Carcinogen: A carcinogen is any substance or agent that can lead to cancer. Associated terms include the adjective "carcinogenic," and the nouns "carcinogenesis" and "carcinogenicity." Ionizing radiation is one of the most widely recognized carcinogens. The International Agency for Research on Cancer has identified around 60 substances and processes as either likely or confirmed to be carcinogenic in humans. These have been classified into three categories: 1. **Agents and groups of agents**: This includes naturally occurring aflatoxins, 4-aminobiphenyl, arsenic compounds, asbestos, azathioprine, benzene, benzidine, beryllium and its compounds, N,N-Bis(2-chloroethyl)-2-naphthylamine (chlornaphazine), bis(chloromethyl) ether and chloromethyl methyl ether, 1,4-dimethanesulfonate (busulfan; Myleran), cadmium and its compounds, chlorambucil, 1-(2-chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (methyl-CCNU; Semustine), [VI] compounds, cyclosporine, cyclophosphamide, diethylstilbestrol, erionite, ethylene oxide, melphalan, methoxypsoralen (methoxsalen) with ultraviolet radiation, MOPP and other combined treatments involving alkylating agents, mustard gas (sulfur mustard), 2-naphthylamine, compounds related to therapy, radon and its decay products, crystalline silica, solar radiation, talc with asbestiform fibers, 2,3,7,8-tetrachlorodibenzo-p-dioxin, thiotepa, treosulfan, and vinyl chloride. 2. **Mixtures**: Examples include alcoholic beverages, mixtures with phenacetin, betel quid combined with tobacco, coal-tar pitches, coal tars, untreated and mildly treated mineral oils, Chinese-style salted fish, shale oils, soots, smokeless tobacco products, tobacco smoke, and wood dust. 3. **Exposure circumstances**: This category covers aluminum production, auramine manufacturing, boot and shoe manufacturing and repair, coal gasification, furniture and cabinet making, underground hematite mining with radon exposure, iron and steel founding, isopropanol production using strong-acid processes, magenta manufacturing, occupational exposure to painting, the rubber industry, and exposure to strong-inorganic-acid mists containing sulfuric acid.