The concept of establishing a threshold limit for contaminants of industrial air is based on the principles of establishing (1) quantitative relationships between the magnitude and duration of exposure to an industrial substance and the nature and magnitude of the response of the worker, and (2) a limiting level of exposure to potentially hazardous agents, when there exists no significant threat to health. This paper focuses on the origin of this concept, and traces the history and development of thought concerning the principles upon which it is based. Threshold limit values have undergone a remarkable evolution, from values denoting concentrations of contaminant producing overt signs of acute toxicity, to those concentrations to which nearly all workers may be exposed for their working lifetime without experiencing adverse health effects. -- AATA