METHYL SILICATE

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: 681-84-5; Chemical Formula: (CH₃O)₄Si

OSHA did not formerly have a limit for methyl silicate; the Agency proposed the adoption of a 1-ppm 8-hour TWA for this substance, and NIOSH (Ex. 8-47, Table N1) concurred with this selection. The final rule establishes this limit, which is consistent with that of the ACGIH. Methyl silicate exists in the form of colorless needles.

Methyl silicate damages the cornea and is associated with a delayed onset of symptoms. In many cases of methyl silicate exposure, the eyes recover completely, but there are reports of damage to the deep layers of the cornea that caused permanent opacification and, in one worker, loss of vision in one eye (Grant 1986/Ex. 1-975). It is estimated that exposing humans to methyl silicate at concentrations of 200 to 300 ppm for 15 minutes will produce lesions, and that exposure to 1000 ppm for this period will produce injury requiring hospitalization (ACGIH 1986/Ex. 1-3, p. 409).

Rabbits exposed to 1000 ppm of methyl silicate in dry air experienced delayed eye burns (ACGIH 1986/Ex. 1-3, p. 409). Exposure of these animals to approximately 15,000 ppm for five minutes caused eye burns, but exposure to this level for four minutes caused no appreciable effect. Guinea pigs showed maximum no-effect levels of 135 ppm for 15 minutes, 90 ppm for one hour, and 20 ppm for 8 one-hour periods. The latency period for ocular changes was 16 hours for serious effects and up to three days for mild involvement (ACGIH 1986/Ex. 1-3, p. 409). Only NIOSH commented on this substance.

Because the onset of response to this toxin is delayed, because exposure in the workplace could have a duration substantially greater than that in the animal bioassays, and because of interspecies variability, it is necessary to establish a PEL considerably below the NOE level in animals to reduce the significant risk of ocular damage to employees. The Agency is therefore establishing a 1-ppm 8-hour TWA limit for methyl silicate to reduce the significant risk of severe ocular effects associated with the uncontrolled exposures formerly possible in the absence of a PEL. The Agency concludes that this limit will substantially reduce this significant risk by protecting workers from the ocular effects of methyl silicate exposure, which constitute material impairments of health.