Carcinogen

A carcinogen is an agent capable of causing cancer. Carcinogens include chemicals, such as those in tobacco smoke, asbestos fibers, benzene and the mold toxin aflatoxin; physical agents, such as ultraviolet light and ionizing radiation; and infectious agents, such as human papillomavirus, the hepatitis B and C viruses and the bacterium Helicobacter pylori. Some are linked to everyday habits or work, including alcoholic drinks and processed meat.

Many carcinogens act by damaging DNA, either directly or after the body's own enzymes convert them into reactive forms, and the resulting mutations disturb genes that control cell growth; these are called genotoxic carcinogens. Others, called nongenotoxic, promote cancer indirectly, for example through chronic inflammation, hormone-like effects or by pushing cells to divide. Exposure does not mean a person will develop cancer: the risk depends on the dose, how long the exposure lasts and the person's genetic makeup, and cancers usually appear years or decades later.

Expert agencies weigh the evidence for each agent. The World Health Organization's cancer agency, IARC, sorts agents into groups from carcinogenic to humans (Group 1) through probably (2A) and possibly (2B) carcinogenic to not classifiable (3), and the US National Toxicology Program publishes its own Report on Carcinogens. These groups reflect how strong the evidence is, not how potent the agent is.