Abstract
In the 1960s, the British introduced respirable mass sampling for coal mine dust using size separation by gravitational elutriation. In the United States, Atomic Energy Commission research showed that cyclones could be used to separate a respirable fraction. One of the cyclones used a low enough flow for the recently developed personal sampling pumps. A colorimetric free silica method permitted analyses in the microgram range. All that prevented personal respirable mass sampling for free silica was a basis for interpretation of results. Two 2-week studies in 1965 and 1966, with a total of 40 sample sets (impinger versus respirable free silica), were conducted in the Barre, Vermont, granite sheds. A respirable free silica concentration of 0.1 mg/m3 appeared equivalent to the granite limit of 10 million particles per cubic foot by the impinger method. Studies in foundries did not indicate that this would significantly change the basis for judgment from the then-current threshold limit value. A formula based on 0.1 and 5 mg/m3 of respirable free silica and total respirable dust, respectively, was proposed to and accepted by the Threshold Limit Value Committee of the American Conference of Governmental Industrial Hygienists in 1968. Subsequent research indicated that this was actually a reduction of the impinger threshold limit value for quartz dust. Ayer, H.E.: Origin of the U.S. Respirable Mass Silica Standard. Appl. Occup. Environ. Hyg. 10(12):1027–1030; 1995.