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Environmental factors may be just as important as genetic factors for cancer. For example, Japanese men and women who leave Japan and settle in Hawaii or the continental United States have a lower risk of stomach cancer than those who remain in Japan. Stomach cancer in the United States has been steadily decreasing with the advent of refrigeration and the consequent removal of carcinogenic chemicals as food preservatives.

Air pollution may be a risk factor for cancer, especially lung cancer. Those living in cities encounter many sources of pollution. More people in cities smoke cigarettes than in rural areas. Carcinogens derived from car emissions, industrial activity, burning of solid wastes and fuels remain in the air from four to forty days and thereby travel long distances. And asbestos, a potent carcinogen, can also be found airborne in cities.

Our drinking water contains a number of carcinogens, including asbestos, arsenic, metals, and synthetic organic compounds. Asbestos and nitrates are associated with gastrointestinal cancers; arsenic is associated with skin cancer; and synthetic organic chemicals are associated with cancers of the gastrointestinal tract and urinary bladder.

As with many carcinogens, the time between exposure to the carcinogen and actual development of cancer may be quite long. Hence, the cause of a cancer initiated by trace amounts of either airborne or waterborne carcinogens years before the cancer appears may be attributed to an unrelated or unknown cause at the time of diagnosis. We are able to detect many carcinogens in our environment, but many others exist in low concentrations. These environmental carcinogens may themselves cause cancer in certain individuals, or they may interact with other risk factors to initiate, or promote, cancers. Therefore, our imperfect environment, a risk factor for cancer, must be modified. If we avoid introducing harmful substances into the environment, it will remain clean.