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Carcinogens are substances or exposures that have the potential to cause cancer in living tissue. They may damage DNA, alter cellular metabolism, or promote uncontrolled cell growth. The cancer risk from a carcinogen depends on dose, duration, route of exposure, and individual susceptibility.

Carcinogens are categorized as chemical, physical, or biological. Chemical carcinogens include polycyclic aromatic hydrocarbons in smoke, aflatoxin B1 in contaminated foods, benzene, and certain pesticides. Physical carcinogens include ionizing radiation from X-rays and radioactive materials, and ultraviolet radiation from the sun. Biological carcinogens include certain infectious agents such as high-risk human papillomavirus types and hepatitis B and C viruses.

Mechanisms include direct DNA damage (mutations, deletions, translocations), epigenetic changes, genomic instability, and promotion of cell proliferation or inhibition of apoptosis. Some agents are initiators that damage DNA; others are promoters that encourage growth of damaged cells. Risk can depend on co-exposures and immune status.

Assessment and regulation: International agencies, such as the International Agency for Research on Cancer (IARC), classify agents into groups reflecting the strength of evidence for carcinogenicity in humans and animals. Classifications guide public health policies, workplace safety standards, labeling, and exposure limits. Not all classified carcinogens pose the same level of risk in every context.

Examples and prevention: Tobacco smoke and other tobacco exposure are leading preventable carcinogens. Ultraviolet radiation from sun or tanning beds, asbestos, certain industrial solvents, processed meats, and alcohol have cancer-linked associations. Prevention focuses on reducing exposure, protective measures, vaccination against oncogenic viruses (HBV, HPV), and cancer screening where appropriate.