Pesticides also pose an environmental hazard. They pollute the rain water of many U.S. states when they are vaporized or when the wind blows soil particles treated with pesticides. (12)

If you were exposed to toxic amounts of pesticide, that is, if a large dose were inhaled or made contact with your skin, you would experience acute effects. These effects usually appear within minutes to hours after contact. However, the effects of low-level or prolonged pesticide exposure, particularly to those that may have carcinogenic potential, are very different. Cancer does not appear immediately after exposure to a pesticide; it may not be apparent until long after exposure has occurred. Unfortunately, by the time the medical and scientific community becomes aware that a particular pesticide causes cancer, a large number of persons could have been exposed without their knowledge. For example, R-11, which is a chemical found in insect repellents, has just been shown to cause cancer in animals.

Dioxin, otherwise known as Agent Orange, and one of its associated contaminants, TCDD, was extensively used toward the end of the United States' conflict in Vietnam. (13) Hundreds of thousands of people were exposed to these agents, and serious allegations by Vietnam veterans and other persons have been raised that Agent Orange and TCDD (14) caused malignant tumors, sterility, spontaneous abortions, birth defects, disfiguring skin diseases, and other illnesses.

We do know that TCDD is very toxic and causes tumors in rats, in which it acts as a promoter of cancer. (15) It can also initiate carcinogenesis in animals. (l6,17) A number of human epidemiological and toxicological studies have suggested an association between TCDD or the chemicals it contaminates, and soft-tissue sarcoma, (18-20) Hodgkin's, (21) non-Hodgkin's lymphoma, (22-24) stomach cancer. (25,26) nasal cancer, (27) and liver cancer. (28,29) However, other studies did not show a correlation between TCDD and other cancers. (30-35) Most of these studies involved a short period of time between exposure and disease. It now appears that the longer the time from exposure to TCDD, the higher the risk for the development of cancer and the higher the incidence of cancer. (36,37)

Table 11.2 lists several different pesticides and their roles as human carcinogens. (38-41) Some are definitely associated with human cancer, some are probably associated with human cancer, and some are possibly associated with human cancer. You will recognize a number of them. Many other pesticides not included in this table, like malathion and Mirex, have been linked to cancers in animal studies. Some of them are also familiar to you because they are commonly used in household and garden settings. Currently there is no definite evidence that the pesticides causing animal cancer also cause human cancer. However, we should continue to be on the alert and await future studies for their possible link to human cancer.