Silica (crystalline, respirable) - OEHHA

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FINAL February 2005 ability of alveolar macrophages to phagocytize Staphylococcus aureus in vitro and impaired the ability to lyse allogeneic tumor cells (from mice other than Balb/c) in vitro. Silica inhalation did not affect antibody-dependent cell-mediated cytotoxic and mitogenic responses by splenic lymphocytes. (Fibrosis was not an endpoint measured, but the effect level is similar to the LOAELs in other animal studies.) Table 18. Animal studies of silica inhalation analyzed by USEPA (1996) Study Species Duration a LOAEL Muhle et al., 1989 Rat 24 mo 1.0 mg/m 3 Scheuchenzuber et al., 1985 Mice 150-570 d 2.0 Burns et al., 1980 Mice 3-39 wk 4.9 Kutzman, 1984a Rat 6 mo 2.0 Kutzman, 1984b Rat 6 mo + 6 mo recovery 2.0 Wagner et al., 1986 Dog Up to 2.5 yr 0.2 a Inhalation exposure was generally for 6 h/day, 5 d/wk. Quartz has the ability to induce the generation of free radicals and to cause oxidative stress in tissues. Many substances that affect the quartz surface can modify this ability. Some of these modifiers could originate from other minerals, which exist together with quartz in nature. Donaldson and Borm (1998) proposed that the hazard posed by quartz may vary widely depending on the origin of the silica sample or on its contact with other chemicals/minerals. Such mechanistic data could assist in the interpretation of epidemiological studies such as those above. Experimentally their group found that DQ12 quartz, a European quartz standard which is often used in experimental studies of silica effects, is much more inflammatory in rat lung than respirable silica collected from two workplaces (Clouter et al., 2001). Humans appear to show adverse effects of silica exposure at lower levels than animals (compare LOAELs in Table 18 to LOAELs/NOAELs in Table 16). Rodents tend to be obligate nosebreathers and to have extensive nasal turbinates, which may result in less silica reaching the lower lung. For silica, results in animals may not be a good predictor of human effect levels. 24
FINAL February 2005 VI. Derivation of Chronic Reference Exposure Level (REL) Key study Hnizdo and Sluis-Cremer, 1993 Study population 2235 white South African gold miners Exposure method Workplace inhalation Critical effects Silicosis (313 miners) (14 %) LOAEL 3 mg/m 3 -years CDE (9 miners with silicosis) NOAEL 2 mg/m 3 -years CDE (0 miners with silicosis) or 600 µg/m 3 -years silica (dust = 30% silica) BMCL01 2.12 (mg/m 3 )–yr CDE or 0.636 (mg/m 3 )–yr silica Exposure continuity 8 h/day, 5 d/wk Exposure duration Average experimental exposure Average of 24 years dust exposure (10-39 years) 235 µg/m 3 -yr silica at BMC01 (636 x 10 m 3 /20 m 3 x 270 shifts/365 days) 235 µg/m 3 -yr/24 yr = 9.8 µg/m 3 Human Equivalent Concentration (HEC) 9.8 µg/m 3 LOAEL uncertainty factor Not needed in BMC approach Subchronic uncertainty factor 1 Interspecies uncertainty factor 1 Intraspecies uncertainty factor 3 Cumulative uncertainty factor 3 Inhalation Reference Exposure Level 3 µg/m 3 (based on 30% silica in mine dust) [respirable, as defined occupationally by ACGIH/ISO] First supportive study Steenland and Brown, 1995 Study population 3330 S. Dakota gold miners Exposure method Workplace inhalation Critical effects Silicosis (170 miners) (5.1%) LOAEL 0-0.2 mg/m 3 -years (5 miners with silicosis) NOAEL Not found BMCL01 0.34 (mg/m 3 )–yr (see text below) Exposure continuity 8 h/day, 5 d/wk Exposure duration 3-36 years (average 9 years underground) Average experimental exposure 112 µg/m 3 -y (340 x 10 m 3 /20 m 3 x 5 d/7 d x 48 wk/52 wk) 112 µg/m 3 -y/9 y = 12.4 µg/m 3 Human Equivalent Concentration (HEC) 12.4 µg/m 3 LOAEL uncertainty factor Not needed in BMC approach Subchronic uncertainty factor 1 Interspecies uncertainty factor 1 Intraspecies uncertainty factor 3 Cumulative uncertainty factor 3 Inhalation Reference Exposure Level 4 µg/m 3 [respirable, as defined occupationally] 25
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