Carcinogens are agents that can cause cancer. Carcinogenic agents include various chemicals, radiation, and viruses. Although exposure to carcinogens can initiate cancer, the conditions that promote the disease process are not fully understood. Chemical carcinogens can be found in tobacco smoke, effluents from industry, environmental pollutants from automobiles, and fumes from factories and refineries. Physical carcinogens include ionizing radiation from X-rays and radioactive materials, as well as ultraviolet rays from sunlight. Some viruses are thought to cause cancer in animals, including humans, and are frequently referred to as oncogenic viruses. It is also possible that recurring irritation to a part of the body might be carcinogenic. Some cancers appear to be heritable in the sense that a predisposition exists. However, in such cases other carcinogenic influences appear to be necessary to trigger the development of cancer.

Many chemical substances that are known mutagens (induce mutations in genes) are...