Toxicological aspects of food safety - carcinogenicity and mutagenicity.

One major problem in the evaluation of potential carcinogenic food additives and contaminants is that of thresholds or, better, of "no-adverse-effect-levels". Arguments in favour of the postulated "irreversibility" of carcinogenic effects are based on dose-response studies, single-dose and multi-generation experiments as well as on the concept of somatic mutation as the first steps in carcinogenesis with subsequent transmittance of induced defects during cell replication. The problem of extrapolation of results of animal experiments using high doses to low exposure and low incidences in man is not yet solved satisfactorily. Possible practical consequences include zero-tolerance, acceptable thresholds at low risk and safety factors. Acceptable intakes never should be considered constants but should be changeable as soon as new facts in regard to the safety evaluation are available. Several systems of short-term tests as screening methods are based on mutagenicity tests and offer many advantages. Their critical evaluation is of utmost importance. Examples of some relevant problems to be discussed include nitrosamines in food products and their formation from ingested precursors; the migration of vinyl chloride from PVC food packing material; the occurrence of low levels of chloroform in drinking water.