A carcinogen is any substance,  radiation or radionuclidethat is an agent directly involved in causing cancer. This may be due to the ability to break 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 example alpha particlesand gamma rays, which they emit. Common examples of non-radioactive carcinogens are inhaled asbestos, certain dioxins, and tobacco smoke. Though the public generally associates carcinogenicity with synthetic chemicals, it is equally likely to arise in both synthetic substances and natural.

Cancer is any disease in which normal cells are damaged and do not undergo programmed cell death as fast they divide by mitosis. Carcinogens may increase the risk of cancer by changing 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 a lot of natural carcinogens. Aflatoxin B1, which is produced by the fungus Aspergillus flavus growing on stored nuts, grains 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, industrial smoke and tobacco 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 thus 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.