What Is a Carcinogen?

The National Cancer Institute says any substance that causes cancer is known as a carcinogen, but that a substance being designated a carcinogen does not mean it will necessarily cause cancer, because many factors influence whether an exposed person develops cancer, including the amount and duration of the exposure and the person's genetic background.

Several agencies classify them. The National Toxicology Program says its congressionally mandated Report on Carcinogens includes 256 listings of substances known or reasonably anticipated to cause cancer in humans, and that the 15th report, released December 21, 2021, is the latest. It lists a substance as known to be a human carcinogen when studies in humans show a causal relationship with human cancer, and as reasonably anticipated to be one when human evidence is limited and chance, bias or confounding cannot be ruled out, or when there is sufficient evidence of cancer in experimental animals.

The EPA's Guidelines for Carcinogen Risk Assessment use five standard hazard descriptors, carcinogenic to humans, likely to be carcinogenic to humans, suggestive evidence of carcinogenic potential, inadequate information to assess carcinogenic potential, and not likely to be carcinogenic to humans.

NCI says the International Agency for Research on Cancer has evaluated more than 1,000 agents since 1971 and identified more than 500 as carcinogenic, probably carcinogenic or possibly carcinogenic to humans. ATSDR explains that IARC Group 1, carcinogenic to humans, rests on sufficient evidence in humans, or on strong evidence in exposed humans that an agent shows key characteristics of carcinogens together with sufficient evidence in experimental animals. Group 2A, probably carcinogenic, needs at least two of limited human evidence, sufficient animal evidence or strong evidence of key characteristics, and Group 2B, possibly carcinogenic, applies when only one of those has been found.