The case for carcinogens having threshold limit values (TLV's) rather than acting on a one hit basis to cause cancer is presented. Background is briefly given on the activity of the TLV Committee of the American Conference of Governmental Industrial Hygienists and definitions are given on low and high potency. It is noted that in the Committee's 1976 TLV Booklet 14 of the 22 listed carcinogens for the industrial worker have been assigned a TLV. Evidence for justifying the use of TLV's is taken from epidemiologic studies of industrial plant experience, from well-designed carcinogenic studies in animals, and from studies showing inherent, built in anticarcinogens and processes in the human body. It is emphasized that predictions of carcinogenesis from animals to man can be misleading in the absence of epidemiologic evidence and that practical thresholds for chemical carcinogens in the workplace can be found that provide no risk to the industrially exposed workers. It is noted that examples of the misleading nature of extrapolating original data on tumorigenesis to man are accumulating almost weekly. The role of cocarcinogens in setting TLV's is mentioned, and the role of glutathione (70188) in the initial metabolic step in detoxication is discussed.
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