A carcinogen is any substance, natural or synthetic, or organism, that causes or promotes the formation of cancer. Several hundred substances are known or suspected to be carcinogenic.
Carcinogens are classified into four categories: known, probable, possible and unknown. A known carcinogen is a substance for which there is overwhelming evidence that it can cause cancer. Examples include aflatoxin, asbestos, benzene, cadmium, dioxins, formaldehyde, some viruses, radon, tobacco, and vinyl chloride.
A probable carcinogen is a substance for which there is still a high possibility of causing cancer but for which the evidence is less strong. Examples include creosotes, malathion, occupational exposure to petroleum refining, and red meat. Most probable carcinogens are chemicals that are used in industrial processes and with which the general public is not familiar, such as ethylene dibromide, mononitrotoluene, and vinyl fluoride.
Several viruses are known to be carcinogenic, and a few bacteria are suspected of being carcinogenic. Fungi themselves may not be carcinogenic, but some species produce aflatoxin, which is a known carcinogen. Some activities might also be carcinogenic: for example, night shift work has been classified as a probable carcinogen.
The variety and volume of synthetic carcinogens produced has increased rapidly as an integral part of industrialization. This has been accompanied by expanding opportunities for exposure to humans and other organisms, including through contamination of the air, water and soil, through proximity in industrial production facilities, and through contact with consumer and other products which incorporate such chemicals.