OSHA formerly had no limit for methyl parathion. The ACGIH has a TLV-TWA of 0.2 mg/m3, with a skin notation. NIOSH also recommends a TWA of 0.2 mg/m3 and a skin notation for methyl parathion. The proposed PEL was an 8-hour TWA limit of 0.2 mg/m 3, with a skin notation; NIOSH (Ex. 8-47, Table N1) concurs with these limits, and they are established in the final rule. Methyl parathion is a tan to brown liquid with a pungent odor like that of garlic.
Methyl parathion is an acetylcholinesterase inhibitor, and excessive exposure can cause sweating, salivation, diarrhea, bradycardia, bronchoconstriction, muscle fasciculations, and coma. Methyl parathion's acute oral LD(50) for male rats is almost identical to that of parathion, (i.e., 10 to 25 mg/kg); for female rats, the LD(50) is 24 mg/kg, or approximately one-sixth that of parathion. By the dermal route, methyl parathion is much less toxic than parathion, with an LD(50) of 67 mg/kg in rats of both sexes (Hayes 1963/Ex. 1-982). Erythrocyte cholinesterase activity was inhibited in dogs fed methyl parathion for 12 weeks at a rate corresponding to approximately 24 mg/day; inhibition of both plasma and erythrocyte cholinesterase activity occurred at doses of 70 mg/day, without accompanying illness (Williams, Fuyat, and Fitzhugh 1959/Ex. 1-810). Dogs fed 6 mg/day methyl parathion for 12 weeks showed no effects from such exposures (Williams, Fuyat, and Fitzhugh 1959/Ex. 1-810). Lifetime feeding studies of rats and mice fed diets containing methyl parathion concentrations of up to 40 ppm and up to 125 ppm, respectively, produced no evidence of cancer (NCI 1979a/Ex. 1-1116).
Plasma and erythrocyte cholinesterase levels did not differ by more than 20 percent in subjects exposed at 7, 7.5, 8, or 9 mg/man/day, compared with controls (Moeller and Rider 1963/Ex. 1-565). Tiess, Wegener, and Tamme (1982/Ex. 1-774) have reported a case of protracted methyl parathion poisoning resulting from both percutaneous and inhalation exposures; Dille and Smith (1964/Ex. 1-549) attribute the long-term neuropsychiatric illness of two pilots to exposure to methyl parathion and other cholinesterase-inhibiting agents. Chronic exposure to small doses of methyl parathion have not caused chromosomal effects (de Cassia Stocco, Becak, Gaeta, and Rabello-Gay 1982/Ex. 1-540). No comments other than those from NIOSH were received on methyl parathion.
In the final rule, OSHA establishes a limit of 0.2 mg/m3 TWA for methyl parathion, with a skin notation. The Agency concludes that this limit will protect workers against the significant risk of acetylcholinesterase inhibition, which constitutes a material impairment of health that is associated with workplace exposures at levels above the new PEL. The skin notation will protect workers from the significant risk of systemic toxicity associated with percutaneous absorption of this substance.