The term "pollution," which carries with it a sense of an impurity, can be defined as a chemical or physical agent in an inappropriate location or concentration. The sources of pollution are varied. Natural sources include those that are not directly under human control, such as volcanoes, which spew forth sulfur oxides and particles; and those people could avoid, such as groundwater with naturally high levels of arsenic, which has caused poisoning in Bangladesh and Taiwan. All human activities have the possibility of polluting the environment by contaminating air, water, food, or soil, The earliest human pollution-control efforts dealt with avoidance of diseases caused by
TYPES OF POLLUTION
Pollution production can be considered under the heading of the four major human activity sectors: industry, energy, transportation, and agriculture. With the marked increase in human population and the industrialization of much of the globe has come a whole new set of pollutants. Scientific advances based upon understanding the chemical and physical forces underlying nature have led to new processes and new products that have transformed society and have had a major positive impact on human health. But these industrial activities also result in air and water emissions and contamination of the soil and of food as by-products of the processes involved in manufacture. The products themselves may be the means by which pollutants are distributed to the general population, such as lead poisoning through the use of lead in house paints. In the United States and other more wealthy countries, there recently has been a marked decline in industrial pollution emissions per unit produced. This has come about through regulatory control of emissions and, in part, through the recognition by industry that emissions represent a loss of raw materials or product that is economically advantageous to retain. As developed countries move into the information era, much of the production of textiles and durable goods has shifted to developing countries, not always with the same level of pollution control or protection of the work force. In developing countries, industrial production often occurs in smaller units, such as backyard smelters, which have significant local effects and are more difficult to control.
The energy sector continues to grow rapidly worldwide. There are basically three types of energy sources: the burning of fossil fuels and biomass; nuclear power; and energy derived from natural processes such as the sun, wind, and the flow of water. Energy from fossil fuels results from the conversion of carbon to carbon dioxide, with the least efficient and most polluting fossil fuels reflecting the extent of components other than carbon and hydrogen in the fuel source. The most plentiful fossil fuel is coal, which is also among the most polluting. Coal contains mineral ashes, nitrogen, and sulfur, which produce particulates, nitrogen oxides and sulfur oxides, when coal is burned. The use of high-sulfur coal for electric power generation and for home heating was a dominant cause of major air pollution episodes in London in 1952, Donora, Pennsylvania, in 1948, and the Meuse Valley in Belgium in 1930. Much of the U.S. electric grid is powered by low-sulfur oil. Natural gas, which is a relatively pure hydrocarbon, is increasing in use and is particularly effective as a source of peak electric power during periods of high demand. The combustion of all fossil fuels produces nitrogen oxides, which are a major precursor of ozone and particulates. One form of nitrogen oxide, nitrogen dioxide, is itself a pollutant of concern. Carbon dioxide, the end product of efficient fossil fuel energy production, is a major contributor to global climate change. Reduction in carbon dioxide emissions requires more efficient production, transmission, and use of fossil fuel-derived energy. A switch to other energy sources will also help to reduce emissions.
Nuclear power has the advantage of not producing carbon dioxide or any of the sulfur oxides, nitrogen oxides, or particulates that are associated with fossil fuels. Its major disadvantages are the release of low-level radiation, the need for major water resources for cooling (with attendant ecological challenges), and, most importantly, the small but not absent risk of an uncontrolled nuclear reaction. The worst such example, and the only one in which there were substantial short-term health impacts from the civilian use of nuclear power, occurred in Chernobyl in the former Soviet Union in 1986. The extent of long-term effects due to the radiation that spread widely over Europe and globally is still being evaluated.
Wind and solar energy are expected to increase in use as the costs of fossil fuels increase and as new technology is developed. These are, in essence, free of pollution emissions. Hydroelectric power is a mainstay in some parts of the world, but dams have significant ecological implications and
The transportation sector worldwide is increasingly dominated by automobile and truck emissions. In the United States there has been a marked decrease in pollutant emissions per mile driven that has been almost counterbalanced by an increase in the number of miles driven. Pollutants from gasoline-powered automobiles include the evaporation of volatile organic compounds and tailpipe emissions such as carbon monoxide, nitrogen oxides, benzene, and polycyclic aromatic hydrocarbons (PAHs). Increased engine efficiency and catalytic converters have been effective in decreasing all but nitrogen oxide emissions. Diesel engines, which in the United States are primarily used on trucks, emit high levels of particulates and PAHs. Two-cycle engines on mopeds and other smaller vehicles are relatively inefficient, with much of the fuel evaporating. This is particularly a problem in developing countries. All internal combustion engines lead to the production of carbon dioxide. Future growth in the use of personal automobiles will be a major threat to global carbon dioxide production unless new engines and power sources are developed. Control of automotive emissions is as much a function of effective planning of transportation systems, including mass transit, as it is of technology. There have been relatively few studies of airport-related pollutant emissions, a segment of transportation that is increasing rapidly.
