5. How are carcinogenic substances classified?

A substance is carcinogenic when it causes an increase in the incidence of malignant tumors or a decrease in the time to onset of tumors.

The assessment of these effects is based essentially on the results of long-term animal studies; it is preferable to have results from two different species.

The preferred species for carcinogenesis tests are rats and mice. The study should generally cover most of the normal lifespan of the test animals, i.e. around 24 months. Results are evaluated using an appropriate statistical method. Secondary information may be included in the assessment of carcinogenic potential, such as data on genotoxicity, metabolic or biochemical studies, induction of benign tumors, structural relationships with other known carcinogens.

Some arguments reduce the significance of experimental tumor induction,...