The contribution of the IARC monographs program to the identification of cancer risk factors

The differences between cancers that occur as a consequence of occupational exposure and other cancers are not only their preventability but, more importantly, their social unacceptability. Occupational cancer occurs, by definition, among individuals who have been exposed to carcinogens because of their occupation, and most, if not all, of these individuals are drawn from the less favoured social classes. This is probably one reason why mortality from cancer and from all causes is greater in people in classes IV and V than in those in I and II. We cannot precisely quantify the proportion of cases, among the total number of cancer cases observed, attributable to occupational exposures. It is very likely, however, in some of the most industrialized countries, although not in certain developing countries, that the number of those cancers that are indisputably due to occupational exposure is not increasing and is perhaps decreasing. This is due to the combined effect of two factors: the banning of certain chemicals, as, for instance, aromatic amines--even if this did not take place in all countries and, where it did, not at the same time; and improved working conditions, as, for instance, in the case of vinyl chloride. We do not know, however, to what extent low levels of exposure to which the general population is commonly exposed, as well as workers in occupations where levels of exposure to carcinogens have recently been significantly reduced, play a role in the causation of human cancer. While it is important to stress that a large proportion of the chemicals to which humans are exposed, either because of their occupation or in the general environment, and for which experimental evidence of carcinogenicity is available, have not been the object of epidemiological surveys, it is also important to realize that epidemiological methods are generally insufficient to provide reliable information on risks generated by low levels of exposure. It is certainly important to encourage epidemiological surveillance, but it should at the same time be made clear that the epidemiological approach will never entirely replace the considered use of experimental data in the implementation of primary prevention.