Evaluation groups - Definitions
Group 1: The agent is carcinogenic to humans.
This category is used when there is sufficient evidence of carcinogenicity in humans. Exceptionally, an
agent may be placed in this category when evidence of carcinogenicity in humans is less than sufficient
but there is sufficient evidence of carcinogenicity in experimental animals and strong evidence in exposed
humans that the agent acts through a relevant mechanism of carcinogenicity.
Group 2.
This category includes agents for which, at one extreme, the degree of evidence of carcinogenicity in
humans is almost sufficient, as well as those for which, at the other extreme, there are no human data but
for which there is evidence of carcinogenicity in experimental animals. Agents are assigned to either
Group 2A (probably carcinogenic to humans) or Group 2B (possibly carcinogenic to humans) on the basis
of epidemiological and experimental evidence of carcinogenicity and mechanistic and other relevant data.
The terms probably carcinogenic and possibly carcinogenic have no quantitative significance and are
used simply as descriptors of different levels of evidence of human carcinogenicity, with probably
carcinogenic signifying a higher level of evidence than possibly carcinogenic.