Classification of chemicals for carcinogenic and mutagenic properties

In the framework of the EC labelling guide, three categories were devised for carcinogenic as well as for mutagenic chemicals. In short, category 1 is for compounds shown to produce these effects in man, whereas category 2 is meant to contain substances that should be regarded as such, generally on the basis of sufficient experimental data. Category 3 is for compounds that cause concern but for which the evidence is not sufficient for 1 or 2. These general statements have been implemented, both for carcinogenicity and mutagenicity, by a set of criteria and considerations of the group of "specialized experts" who are supporting the "national experts" in deciding upon the classification of individual compounds. Essentially, this classification relies on the strength of the experimental or epidemiological evidence rather than on considerations of risk. In other words, it is based on proven intrinsic properties of the substances. Compared to the IARC classification for carcinogens, the EC criteria rely more on mechanistic data and on qualitative considerations concerning the relevance for man, and less on a strict count of the amount of positive experimental evidence. Difficulties of this (and any) classification system include: (1) the problem of classifying with limited data and (2) the fundamental problem involved in forcing sets of multi-facetted, and often unsatisfactory, scientific data into a simple one-dimensional scheme that may have huge economic consequences.