OSMIUM TETROXIDE

OSHA comments from the January 19, 1989 Final Rule on Air Contaminants Project extracted from 54FR2332 et. seq. This rule was remanded by the U.S. Circuit Court of Appeals and the limits are not currently in force.

CAS: 20816-12-0; Chemical Formula: OsO₄

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.