POLLUTION

also called environmental pollution the addition of any substance or form of energy (e.g., heat, sound, radioactivity) to the environment at a rate faster than the environment can accommodate it by dispersion, breakdown, recycling, or storage in some harmless form. A pollutant need not be harmful in itself. Carbon dioxide, for example, is a normal component of the atmosphere and a by-product of respiration that is found in all animal tissues; yet in a concentrated form it can kill animals. Human sewage can be a useful fertilizer, but when concentrated too highly it becomes a serious pollutant, menacing health and causing the depletion of oxygen in bodies of water. By contrast, radioactivity in any quantity is harmful to life, despite the fact that it occurs normally in the environment as so-called background radiation. Pollution has accompanied mankind ever since groups of people first congregated and remained for a long time in any one place. Primitive human settlements can be recognized by their pollutants-shell mounds and rubble heaps. But pollution was not a serious problem as long as there was enough space available for each individual or group. With the establishment of permanent human settlements by great numbers of people, however, pollution became a problem and has remained one ever since. Cities of ancient times were often noxious places, fouled by human wastes and debris. In the Middle Ages, unsanitary urban conditions favoured the outbreak of population-decimating epidemics. During the 19th century, water and air pollution and the accumulation of solid wastes were largely the problems of only a few large cities. But, with the rise of advanced technology and with the rapid spread of industrialization and the concomitant increase in human populations to unprecedented levels, pollution has become a universal problem. The various kinds of pollution are most conveniently considered under three headings: air, water, and land. also called Environmental Pollution, the addition to the environment of any substance or energy form (e.g., heat) at a rate faster than the environment can accommodate it by dispersion, breakdown, recycling, or storage in some harmless form. Pollution of the natural environment is a largely unintended and unwanted consequence of human activities in manufacturing, transportation, agriculture, and waste disposal. High levels of pollution are largely a consequence of industrialization, urbanization, and the rapid increase of human populations in modern times. Pollutants commonly are classified according to the part of the environment primarily affected by them: either the air, water, or land. Subgroupings depend on characteristics of the pollutants themselves: chemical, physical, thermal, and others. Many pollutants affect more than one resource. The substances that pollute the atmosphere are either gases, finely divided solids, or finely dispersed liquid aerosols. Five major classes of pollutants are discharged into the air: carbon monoxide, sulfur oxides, hydrocarbons, nitrogen oxides, and particulates (dust, ash). The principal source of air pollution is the burning of fossil fuels-e.g., coal, oil, and derivatives of the latter, such as gasoline-in internal-combustion engines or for heating or industrial purposes. The single largest source of air pollution is automotive exhaust fumes, which contain carbon monoxide and various hydrocarbons and nitrogen oxides. Most sulfur oxide emissions are from utility and industrial plants that burn coal and oil, both of which contain sulfur as an impurity. Air pollution is a particular problem in urban areas, where the ultraviolet rays in sunlight combine with hydrocarbons and nitrogen oxides to form photochemical smog. On a somewhat wider scale, sulfur dioxides and nitrogen oxides from the burning of fossil fuels can combine with atmospheric water vapour to form acid rain (q.v.), which is damaging to water, forest, and soil resources. As a result of the increased consumption of fossil fuels, levels of carbon dioxide in the atmosphere have risen steadily in the 20th century and show signs of increasing atmospheric temperatures worldwide owing to the greenhouse effect (q.v.). Air pollution is reduced or controlled by various methods. Automotive emissions are reduced by redesigning engines, installing devices to control their exhaust emissions, and improvements in fuel and fuel additives. Coal-burning power plants can be replaced by less polluting ways to generate electricity that involve the use of natural gas, nuclear fuels, or flowing water. Pollutants can be removed, or scrubbed, from the gases emitted in industrial smokestacks by electrostatic precipitation. Water pollution includes the accumulation in oceans, lakes, streams, and groundwater of substances that are either directly harmful to life or that have harmful secondary or long-term effects. The principal sources of water pollution are sewage, industrial waste, garbage and refuse, and agricultural fertilizers, pesticides, and herbicides. Any body of water has the capacity to absorb or break down introduced materials, and sewage and some organic industrial wastes are broken down naturally by microorganisms into forms in which they are useful to aquatic life. But if the capacity of a body of water to dissolve, disperse, or recycle is exceeded, all additional substances become pollutants. The major sources of water pollution are untreated sewage from cities and towns, chemical fertilizers and pesticides that have run off farmland into rivers and streams, and chemicals from industrial plants located along waterways. The domestic waste water in sewage systems can be artificially treated and purified to remove its pollutants before the water is discharged back into the environment in a process called water treatment. Industrial waste water can likewise be purified, or else production methods at the plant can be changed to minimize the production of wastes or recycle them for further industrial use. Agricultural wastes are generally less concentrated and take longer to produce aggravating effects than industrial and municipal sewage wastes originating in or near cities. Land pollution mainly involves the deposition on land of solid wastes-such as cans, bottles, plastic containers, paper, and used cars-that cannot be broken down quickly or, in some cases, at all. Aside from recycling, disposal methods include concentrating such materials in landfills, burning them in incinerators, or dumping them in the ocean. The term "land pollution" also includes the accumulation on land of toxic chemicals (in solid or liquid form) produced by industry and of radioactive wastes from nuclear processing facilities. Additional reading Rachel Carson, Silent Spring (1962, reissued 1982), a popular best-seller that first alerted the general public to the dangers inherent in the widespread use of persistent pesticides; Gino G. Marco, Robert M. Hollingworth, and William Durham (eds.), Silent Spring Revisited (1987), a collection of essays on the topics posed in Rachel Carson's book; Robert L. Rudd, Pesticides and the Living Landscape (1964, reprinted 1970), a scholarly review of the ways in which pesticides enter into the ecological networks in natural and man-made environments, with a discussion of the dangers involved in their continued use; Environmental Quality (annual), a regular report of the Council on Environmental Quality, which reviews the state of the U.S. environment with particular emphasis on problems of pollution; American Chemical Society, Cleaning Our Environment: A Chemical Perspective, 2nd ed. (1978); a report by the Council on Environmental Improvement on the technological state of the art of pollution control; Barry Commoner, Science and Survival (1966), a discussion of how science and technology can cause unanticipated changes in the environment, with particular reference to pollution from nuclear energy; Donald L. Barlett and James B. Steele, Forevermore, Nuclear Waste in America (1985), an investigative report on the environmental hazard; and Scientific Committee on Problems of the Environment (SCOPE), Environmental Consequences of Nuclear War, 2 vol. (1985-86), an international scientific review of the likely effects of nuclear war, prepared by a committee of the International Council of Scientific Unions. Raymond F. Dasmann