Reference values for the study of low doses.

At present, the quantitative differences between environmental exposure and occupational exposure to toxic metals are steadily decreasing, it becomes important to establish reference values of the tests used for biological monitoring. However, the word 'reference values' is often used inappropriately and in particular its significance is often confused with that of 'limit values' or 'normal values'. The term reference values defines the levels of the biological indicators in the general population which are not occupationally exposed. It is necessary to refer to these values in order to compare the data obtained through biological monitoring programmes in workers and in groups of the general population which are presumed to be exposed. The most essential aspects for identifying reference values for toxic metals, using a rigorous methodology, are taken into consideration and in particular, the requisites needed for identifying the subjects which must be included in the reference groups are also examined. The modalities for using reference values for individual subjects as well as for groups are then discussed and the main points of research which must be faced in the near future regarding reference values are highlighted.