OSHA previously had an 8-hour TWA limit of 0.002 mg/m3 for osmium tetroxide. Based on the ACGIH recommendation, OSHA proposed to revise this limit to 0.002 mg/m3 as a TWA and to add a STEL of 0.006 mg/m3; NIOSH (Ex. 8-47, Table N1) concurred with this proposal. The final rule establishes these limits for this substance. Osmium tetroxide is a noncombustible, colorless to pale yellow solid with a disagreeable, chlorine-like odor.
Exposure to osmium tetroxide is known to produce ocular effects and respiratory irritation. In 1933, Brunot (Ex. 1-776) reported that rabbits died from pulmonary edema four days after a 30-minute exposure to osmium tetroxide at 130 mg/m3 or higher. Visual problems (e.g., delayed lacrimation and "halo" effects) were reported by this investigator after a brief exposure to osmium tetroxide at a significantly lower concentration (Brunot 1933/Ex. 1-776). A four-hour LC(50) value of 40 ppm has been reported in rats and mice (NIOSH 1977i/Ex. 1-1182). Toxic effects to bone marrow have been reported in guinea pigs (Hamilton and Hardy 1974a/ Ex. 1-957).
Industrial experience indicates that concentrations in a precious metal refining plant ranged from 0.1 to 0.6 mg/m3; intermittent exposures produced symptoms (sometimes delayed) of lacrimation, vision disturbances, headache, conjunctivitis, and cough (McLaughlin, Milton, and Perry 1946/Ex. 1-749). Complaints of persistent and severe nose and throat irritation have been reported (Hamilton and Hardy 1974a/Ex. 1-957). Fairhall (1949d, as cited in ACGIH 1986/Ex. 1-3, p. 450) reported a human fatality resulting from inhalation exposure to OsO4. Flury and Zernik (1931i, as cited in ACGIH 1986/Ex. 1-3, p. 450) reported that 0.001 mg/m3 is the highest concentration of osmium tetroxide that can be tolerated for six hours without harmful effects.
Except for NIOSH, no rulemaking participants commented on the proposed addition of a STEL for osmium tetroxide. The study by McLaughlin, Milton, and Perry (1946/Ex. 1-749) used a calibrated, calorimetric procedure, together with well-described case reports, to assess the dose-response relation-ship. OSHA finds this study superior to the report by Flury and Zernik (1931i, as cited in ACGIH 1986/Ex. 1-3, p. 450), which is more anecdotal. The McLaughlin et al. (1946/ Ex. 1-749) study demonstrates serious, acute effects resulting from intermittent and short-term exposure. OSHA concludes that, in the absence of a limit on short-term exposures, the 0.002-mg/m3 8-hour TWA PEL alone is not sufficient to protect employees from experiencing these effects, which are deemed to constitute material impairments of health. Therefore, to reduce the risk from short-term elevated exposures to osmium tetroxide, OSHA is establishing a 15-minute STEL of 0.006 mg/m3 to supplement the 0.002-mg/m3 TWA limit.