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 contamination of water and food by human excreta and with the control of smoke from fires used for cooking and heating. Sanitary engineering to manage human wastes remains a central public health need. Indoor air pollution due to the use of wood and fossil fuels in poorly ventilated residences also remains a major source of exposure to pollutants and a cause of respiratory disease in much of the world.
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 there is a growing movement against them. The most effective means of decreasing energy use is by lessening demand.
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.
BERNARD D. GOLDSTEIN, The Gale Group Inc., Macmillan Reference USA, New York,