UPdate Fall 2002

We often read that a chemical, group of chemicals, or industrial process is a "known", "probable" or "possible" human carcinogen. These terms have very
specific meanings. The International Agency for Research on Cancer (IARC)
categorizes substances which have been studied for their possible contribution to cancers into five categories. The categories are based on whether the research was done on humans or animals, how much research has been done, and how strong the evidence is.

Known human carcinogens:This ranking is used only if there is "sufficient" evidence from epidemiological studies to support a cause and effect association between an exposure and cancer. Epidemiological studies are those which look at large groups of people and compare those who have been exposed to a potential carcinogen with those who have not.

Probable human carcinogens are substances in which there is some evidence of carcinogenicity from studies of humans, and additional evidence from studies in animals.

Possible human carcinogens are ones where there is "sufficient" evidence from animal studies, but limited data from human studies.

Substances which "cannot be classified" as to their carcinogenicity to humans are ones for which there is some suspicion as to their cancer causing potential. However, despite an investigation, a definitive conclusion cannot be made.  This term can sometimes be misused by people who use it to imply that a substance is not carcinogenic. In fact, it means that the evidence is not in, but suspicion remains.

The IARC classifies a substance not carcinogenic when, on the basis of human and animal studies, the substance is not suspected of being a carcinogen.