Agriculture is also a major source of pollution. World population growth has been accompanied by increased crop yields, which have been made possible by heavy use of fertilizers and pesticides. Nitrogenous fertilizers, an important part of the increased yield, result in nitrite contamination of drinking water, to which infants are particularly vulnerable. Nitrogenous fertilizers contribute to oxygen problems in water bodies and to greenhouse gas emissions. Phosphate fertilizers are of concern because of trace amounts of cadmium and other heavy metals that sometimes are part of natural phosphates. Cadmium can be taken up into certain crops, can cause renal toxicity, and is a potential carcinogen.
There are a wide range of pesticides and herbicides that are central to modern agriculture. Each of these is chosen because of its ability to have a biological effect on a plant or insect, and there is always a possibility that the biological effect will extend to humans or to other species. Major problems have been caused by pesticides that persist in the environment, such as heptachlor. This has led to bans on persistent organic pollutants and to testing protocols to avoid developing new ones.
OTHER POLLUTION CATEGORIES
Categorizing pollution in terms of the four sectors of industry, energy, transportation, and agriculture obscures the fact that some of the most important sources of pollution are intersectorial. As just one example, the Aswan High Dam provides Egypt with an important hydroelectric source and is effective in controlling flooding and providing irrigation for agriculture. But by retaining silt it decreases the nutrient load to the Nile Delta, which leads to a much heavier requirement for chemical fertilizers for agriculture as well as loss of sardine and salmon fisheries. The lack of the flushing effect of Nile floods has led to increased salinization of the land and has optimized breeding conditions for snails that carry schistosomiasis, an ancient scourge of this area. Similarly, the use of wood for local energy in developing countries is more than just a potential source of indoor and outdoor air pollution. Loss of forests can lead to soil erosion, flooding, and desertification, and have a negative impact on global climate.
Activities that lead to human development within and across each of these major sectors have the potential for producing a pollution impact that outweighs any benefit. There is, unfortunately, one common human activity that has an enormous environmental impact with no redeeming developmental consequences: war.
Pollutants can also be characterized by chemical or physical class; by use; by industrial source; by whether they are likely to be present in air, water, food, or other media; by the organs they attack or the effects they have; by the laws that control their use; and by whether they present a local, regional, or global problem. All of these categorization schemes are valuable, but none are without its faults. Chemicals have multiple properties and uses, and are able to move across environmental
A more holistic approach to environmental pollution is particularly important during the current transition period. Pollution control techniques have been largely successful in dealing with end-of-pipe emissions. Through regulatory command and control of major pollution sources there has been a steady diminution of measured emissions to air and water in developed countries, and an improvement in air and water quality. Yet major problems remain, and in some instances they are getting worse. Two interrelated categories of particular concern are global climate change and pollutants from nonpoint sources.
Our planet maintains itself through a series of feedback loops involving interconnected biological, geological, and physical processes. The science that has enhanced our understanding of these processes has also demonstrated their vulnerability to the increasing dominance of human activities, including the effect of pollutants. One example is the diminution of the stratospheric ozone layer that protects humans against the harmful effects of short-range ultraviolet light. A major source of this diminution is chlorofluorocarbons (CFCs). These compounds were seemingly ideal for refrigeration and a variety of other industrial purposes, in part because they are inert and cause little or no direct biological effects. But this lack of reactivity allows CFCs to persist and rise into the stratosphere where they enter into a reaction that decomposes ozone. An international treaty, the Montreal Protocol, has led to a decrease in this particular threat to the ozone layer. The feedback loops involved in global climate change, including the greenhouse effect which is now warming the earth, are far more complex and less well understood. Further, competitive economic and nationalistic interests have made it more difficult to deal with carbon dioxide and nitrogenous greenhouse gases.
Nonpoint source emissions refer to pollution for which there is no readily obvious target, or source. An example is damage to the Chesapeake Bay due to runoff of nitrogenous fertilizer compounds from farms along the Susquehanna River, including a heavy contribution from farms using natural fertilizing techniques. Agricultural practices and energy and transportation decisions contribute heavily to regional air and water pollution and to global warming.
