Experts estimate that environmental factors account for nearly three-quarters of cancers, while only a small percentage can be explained by genetics alone. Tobacco is far and away the most important environmental cause of cancer; occupational and community exposure to radiation and chemicals are another source of risk. Poor diet and lack of exercise may also contribute to cancer.
Among chemical carcinogens, persistent organic pollutants pose an important environmental cancer risk. POPs, which include dioxin, DDT, and PCBs, are synthetic chemicals that resist normal environmental degradation. In addition to their persistence in the environment, they are characterized by high toxicity, a special affinity for fat, and the ability to travel long distances. Some POPs have been linked to breast cancer and to non-Hodgkins lymphoma. Others are known carcinogens.
While the evidence is not always conclusive, many links have been found between environmental factors and some cancers, as described below.
- Brain Cancer: Studies show that excessive exposure to radiation can lead to brain cancer. Childhood exposure to pesticides and use of electric blankets have been associated with brain cancer, although the evidence on these links is conflicting. Workers in oil refineries, rubber, and drug manufacturing facilities, and those who have high exposures to pesticides, are at higher risk for brain cancer.
- Breast Cancer: Breast cancer rates in the Bay Area are among the highest in the world. Rates in white women are almost 50 percent higher than rates in most European countries, according to the Women's Environment and Development Organization. Bay Area African-American women have the fourth highest rate in the world. Although the reasons for these high rates are unclear, experts suspect that environmental factors may play a role. Recent studies have linked breast cancer with exposure to certain pesticides and to chemicals in the environment that mimic estrogen. These estrogen-mimicking chemicals (called endocrine disruptors) are found in certain pesticides, plastics, and the byproducts of combustion, including diesel exhaust and incinerator emissions. Studies also indicate that women who have diets high in fat and who consume high levels of alcohol are at greater risk. Because many endocrine-disrupting chemicals accumulate in animal fat, the link between fat and breast cancer may be related to the fat itself or to chemicals in the fat. One recent study showed a possible link between breast cancer and the dry-cleaning solvent perchloroethylene.
- Leukemia: Exposure to radiation or to benzene and other gasoline-related chemicals is known to cause leukemia. Other factors that have been associated with the disease in some studies include exposure to pesticides and to electromagnetic fields -- radiation emitted by power lines and other electrical sources.
- Lung cancer: Use of tobacco is a primary cause of lung cancer. Other known causes include asbestos, diesel exhaust, and other chemicals that are products of combustion.
- Non-Hodgkin's Lymphoma: People with weaker immune systems due to HIV, the Epstein-Barr virus, or inherited immune deficiency disorder are at higher risk of developing non-Hodgkin's lymphoma. Studies have found that people who work with pesticides or live in areas of pesticide use or drift have an increased risk.
- Prostate cancer: Many studies show small but significant correlations between prostate cancer and jobs involving exposure to pesticides and herbicides.