Table 1. Classification of carcinogenic agents according to the International Agency for Research on Cancer.

Group	Classification	Parameter	Number of agents
1*	Carcinogenic to humans	Sufficient evidence of carcinogenicity in humans and in experimental animals	111
2A	Probably carcinogenic to humans	Limited evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals	65
2B	Possibly carcinogenic to humans	Limited evidence of carcinogenicity in humans and less than sufficient evidence of carcinogenicity in experimental animals	274
3	The agent is not classifiable as to its carcinogenicity to humans	Inadequate evidence of carcinogenicity in humans and in experimental animals	504
4	The agent is probably not carcinogenic to humans	Evidence suggesting lack of carcinogenicity in humans and in experimental animals	1

*An agent can be included in Group 1 in the absence of sufficient evidence for carcinogenicity in humans but there is sufficient data of carcinogenicity in experimental animals and strong evidence that the agent acts through a similar mechanism of carcinogenicity in humans.