Was There Sufficient Justification for the 10-Fold Increase in the TLV for Silica Fume? A Critical Review

In 1992, the Threshold Limit Value (TLV) for amorphous silica fume produced as a by-product of metallurgical processes was revised upwards from 0.2 mg/m3 (respirable dust) to 2.0 mg/m3. Comparison of the documentation justifying the lower TLV published by the ACGIH in 1989, with the subsequent documentation justifying the higher value published in 1992, does not support this increase. Following an outline of the problem areas existing in interpretational difficulties of experimental and review material in the silica fume bibliography, this paper provides a detailed examination of the six additional references cited in the 1992 documentation. All additional material suggests a need for extra caution, particularly with respect to recent experimental work in Australia on the sizing of silica fume. This paper concludes that the health evidence supports a TLV for silica fume closer to 0.3 mg/m3 rather than the current 2.0 mg/m3 now adopted in the U.S. and Australia.