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: 16752-77-5; Chemical Formula: C5H10N2O2S

OSHA formerly had no limit for methomyl. The ACGIH has a TLV-TWA of 2.5 mg/m3 for this white crystalline solid with a slightly sulfurous odor. The proposed PEL for methomyl was 2.5 mg/m3 as an 8-hour TWA; NIOSH (Ex. 8-47, Table N1) concurs, and the final rule establishes this limit.

Methomyl is a cholinesterase-inhibiting insecticide. The oral LD(50) in rats is reported to be between 25 and 40 mg/kg (Dashiell and Kennedy 1984/Ex. 1-548). Studies of dermal effects have reported no appreciable irritation or sensitization in guinea pigs. Instillation of a 10-percent solution of methomyl in propylene glycol or of the dry material into rabbit eyes caused mild conjunctivitis without corneal injury. However, marked pupillary constriction, a health effect produced commonly by cholinesterase inhibitors, was observed (E.I. du Pont de Nemours and Co., Inc., as cited in ACGIH 1986/Ex. 1-3, p. 363). The LC(50) of unformulated methomyl as mist is 0.3 mg/L at four-hour exposures; the lethal concentration in rats exposed to a 90-percent water-soluble formulation with a particle size of less than 10 microns was approximately 0.45 mg/L.

Inhalation studies have reported no fatalities resulting from four-hour exposures to the saturated vapor. There is no clinical evidence of cumulative toxicity resulting from 10 doses of 5.1 mg/kg/day over a 14-day period (Harvey, Jelinek, and Sherman 1973/Ex. 1-486). Methomyl is rapidly metabolized and excreted in the urine, and cholinesterase inhibition is thus quickly reversed. In dogs, a dose of 20 mg/kg (one-half the lethal dose) produced symptoms of intoxication and cholinesterase inhibition that disappeared within two to four hours after cessation of exposure (E.I. du Pont de Nemours and Co., Inc. as cited in ACGIH 1986/Ex. 1-3, p. 363). No depression of cholinesterase activity could be detected in rats fed at levels of 0, 200, 400, or 800 ppm methomyl for 79 days. In dogs, 90-day and two-year feeding studies showed no effects at 0, 50, 100, or 400 ppm; however, animals fed at 1000 ppm did demonstrate toxicity. Similar studies of rats have shown kidney, liver, and spleen damage at higher feeding levels, but the no-effect level for both rats and dogs has been reported to be 100 ppm (Kaplan and Sherman 1977/Ex. 1-337). Only NIOSH submitted comments on methomyl.

In the final rule, OSHA is establishing a PEL of 2.5 mg/m3 (8-hour TWA) for methomyl. The Agency concludes that this limit will protect exposed workers against the risk of cholinesterase inhibition to which they could formerly have been exposed in the absence of any OSHA limit. The Agency has determined that this limit will substantially reduce the significant risk of cholinergic effects, which constitute material impairments of health.

Page last reviewed: September 28, 2011