A carcinogen is any substance, radiation, or radionuclide that is an agent directly involved in causing cancer. This is because to the ability to damage the genome or to the disruption of cellular metabolic processes. Various radioactive substances are considered carcinogens, but their carcinogenic activity is attributed to the radiation, for instance gamma rays and alpha particles, which they emit. Common examples of non-radioactive carcinogens are inhaled asbestos, tobacco smoke and certain dioxins. Although the public generally associates carcinogenicity with synthetic chemicals, it is equally likely to arise in both natural and synthetic substances.
Cancer is any disease in which normal cells are injured and do not undergo programmed cell death as fast they divide via mitosis. Carcinogens may enhance the risk of cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with biological processes, and induces the uncontrolled, malignant division, ultimately leading to the formation of tumors. generally, severe DNA damage leads to apoptosis, but if the programmed cell death pathway is damaged, then the cell cannot prevent itself from becoming a cancer cell.
There are many natural carcinogens. Aflatoxin B1, which is formed by the fungus Aspergillus flavus growing on stored grains, nuts and peanut butter, is an example of a potent, naturally occurring microbial carcinogen. Certain viruses such as Hepatitis B and human papilloma virus have been found to cause cancer in humans. The first one shown to cause cancer in animals is Rous sarcoma virus, discovered in 1910 by Peyton Rous.
Dioxins and dioxin-like compounds,kepone,  benzene, EDB, and asbestos have all been classified as carcinogenic. As far back as the 1930s, tobacco smoke and industrial smoke were identified as sources of dozens of carcinogens, including benzo[a]pyrene, tobacco-specific nitrosamines such as nitrosonornicotine, and reactive aldehydes such as formaldehyde—which is also a hazard in embalming and making plastics. Vinyl chloride, from which PVC is manufactured, is a carcinogen and therefore a hazard in PVC production.
Co-carcinogens are chemicals that do not necessarily cause cancer on their own, but promote the activity of other carcinogens in causing cancer.
After the carcinogen enters the body, the body makes an attempt to eliminate it through a process known as biotransformation. The purpose of these reactions is to make the carcinogen more water-soluble so that it can be removed from the body. But these reactions can also convert a less toxic carcinogen into a more toxic carcinogen.
DNA is nucleophilic, thus soluble carbon electrophiles are carcinogenic, because DNA attacks them. For example, some alkenes are toxicated by human enzymes to produce an electrophilic epoxide. DNA attacks the epoxide, and is bound permanently to it. This is the mechanism behind the carcinogenicity of benzo[a]pyrene in tobacco smoke, other aromatics, aflatoxin and mustard gas.