The relationship between TLV-TWA compliance and TLV-STEL compliance

The purpose of this study was to investigate the relationship between the American Conference of Governmental Industrial Hygienists' (ACGIH) time-weighted average (TLV-TWA) and short-term exposure limit (TLV-STEL) Threshold Limit Values (TLVs). It is of value to determine if one or the other of these exposure limits is inherently more stringent so that exposure monitoring strategies may be devised which efficiently use available resources and effectively control exposures to meet both exposure limits. The ACGIH short-term exposure limit (TLV-STEL) imposes three conditions on short-term (15-minute) exposures. These conditions involve exceeding the TLV-TWA and TLV-STEL levels and the time-separation of those short-term exposures that exceed the TLV-TWA level. These conditions were analyzed to produce eight unique component probabilities for TLV-STEL non-compliance. The sum of these eight components is the total probability of TLV-STEL non-compliance. Mathematical expressions for the eight probability components are derived in terms of the probability that a single 15-minute exposure exceeds the time-weighted average threshold limit value (TLV-TWA) and the geometric standard deviation of these 15-minute exposures. These expressions were applied to various hypothetical workplaces, and the relationship between TLV-TWA and TLV-STEL compliance is presented. The results showed that non-compliance of 15-minute exposures with the TLV-STEL level is only one part of overall TLV-STEL non-compliance. The additional ACGIH provisions for TLV-STEL compliance--the number of 15-minute exposures exceeding the TLV-TWA level and the number of clean periods separating such exposures--can be important factors in determining TLV-STEL compliance. It is concluded that compliance with all provisions of the ACGIH TLV-STEL adds a degree of stringency that greatly enhances the likelihood of TLV-TWA compliance for most workplace environments.