Air and Water Pollutants Kenneth P. Cantor, Ph.D.*	Exposure to environmental pollutants is nearly unavoidable, but while the number of polluting substances in the general environment is large, their concentrations are generally low compared to exposures encountered in certain workplaces. The effects of exposure to general pollutants in air and water are not clear, although some estimates cite pollution for causing from 1 to 5 percent of all cancer deaths (Doll and Peto, 1981). Air Pollution Pollutants in the urban air have long been suspect as causes of lung cancer (Blot and Fraumeni, 1992), but evaluation is difficult because of the problems of defining and measuring air pollution and of measuring effects of low-level exposures in large populations (National Research Council, 1979). Of special concern are products from the combustion of fossil fuels found principally in motor vehicle exhausts (especially diesel engines), residential and commercial space heating, oil and coal-fired power plants, and industrial emissions. Several studies have found a statistical link between measured levels of specific combustion products in urban air and lung cancer rates, but a major confounding factor appears to be differences in urban and rural cigarette smoking, with some effect from occupational hazards. Another approach has been to extrapolate from studies of workers heavily exposed to air pollutants found at lower levels in the environment. Using this technique, researchers have suggested that the causative effect of urban air pollution on lung cancer is about one-tenth the effect of smoking on men with average smoking habits (Doll, 1978). Some research suggests that the carcinogenic effects of smoking on the lung are enhanced in urban areas, indicating that pollutants in the urban atmosphere may increase the carcinogenic potency of tobacco smoke (Friberg and Cederlof, 1978). Recent studies in areas of China with unusually high lung cancer rates provide some of the strongest evidence that specific air pollutants can increase lung cancer risk. In one investigation, women and men living in chimneyless houses heated by a local soft, smoking coal had especially high risk (Mumford et al., 1987). Indoor coal-burning devices that release smoke and fumes into the indoor environment contribute to high lung cancer rates in urban Shenyang and Harbin (Xu et al., 1989; Wu-Williams et al., 1990). In Shenyang, levels of some specific pollutants in indoor and outdoor environments exceeded recommended standards for U.S. cities by more than 60-fold. Products of combustion are not the only carcinogenic air pollutants. Lung cancer occurs excessively in neighborhoods adjacent to arsenic-emitting smelters in the United States, Sweden, and China (Brown et al., 1984; Pershagen, 1986; Xu et al., 1989). Asbestos bodies are present in the lungs of large segments of the urban population, although it is not directly established that lung cancer occurs excessively following low-dose, non-occupational exposures to asbestos (Churg and Warnock, 1979). Nevertheless, the well-known link between occupational exposure to asbestos and lung cancer has led to concern about airborne exposures to asbestos in consumer products, schools, office buildings, and other public places (Mossman et al., 1990). Exposure to radioactive emissions from radon in uranium mines has been shown to be responsible for the increased risk of lung cancer among miners. Recent concern has focused on radon exposures in homes, particularly those that have been made air-tight by efficient insulation. Radon can enter the home from soil, water, or building materials. Scientists at the National Cancer Institute (Lubin and Boice, 1989) and the Environmental Protection Agency (EPA) (U.S. Environmental Protection Agency, 1992) have estimated that about 13,000 lung cancers a year may result from exposure to radon in the home environment. The Environmental Protection Agency has suggested that homes with indoor radon levels above 4 picocuries per liter be subject to modifications, although others recommend consideration of remediation only at levels of 5 to 10 picocuries per liter or higher. Lowering indoor radon in this way would protect the persons at greatest risk. However, the total number of lung cancers that result from this exposure would likely be decreased only by a few percent, because such a large part of the U.S. population is exposed to lower levels of indoor radon (Lubin and Boice, 1989). Cigarette smoking is a well-established cause of a number of cancers. Numerous studies have also shown a small, but measurable, increase in the risk of lung cancer among non-smoking spouses of smokers, presumably the result of their passive exposure to tobacco smoke in the home environment (National Research Council, 1986; International Agency for Research on Cancer, 1986; Environmental Protection Agency, 1992).
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* From the Environmental Epidemiology Branch, Division of Cancer Etiology, National Cancer Institute, Bethesda, Maryland