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Threshold Limit Values (TLVs) for industrial chemicals have been in existence since 1946. It has been proposed that the rate of change of TLVs over the years reflects the rate of change of exposure within industry (Roach and Rappaport, 1990). In this paper, a detailed analysis of TLVs as a whole, industry specific TLVs, and TLVs for known or suspected carcinogens was made lo determine the rate of change of TLVs between 1946 and 1990. It was found that the median fold reduction for all TLVs was 2.0. For most industries in this analysis it was also 2.0-fold, and for known or suspected carcinogens it was 2. 4-fold. The rate of change for all TLVs decreased with time (21.7% reductions per year in 1951 to 13.3% per year in 1990). For most industries the reduction rate was 13.3% reductions per year, while for known or suspected carcinogens it was 16% per year. It is postulated that the results of this analysis can be used to infer past exposures.... TLV, Change of TLVS, Exposure, History of reductions