Carcinogens cause cancer by altering DNA in cells, interfering with normal biological processes. Usually cells are able to detect this and attempt to repair the DNA; if they cannot, they undergo Programmed cell death to protect the organism. When the damage interferes with cell death or encourages Cell division, cancer may occur. Rapidly dividing cells, such as in Skin, the Stomach and intestinal lining, Breast tissue, and reproductive organs, are particularly sensitive to carcinogens due to any harmful DNA being quickly copied.
Most carcinogens consumed by humans are produced by plants to prevent animals from eating them (as are alkaloids). Plants containing large amounts of carcinogens include Aristolochia and Bracken. Aflatoxin B1, which is produced by the Fungus Aspergillus flavus growing on stored grains, nuts and Peanut butter, is an example of a potent, naturally-occurring microbial carcinogen.
DDT, Benzene, Kepone, EDB, Asbestos, and the waste rock of oil-shale mining have all been classified as carcinogenic. As far back as the 1930s, industrial and Tobacco smoke were identified as sources of dozens of carcinogens, including Benzopyrene, tobacco-specific nitrosamines such as nitrosonornicotine (NNN), and reactive aldehydes such as Formaldehyde - which is also a hazard in Embalming and making plastics. Vinyl chloride from PVC is a carcinogen. Certain viruses such as Hepatitis B and human papilloma viruses have been found to cause cancer in humans. The first one shown to cause cancer in animals was chicken sarcoma virus, discovered in 1910 by Peyton Roux.
CERCLA identifies all radionuclides as carcinogens, although the nature of the emitted Radiation (alpha, beta, or gamma, and the energy), its consequent capacity to cause ionization in tissues, and the magnitude of radiation exposure, determine the potential hazard. For example, Thorotrast, an (incidentally-radioactive) suspension previously used as a Contrast medium in X-ray diagnostics, is thought by some to be the most potent human carcinogen known because of its retention within various organs and persistent emission of alpha particles. Both Wilhelm Röntgen and Marie Curie died of cancer caused by radiation exposure during their experiments. The non-reproducing cells of the (non-gametogenic) tissues of adult insects are particularly resistant.
Recent reports have implicated Acrylamide in fried or overheated Carbohydrate foods (such as French fries and Potato chips) as a possible carcinogen. Studies are underway at the FDA and European regulatory agencies to assess its potential risk. The charred residue on barbecued meats has been identified as a carcinogen, along with many other tars.
Co-carcinogens are chemicals which do not separately cause cancer, but do so in specific combinations.