SILICON TETRAHYDRIDE
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: 7803-62-5; Chemical Formula: SiH4
OSHA formerly had no limit for silicon tetrahydride. The ACGIH limit of 5 ppm as an 8-hour TWA was established in 1983. The proposed PEL was 5 ppm as an 8-hour TWA, a limit with which NIOSH concurs (Ex. 8-47, Table N1). The final rule establishes an 8-hour PEL for silicon tetrahydride of 5 ppm. Silicon tetrahydride, a colorless gas, is used in the manufacture of semiconductors.
Studies of rats exposed to silicon tetrahydride at levels of 126 ppm for one hour (Matheson Gas Products 1971, as cited in ACGIH 1986/Ex. 1-3, p. 528) and at 1400 ppm for six hours (Union Carbide Corporation 1980, as cited in ACGIH 1986/Ex. 1-3, p. 528) have failed to identify any systemic effects associated with exposure to this chemical. Sax ( Dangerous Properties of Industrial Materials, 6th ed., 1984) lists the effects of acute exposure to silicon tetrahydride as moderate irritation of the eyes, skin, and mucous membranes.
In addition to NIOSH’s comment on silicon tetrahydride, Grace Ziem, an occupational physician, stated that she believed OSHA’s reference to Sax (1984) in the proposal’s discussion of silicon tetrahydride’s irritant effects was incorrect. However, OSHA notes that this notation was correct; Dr. Ziem did not realize that Sax (1984, p. 2394) has an entry for silicon tetrahydride under silane, a synonym.
OSHA is establishing an 8-hour TWA limit of 5 ppm for silicon tetrahydride in the final rule. The basis of this limit is analogy to the toxicity of silicon tetrahydride and other tetrahydrides. The Agency concludes that this limit will protect exposed workers from the significant risk of eye, skin, and upper respiratory tract irritation, which are material health impairments that are associated with exposure to this substance at levels above the new PEL.