MONOCROTOPHOS

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: 6923-22-4; Chemical Formula: C₇H₁₄NO₅P

OSHA formerly had no limit for the systemic insecticide monocrotophos. The ACGIH has a TLV-TWA of 0.25 mg/m³ for this reddish-brown solid with a mild ester odor. The proposed PEL was 0.25 mg/m³ as an 8-hour TWA; NIOSH (Ex. 8-47, Table N1) concurs with this limit, which is established in the final rule.

Monocrotophos is a highly toxic, direct acting cholinesterase inhibitor that penetrates the intact skin (ACGIH 1986/Ex. 1-3, p. 416). The acute oral LD(50) values in rats and mice range from 5.7 to 17 mg/kg in a water formulation (Brown et al. 1970, Shellenberger and Newell, both as cited in ACGIH 1986/Ex. 1-3, p. 416) and from 10 to 23 mg/kg in an oil formulation (Shellenberger and Newell, as cited in ACGIH 1986/Ex. 1-3, p. 416). These authors also report a percutaneous LD(50) in the rabbit that ranges from 112 to 709 mg/kg, depending on the vehicle used. A two-year dietary study of rats ingesting 0, 1, 10, or 100 ppm monocrotophos revealed that both sexes in the 100 ppm group failed to gain as much weight as the controls, but autopsy showed no significant findings; plasma, erythrocyte, and brain cholinesterase decreased at the two highest dose levels but were unaffected at 1 ppm (Johnston 1966-67, as cited in ACGIH 1986/Ex. 1-3, p. 416). Another two-year feeding study in dogs administered doses of up to 16 ppm monocrotophos revealed no adverse effects at levels of 0.16 and 1.6 ppm, but serious cholinesterase reduction was observed at the 16-ppm level (Johnston 1966-67, as cited in ACGIH 1986/Ex. 1-3, p. 416). Metabolism studies in rats and goats indicate that monocrotophos is excreted rapidly in the rat and does not accumulate in the body (Bull and Lindquist 1966/Ex 1-719); goats given labeled monocrotophos by mouth showed only traces of the material in their milk (Menzer and Casida 1965/Ex. 1-986; Potter, as cited in ACGIH 1986/Ex. 1-3, p. 416). Inhalation exposure of rats to an unknown concentration of 75 percent monocrotophos in air for one hour was not lethal; a four-hour exposure to an unknown concentration of the aerosol (0.4 and 0.75 percent) was fatal to two out of six (0.4 percent aerosol) and five out of eight rats (0.75 percent aerosol). Head-only exposure to the 0.4-percent aerosol resulted in the death of one of eight animals (Wilson, as cited in ACGIH 1986/Ex. 1-3, p. 416).

Intravenous injection of radiolabeled monocrotophos in human volunteers showed maximum excretion at 4 to 8 hours, with 67 + 5 percent of the material in the urine; absorption of 14 + 7 percent occurred when the radiolabeled material was applied to the forearm; 33 + 9 percent of the applied dose was absorbed when it was covered with a vapor-proof film for 72 hours (Maibach 1970, as cited in ACGIH 1986/Ex. 1-3, p. 416). Although gauze patches attached to the clothing and skin of field workers attested to the presence of monocrotophos, no cholinesterase inhibition was observed in post-exposure examinations at three hours and at three and seven days (Maibach, as cited in ACGIH 1986/Ex. 1-3, p. 416). Only NIOSH commented on this substance.

OSHA is establishing a PEL of 0.25 mg/m³ (8-hour TWA) for monocrotophos in the final rule.