Page:NIOSH Manual of Analytical Methods - Chapter R.pdf/4

In 1974, NIOSH reviewed the available health effects data on occupational exposure to respirable crystalline silica and determined that the principal adverse health effect was silicosis [9]. On the basis of the evidence from the animal studies published by 1986, the International Agency for Research on Cancer (IARC) concluded that "sufficient evidence" existed for the carcinogenicity of respirable crystalline silica in experimental animals but only "limited evidence" existed for carcinogenicity in humans [15]. During the 1988 OSHA rulemaking activity on air contaminants, NIOSH recommended an exposure limit of 0.05 mg/m³ "as respirable free silica for all crystalline forms of silica" to protect workers from silicosis and cancer [16]. In addition, NIOSH testimony referred to the IARC [1987] review and recommended that OSHA label crystalline silica a potential occupational carcinogen [16]. IARC recently concluded that there is "sufficient evidence in humans for the carcinogenicity of inhaled crystalline silica in the form of quartz or cristobalite from occupational sources" (i.e., IARC category Group 1 carcinogen) [8]. The American Conference of Governmental Industrial Hygienists (ACGIH) has classified crystalline silica as a "suspected human carcinogen" (i.e., category A2 carcinogen) [17]. The National Toxicology Program reclassified respirable crystalline silica from "reasonably anticipated to be a carcinogen" to "known carcinogen" in the 9^th Report on Carcinogens [18]. Furthermore, experimental research has shown that crystalline silica is not an inert dust. The toxicity of crystalline silica particles may be related to reactive sites on the surfaces of silica particles and the crystallinity index [19]. Table 2 lists the current U.S. and U.K. guidelines and limits for occupational exposure to crystalline silica.