TEMEPHOS

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: 3383-96-8; Chemical Formula: C₁₆H₂₀O₆P₂S₃

The former OSHA Z tables had no specific limit for exposure to temephos, a cholinesterase-inhibiting insecticide. Temephos was formerly regulated under OSHA’s generic particulate limit of 15 mg/m³. The ACGIH limit is 10 mg/m³ (total dust) as an 8-hour TWA. The proposed PEL was 10 mg/m³ (total particulate), and this is the limit promulgated in the final rule; the 5-mg/m³ limit for the respirable fraction of temephos dust is retained. NIOSH (Ex. 8-47, Table N4) concurs with the selection of these PELs. Temephos may be a white crystalline solid or a viscous brown liquid.

In rats and mice, temephos has an acute oral LD(50) of 400 mg/kg or greater. Various animal species tolerated doses of 10 mg/kg without clinical effect and 1 mg/kg without effect on cholinesterase activity (Gaines, Kimbrough, and Laws 1967/ Ex. 1-553). Laws, Morales, Hayes, and Joseph (1967/Ex. 1-562) revealed that human volunteers consuming oral doses of temephos at levels of either 256 mg/man/day for five days or 64 mg/man/ day for four weeks evidenced no detectable effects on erythro-cyte or plasma cholinesterase levels. Murphy and Cheever (1972/Ex. 1-567) reported that 1 mg of temephos per liter of drinking water produces no effect. These authors found that rat liver carboxylesterases were at least 30 times more sensitive to inhibition from temephos than were rat cholin-esterases. Assuming that human liver carboxylesterases are proportionately more sensitive than cholinesterases, it is estimated that significant inhibition of these carboxylesterases could occur as a result of consuming 2 liters of drinking water containing 1 mg/L of temephos. Although nonspecific liver carboxylesterase is not critical for normal physiologic function, adverse effects on this enzyme could increase the susceptibility of exposed individuals to chemicals and drugs that contain carboxylesterase linkages (ACGIH 1986/Ex. 1-3, p. 557).

The ACGIH derived its limit of 10 mg/m³ TWA for temephos from studies of malathion, which has an acute LD(50) of 2100 mg/kg in rats, or roughly one-half that of temephos. Because humans tolerate 16 mg/day oral doses of malathion without effects on blood cholinesterase levels, the ACGIH believes the 10-mg/m³ limit is appropriate for temephos (ACGIH 1986/Ex. 1-3, p. 557).

OSHA agrees with the ACGIH’s reasoning in this matter and is establishing limits in the final rule of 10 mg/m³ (total particulate) and 5 mg/m³ (respirable fraction) for temephos. The Agency concludes that these limits will protect workers from the significant risk of cholinesterase inhibition and reduction in carboxylesterase activity, which together constitute material health impairments within the meaning of the Act and are potentially associated with exposure to this substance.