smog reduces natural visibility and often irritates the eyes and respiratory tract. In dense urban areas, the death rate usually goes up considerably during prolonged periods of smog, particularly when a process of heat inversion creates a smog-trapping ceiling over a city.
Smog occurs most often in and near coastal cities and is an especially severe problem in Los Angeles and Tokyo. Smog prevention requires control of smoke from furnaces; reduction of fumes from metal-working and other industrial plants; and control of noxious emissions from automobiles, trucks, and incinerators. In the U. S. internal-combustion engines are regarded as the largest contributors to the smog problem, emitting large amounts of contaminants, including unburned hydrocarbons and oxides of nitrogen. The number of undesirable components in smog, however, is considerable, and the proportions highly variable.
They include ozone, sulfur dioxide, hydrogen cyanide, and hydrocarbons and their products formed by partial oxidation. Fuel obtained from fractionation of coal and petroleum produces sulfur dioxide, which is oxidized by atmospheric oxygen, forming sulfur trioxide (SO3). Sulfur trioxide is in turn hydrated by the water vapor in the atmosphere to form sulfuric acid (H2SO4). The so-called photochemical smog, which irritates sensitive membranes and damages plants, is formed when nitrogen oxides in the atmosphere undergo reactions with the hydrocarbons energized by ultraviolet and other radiations from the sun.