On February 1, 2010, I received a letter from the American Conference of Governmental Industrial Hygienists (ACGIH) that definitely got my attention. As chair of the ANSI/ASSE Z390.1, Accepted Practices for Hydrogen Sulfide (H2S) Training Programs, I was being formally notified that the ACGIH Board of Directors had finally acted on their intentions to reduce the Threshold Limit Value/Time Weighted Average (TLV/TWA) and Short-Term Exposure Limit (STEL) for hydrogen sulfide. The TLV at the time was 10 parts per million (ppm), and the STEL was 15 parts per million (ppm). The new levels were 1 ppm and 5 ppm, respectively, and were effective immediately.

Although our committee had been anticipating this reduction for a number of years, and we had objected to such a radical change based on what we felt was flawed science, we were still stunned to actually see the change approved by the ACGIH. Their response to us was that they didn't find our objections compelling, and their TLVs were not consensus-based values, but based on scientific opinion.

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