Exposure limits have been a widely discussed and controversial subject. As was stated by Parmeggiani [ 19881, the concept of exposure limits (TLVs in the United States) was rather simple as long as health meant absence of clinical disease based on organic pathology. In the course of time, however, major differences in the permissible levels of particular compounds coming from various sources became evident (especially between those from the U.S. and the U.S.S.R.). Those differences led to discussions of exposure limits by such organizations as the International Labour Office (ILO) and the World Health Organization (WHO).
Owing to advances in the detection of early effects of various types of exposure, e.g., chemicals, sometimes long before the onset of clinical signs, defining an adverse health effect has become a matter of topical importance. In the meantime, the concept of health-based occupational exposure limits was introduced by the WHO. Acquisition of new information regarding possible distinction between the non-adverse and adverse effects, supported by scientific evidence, has turned out to be the source of permanent need for revising health-based exposure limits. In spite of the fact that distinction between adverse and non-adverse effects poses considerable difficulties, and the existence of defined criteria for selection of a lowest observed adverse effect level, the "recommended" concentrations, almost without exception, tend to be set at an ever lower level. The matter appears to be even more complicated if the existence or non-existence of a "threshold" for specific toxic effects, the significance of interactions, and the variation in sensitivity and exposure levels within the population are considered, or if attention is correctly paid to the questions of toxic effects not only of specific pollutants but also of their mixtures. Various pollutants enter the body, in meaningful concentrations, not only through exposure in the workplace but also through environmental exposure, which includes indoor exposure as well as food and drinking water. It remains to be seen to what extent such exposure