Cancer is a disease in which abnormal cells in the body grow out of control. There are more than 100 different types of cancer. Cancers are named after the part of the body where the abnormal cell growth begins. Exposures to natural and man-made substances in the environment (environmental factors, including tobacco smoke) are estimated to account for at least two-thirds of all cases of cancer in the United States. These environmental factors can include lifestyle choices such as: smoking, alcohol consumption, diet, exercise, and exposure to certain medical drugs and hormones; environmental exposure to radiation, viruses, or bacteria; and chemicals that may be present in the air, water, food, and the workplace. Chemicals are classified according to research studies that evaluate whether they are carcinogens (cancer-causing substances). These studies are evaluated by the International Agency for Research on Cancer (IARC) and the US National Toxicology Program (NTP), both of which publish lists of chemicals and their level of carcinogenicity (known, probable, etc).
Cancer is a disease with many risk factors. These factors can affect the illness in ways that are not fully understood. Most cancers take a long time to develop. Studies have documented that it may take as long as 40 years for some cancers to develop after exposure to a cancer-causing substance, depending on the type of cancer.
Genetic factors also appear to play a role in some types of cancer. However, the cause or origin of many cancer types is unknown and may be determined by the combined effects of multiple factors.
Although environmental pollution has been a source of great public concern for decades, few community-level environmental exposures have been well-studied. The cancer risks associated with many environmental chemicals have been identified through studies of workers who were exposed to chemicals at much higher levels than the general public typically is exposed to in an environmental setting (like in drinking water).
Occupational substances categorized by the International Agency for Research on Cancer (IARC) as known lung carcinogens include: arsenic, asbestos, bischloromethlyl ether, chromium, nickel, polycyclic aromatic compounds, radon and vinyl chloride.
The evidence is building to support a link between cancer and exposures to high levels of environmental pollutants. Some environmental exposures are potentially avoidable, such as smoking tobacco, using alcohol or eating an unhealthy diet. Other risk factors, such as age, race, or genetics, are beyond our control. Having a risk factor does not mean that cancer will develop and many people who get the disease may not have had any known risk factors.