So, all told, how much do these environmental hazards contribute to the global burden of ill health? The answer is impossible to pin down with any precision because of the complex etiology of most diseases. Asthma is a case in point. Although the disease is on the upswing and environmental factors appear to be involved, their exact contribution is difficult to discern. (See Why the Increase in Asthma?.) Clearly, however, the risk posed by environmental factors varies enormously depending upon where one lives. A major portion of death and disease in developing countries can be directly tied to poor environmental conditions, especially at the household or local level. Indeed, the two major sources of death and disability in the developing world – ARI and diarrhea – have their origins in poverty and degraded household environments. Together, these two diseases account for up to 7 million deaths per year . The Global Burden of Disease study calculates that they contribute more than 21 percent to the burden of disease in developing countries. Similarly, malaria accounts for 2.6 percent, and the tropical disease cluster, which includes trypanosomiasis, Chagas disease, schistosomiasis, leishmaniasis, filariasis, and onchocerciasis, accounts for 0.87 percent . Improving the household environment – providing improved housing, clean fuels, improved access to water and sanitation, and better waste removal – would do much to reduce the burden of these diseases. But because these diseases are influenced by multiple factors, including nutrition, hygiene, and education, it would be simplistic to think environmental interventions could eliminate them entirely. (See, Improving Health Through Environmental Action.)
The environmental contribution to health risks is less clear in the developed countries, where many of the most obvious and immediate threats, such as fecal contamination or lack of water, have been virtually eliminated, and the burden of infectious diseases is relatively small. (AIDS and TB are important exceptions.) Environmental factors clearly play a role in chronic conditions such as cancer and heart disease, which predominate in the wealthier countries. The question is: how large a role?
The debate over cancer illuminates some of these difficulties. Cancers of all types claimed some 6 million lives in 1996 including 3.8 million in developing countries . Moreover, cancer’s links to the environment are unequivocal, as increasing evidence about the process of carcinogenesis has made clear. Cancer arises through a multistep process of accumulated damage to genes in a single cell. Several distinct mutations (genetic alterations) are required – some to start the process, some to release the normal controls on cell growth. In a small number of cases, perhaps 5 percent of breast cancers, for instance, one or more mutations may be inherited. But additional mutations are still required before cancer ensues, and many of these can be triggered by external agents. The list of known carcinogens is long and includes radiation, natural or synthetic chemicals, tobacco smoke, some viruses, agents in the diet, and sunlight. Whether any agent contributes to cancer depends not just on the timing of the exposure but also on the effectiveness of the body’s defenses.
Based on this same understanding, estimates of the environmental contribution to cancer vary widely. In their classic 1981 study, epidemiologists Richard Doll and Richard Peto estimated that 1 to 5 percent of cancers derive from environmental pollutants in air, water, and food. They estimated that behavioral factors such as tobacco use and unhealthy diets play a much larger role, accounting for 30 and 35 percent, respectively . (See Smoking-Related Deaths.) Similarly, a 1996 study estimated that just 2 percent of all cancers arise from exposure to air pollution . In contrast, WHO recently estimated the contribution of environment – including the workplace – to cancer at 25 percent .
Investigators are experimenting with approaches to better assess the environment’s contribution to human death and disease. Some estimates are reported here, with the caveat that they are all only crude approximations. Even so, these estimates can provide useful guides in designing policies to improve public health .
One approach is to estimate the attributable risk of certain factors in disease. The 1996 Global Burden of Disease study tried to tease apart the contribution of 10 different risk factors, including some key environmental hazards, by determining what proportion of the disease burden would not have occurred in the absence of specified exposures. In the interest of scientific caution, the study authors have interpreted their analysis conservatively, so these estimates are more likely to be underestimates than overestimates .
145. Christopher J. L. Murray and Alan D. Lopez, eds., The Global Burden of Disease: Volume 1 (World Health Organization, Harvard School of Public Health, and The World Bank, Geneva, 1996), p. 464.
146. Christopher J. L. Murray and Alan D. Lopez, eds., The Global Burden of Disease: Volume 1 (World Health Organization, Harvard School of Public Health, and The World Bank, Geneva, 1996), pp. 549-572.
147. Dr. Annie J. Sasco, Head, Programme of Epidemiology for Cancer Prevention, International Agency for Research on Cancer, September 1997 (personal communication).
148. Richard Doll and Richard Peto, The Causes of Cancers: Quantitative Estimates of Avoidable Risks of Cancer in the United States Today (Oxford University Press, Oxford, U.K., 1981), p. 1256.
149. World Health Organization (WHO), Health and Environment in Sustainable Development: Five Years After the Earth Summit (WHO, Geneva, 1997), p. 161.
50. World Health Organization (WHO), Health and Environment in Sustainable Development: Five Years After the Earth Summit (WHO, Geneva, 1997), p. 173.
151. Christopher J. L. Murray and Alan D. Lopez, eds., The Global Burden of Disease: Volume 1 (World Health Organization, Harvard School of Public Health, and The World Bank, Geneva, 1996), p. 321.
152.Christopher J. L. Murray and Alan D. Lopez, eds., The Global Burden of Disease: Volume 1 (World Health Organization, Harvard School of Public Health, and The World Bank, Geneva, 1996), p. 321.