Substances, mixtures and exposure circumstances in this list have been classified by the IARC as Group<unk> : The agent (mixture) is possibly carcinogenic to humans. The exposure circumstance entails exposures that are possibly carcinogenic to humans. This category is used for agents, mixtures and exposure circumstances 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, mixture or exposure circumstance for which there is inadequate evidence of carcinogenicity in humans but limited evidence of carcinogenicity in experimental animals together with supporting evidence from other relevant data may be placed in this group. 
Further details can be found in the IARC Monographs.
List of IARC Group 2B carcinogens
List of IARC Group 2A carcinogens
List of IARC Group 4 carcinogens
List of IARC Group 3 carcinogens
List of IARC Group 1 carcinogens
List of cancer mortality rates
List of cancer clusters
List of cancer types