A threshold limit for toxic substances in industrial air is, mathematically, a nonlinear relationship between dose and response at the initiation of the response. Developments in radiation biology and carcinogenesis have cast doubt on the validity of the threshold concept; injurious agents may be so at any concentration, either per se or as factors in diseases of multiple causation. Evidence for the adaptive mechanisms of the human body (e.g., development of tolerance to foreign substances) is also evidence for the existence of threshold limits. Experimentally derived thresholds for substances in industrial air must be adjusted for the differences between animals and humans, if test animals were used, and, if based on healthy humans, for the differences between them and the more average, less healthy, drinking and smoking worker. Preexisting systematic disease also modifies thresholds. Thus a factor of safety must be built into industrial threshold standards; such judgmental values are set by the Threshold Limit Value (TLV) Committee of the American Conference of Governmental Industrial Hygienists. Safety factors do not allow for the worker who is genetically hypersusceptible due to metabolic error; he must be identified in the preplacement job examination.