UNDERSTANDING POLLUTION EFFECTS
A transition is also occurring in our understanding of the health effects of pollutants. It is now recognized that there are subtle health effects of environmental pollutants, such as endocrine disruption and neurobehavioral changes, for which newer toxicological paradigms are being developed. The unraveling of the human genome may provide a better understanding of the role of genetic susceptibility factors in response to pollution.
Understanding the effects of pollutants requires understanding how pollutants change following their release from a source, and how they can have effects many miles from their sources. For example, there are no significant direct emitters of air pollutant ozone. Rather, this major component of oxidant smog is formed in the air through the action of sunlight on a mixture of nitrogen oxides and hydrocarbons coming from many different sources, primarily automobiles. The precursors may have been emitted hundreds of miles upwind of where the ozone is eventually formed. For the northeastern United States, this means that statewide control strategies, which are the major enforcement focus of the U.S. Clean Air Act, are an inadequate approach to a regional issue. Similarly, acid rain and other forms of particulate air pollution can be derived from atmospheric reactions of gaseous sulfur dioxide and nitrogen oxides precursors occurring many hundreds of miles downwind. Agents released into water can also undergo significant changes. For example, methyl mercury, which is far more toxic than elemental mercury, is formed in water through the action of bacteria and makes its way into the
Understanding the effects of pollutants on human health requires not only an understanding of the intrinsic hazard of the chemical or physical agent, but also the extent of human exposure. Exposure is often determined by local pathways within a community, such as whether drinking water comes from wells or from surface sources or whether individuals consume vegetables grown in their backyards or brought to market from far away. Individual activities can also alter pollutant intake; exercise, for example, increases respiratory uptake of air pollutants. Health effects due to pollutants are heavily dependent upon susceptibility factors, including age, gender, and genetic predisposition.
A variety of approaches have been developed to manage existing pollution. These include punishment of polluters through regulation, taxation, fines, toxic tort suits, and other disincentives; encouragement of nonpolluting approaches through tax and other incentives; and education of the public. The increased awareness of the potential harmful effects of pollution has had a major impact on industries and on individuals, particularly the young, who have led the way in activities such as recycling. Risk assessment has developed as a useful technique to estimate the risks of environmental pollutants and to establish priorities for environmental control and remediation efforts. These efforts to manage existing pollution are largely a form of secondary prevention in that the pollution already exists and the focus is on lessening the extent or the effects.
Primary prevention of pollution has occurred through approaches that, like any form of primary prevention, are both highly effective and difficult to quantify. The United States National Environmental Policy Act of 1969 was the first major action arising out of the new environmental movement aimed at avoiding unwanted environmental consequences. It contained the requirement that significant newly proposed federal activities have an environmental impact statement prepared in advance, the goal being the incorporation of environmental concerns into all planning processes and the avoidance of those activities that would have an adverse impact. Advances in science have had a significant primary preventive effect, in part through providing assessment tools of use in preventing the development of new harmful products by the chemical industry. As examples, a basic understanding of the role of mutation in cancer and recognition of the structural aspects resulting in the environmental persistence of chemicals have led the chemical industry to detect and quickly drop out of its development programs those new chemicals that are mutagens or are likely to persist in the environment. The Precautionary Principle is basic to public health practice, but is also now being advocated as a form of primary prevention of environmental pollution.
Control of the more challenging insidious pollutant effects related to the health of the planetary biosphere and to nonpoint sources cannot depend solely upon standard command and control regulatory approaches. Central to avoiding significant long-term consequences to health and the environment is the development of innovative pollution prevention and control strategies, including emissions trading, taxation of consumption and international compacts; better targeting of controls through improved scientific understanding of the processes involved; and a more informed public.
BERNARD D. GOLDSTEIN
(SEE ALSO: Acid Rain; Airborne Particles; Ambient Air Quality [Air Pollution]; Ambient Water Quality; Arsenic; Automotive Emissions; Benzene; Carcinogen; Chlorofluorocarbons; Clean Air Act; Clean Water Act; Climate Change and Human Health; Ecosystems; Emissions Trading; Endocrine Disruptors; Environmental Impact Statement; Exposure Assessment; Groundwater; Human Genome Project; Lead; Mercury; National Environmental Policy Act of 1969; Nuclear Power; PCBs; Persistent Organic Pollutants [POPs]; Pesticides; Precautionary Principle; Radiation, Ionizing; Risk Assessment, Risk Management; Sulfur-Containing Air Pollutants [Particulates]; War)
United Nations Environment Programme (1999). Global Environment Outlook 2000—UNEP's Millennium Report on the Environment. London, UK: Earthscan Publications Ltd.
World Health Organization (1992). Report of the WHO Commission on Health and Environment. Geneva: Author.