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SILICA, AMORPHO

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: 68855-54-9; Chemical Formula: SiO2

OSHA's current limit for amorphous silica is 20 mppcf, which is equivalent to 6 mg/m3 TWA (ACGIH 1984), measured as total dust. The ACGIH has established a limit for this dust (measured as total dust) of 10 mg/m3 (8-hour TLV-TWA). Amorphous silica (diatomaceous earth) is composed of the skeletons of prehistoric plants known as diatoms. These skeletons are largely noncrystalline, although diatomaceous earth can contain varying amounts of crystalline quartz, which has led, in the opinion of the ACGIH (1986/Ex. 1-3, p. 520), to conflicting results in studies of the pulmonary effects of exposure to this colorless to gray, odorless powder.

Cooper and Cralley (1958/Ex. 1-1145) reported "doubtful" linear-nodular changes in the lungs of workers exposed only to amorphous (noncrystalline) silica for five years or more. Other studies (Vigliani and Mottura 1948/Ex. 1-534; Gardner 1942, as cited in ACGIH 1986/Ex. 1-3, p. 520) either found mild silicosis only or no evidence of serious lung pathology in diatomite workers. Kovalevich (1957, as cited in ACGIH 1986/Ex. 1-3, p. 520) reported silicosis in diatomite workers, but intratracheal instillation of diatomaceous earth dust in animals caused evidence of fibrosis (Gardner 1942, as cited in ACGIH 1986/Ex. 1-3, p. 520) and silicosis (Kovalevich 1957, as cited in ACGIH 1986/Ex. 1-3, p. 520). Another study (Tebbens and Beard 1957/Ex. 1-531) exposed guinea pigs to this substance at an average concentration of 60 mg/m3 for 37 to 50 weeks and found both accumulations of dust-laden macrophages and alveolar epithelialization but no fibrosis.

In setting its limit for diatomaceous earth, the ACGIH (1986/Ex. 1-3, p. 520) assumed that this substance itself is either "weakly fibrogenic or nonfibrogenic," and thus that those studies discussed above that report adverse pulmonary effects actually involved exposure to diatomaceous earth having an unmeasured but significant crystalline quartz content. Based on this reasoning, the ACGIH considers amorphous silica (diatomaceous earth) to have low biological activity.

OSHA received few comments on its proposal to retain the 6 mg/m3 PEL for diatomaceous earth. The Synthetic Amorphous Silica and Silicates Industry Association (SASSI) (Ex. 1-630) requested that OSHA revise its entry for "silica, amorphous, diatomaceous earth" to "silica, crystalline, diatomaceous earth" to reflect the fact that diatomaceous earth frequently contains crystalline silica. OSHA intends the PEL for crystalline quartz of 0.1 mg/m3 to apply to diatomaceous earth containing more than 1 percent crystalline silica. For clarification, OSHA has added the designation "containing less than 1 percent crystalline silica" to the entry for diatomaceous earth on Table Z-1-A of the final rule, for which the 6 mg/m3 limit is applicable.

SASSI also suggested that the crystalline silica PEL apply to any silicates containing more than 0.1 percent, rather than 1 percent, crystalline silica because of recent concerns regarding the potential carcinogenicity of silica. As discussed in the section on crystalline silica (see Section VI.C.6), OSHA has not made a final determination on the carcinogenicity of silica; therefore, at this time, OSHA will apply the limits for silicates to those materials containing less than 1 percent silica.

NIOSH (Ex. 8-47) concurred with the 6 mg/m3 TWA PEL for diatomaceous earth, provided the silica content does not exceed 1 percent. Chevron Corporation (Ex. 3-896) also agreed with OSHA's proposal. Both Chevron (Ex. 3-896) and SASSI (Ex. 3-630) agreed that the former mppcf limit should be revised to a limit expressed as mg/m3, since the use of mppcf units is outdated.

OSHA is retaining an 8-hour TWA of 6 mg/m3 (equivalent to 20 mppcf) for this form of silica. OSHA finds that the health evidence for this substance is not sufficiently persuasive to permit an increase in the limit at the present time. The Agency is revising the units in which its permissible exposure limit for diatomaceous earth is expressed; this change is being made to facilitate the accurate monitoring of employee exposures and does not represent a change in the value of the limit.

 
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  • Page last reviewed: September 28, 2011
  • Page last updated: September 28, 2011
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