Complexities of carcinogenicity classification


A partner organisation, running a large REACH Consortium.


Working Group experts from the International Agency for Research on Cancer classify compounds according to their likely carcinogenicity to humans based on available cancer data from humans and laboratory animals, and mode-of-action (MOA) considerations. Some chemicals are classified individually; others may be classified together as part of a group. The implication is that IARC-classified carcinogens should also be subject to classification and labelling as carcinogenic to humans under the GHS (Globally Harmonised System) and EU CLP Regulation (EC) 1907/2006.

Project goals

Bibra was asked to inform on whether the inclusion of a certain acid by IARC in a group of compounds classified as “carcinogenic to humans” (Group 1) is scientifically justified, and whether classification of this specific acid as carcinogenic under CLP is warranted.


The available (limited) epidemiology data were assessed and summarised. No long-term laboratory animal bioassays were available on the acid itself, so read-across data were evaluated. Bibra considered whether more short-term exposures caused pre-neoplastic effects (e.g. hyperplasia), and looked at the acid’s local irritant potential, and also its genotoxic character.

The group of compounds classified by IARC are implied to have a common MOA. In this case, respiratory irritation was considered to be a potential key event in inducing respiratory tract tumours. The relevance of the MOA to the chemical of interest was assessed to determine whether its inclusion in IARC’s Group 1 classification is scientifically justified. Our findings were discussed in the context of current guidance on the application of the CLP criteria, and recommendations were made on CLP classification and labelling for carcinogenicity.

Project team

Richard Young

James Hopkins

Beth O’Connell


Classification and Labelling

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