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. 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. Environmental factors clearly play a role in chronic conditions such as cancer and heart disease, which predominate in the wealthier countries. The debate over cancer illuminates some of these difficulties. 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.
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.