Epidemiology and the identification of metals as human carcinogens.

Classification of substances as probable human carcinogens under the current IARC classification scheme is dependent on epidemiological evidence. The epidemiological data relating to the four metals currently identified as probable human carcinogens, in the metallic form or in the compounds, are reviewed and the weaknesses identified. These weaknesses lie mainly in exposure assessment. The weaknesses may be overcome to some extent by the use of metademographic methods as applied recently to the respiratory cancers that occurred at the Clydach Nickel Refinery in the first 30 years of this century. The general conclusion is that the epidemiological data relating to metals are unsatisfactory bases for the IARC classifications. There is a need to revise these classifications and to make them more precise by identifying exactly the substances which have caused human cancers.