http://www.iarc.fr/en/media-centre/pr/2011/pdfs/pr208_E.pdf
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.
Group 2A: The
agent is probably carcinogenic to humans.
This category is
used when there is limited evidence of carcinogenicity in humans and sufficient
evidence of carcinogenicity in experimental animals. In some cases, an agent
may be classified in this category when there is inadequate evidence of
carcinogenicity in humans and sufficient evidence of carcinogenicity in
experimental animals and strong evidence that the carcinogenesis is mediated by
a mechanism that also operates in humans. Exceptionally, an agent may be
classified in this category solely on the basis of limited evidence of
carcinogenicity in humans. An agent may be assigned to this category if it
clearly belongs, based on mechanistic considerations, to a class of agents for
which one or more members have been classified in Group 1 or Group 2A.
Group 2B: The
agent is possibly carcinogenic to humans.
This category is
used for agents for which there is limited evidence of carcinogenicity in
humans and less than sufficient evidence of carcinogenicity in
experimental animals. It may also be used when there is inadequate evidence
of carcinogenicity in humans but there is sufficient evidence of
carcinogenicity in experimental animals. In some instances, an agent for
which there is inadequate evidence of carcinogenicity in humans and less
than sufficient evidence of carcinogenicity in experimental animals
together with supporting evidence from mechanistic and other relevant data may
be placed in this group. An agent may be classified in this category solely on
the basis of strong evidence from mechanistic and other relevant data.
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