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There is no question that there are harmful substances, including analine dyes, asbestos, coal tars, and other chemicals, that apparently are able to affect the genetic information in cells and thus produce cancer. Research using laboratory animals has demonstrated that when they are exposed to large amounts of these harmful substances over a period of time, the substances have become known as "carcinogens," or cancer-producing agents.

One fact offered to support the argument that these substances are the cause of cancer is that the incidence of cancer has been found to climb markedly with increased levels of industrialization. Cancer is very prevalent in the United States, Western Europe, and other industrialized nations. Since a frequent by-product of industrialization is environmental pollution, which exposes people to an increasing amount of these carcinogens, it is argued that the increase in cancer is the result of the environmental pollution that accompanies industrialization. Indeed, the cancer incidence rates for the Soviet Union, which is not yet as industrialized as the United States, are virtually identical to those in the United States twenty years ago, and so it is suggested this time lag corresponds to the lag in industrialization.

Other researchers argue, on the other hand, that industrialized countries also have better medical care. Thus, people in less-developed countries die of other diseases or illnesses that are cured or prevented in industrialized countries, and so do not live long enough to contract cancer. While the fact that people live longer with better medical care may account for some of the increase in cancer deaths in industrialized societies, it doesn't satisfactorily explain the entire phenomenon.

If there were a direct, simple cause-and-effect relationship between harmful substances, chemicals, chronic irritants, and cancer, then increased exposure to these substances should cause an increase in cancer. On a broad statistical basis, there is an increased incidence of cancer with exposure to these substances, yet the vast majority of people exposed still do not get the disease and people who are apparently not exposed to exceptionally high levels of harmful substances still do get it.

In other words, exposure to carcinogens alone is not sufficient to cause cancer, nor does reduced exposure automatically eliminate cancer. On a person-by-person basis, additional explanations are needed.