In pathology, a carcinogen is any substance or agent that promotes cancer. Carcinogens are also often, but not necessarily, mutagens or teratogens.
Carcinogens may cause cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with normal biological processes. Usually cells 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. Cooking protein-rich food at high temperatures, such as broiling or barbecuing meats, can lead to the formation of many potent carcinogens that are comparable to those found in cigarrette smoke (i.e., benzo[a]pyrene). Pre-cooking meats in a microwave oven for 2-3 minutes before broiling can help minimize the formation of these carcinogens.
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.
IARC classification of carcinogens
- Group 1: the agent (mixture) is carcinogenic to humans. The exposure circumstance entails exposures that are carcinogenic to humans.
- Group 2A: the agent (mixture) is probably carcinogenic to humans. The exposure circumstance entails exposures that are probably carcinogenic to humans.
- Group 2B: the agent (mixture) is possibly carcinogenic to humans. The exposure circumstance entails exposures that are possibly carcinogenic to humans.
- Group 3: the agent (mixture or exposure circumstance) is not classifiable as to its carcinogenicity to humans.
- Group 4: the agent (mixture) is probably not carcinogenic to humans.
Further details can be found in the IARC Monographs.
EPA classification of carcinogens
- Group A: Human Carcinogen
- Group B: Probable Human Carcinogen
- Group C: Possible Human Carcinogen
- Group D: Not classifiable as to Human Carcinogenicity
- Group E: Evidence of Non-Carcinogenicity for Humans