OSHA had no former limit for chlorpyrifos. The ACGIH has a TLV-TWA of 0.2 mg/m3 and a 0.6 mg/m3 STEL, with a skin notation, for this white, crystalline solid. The proposed PELs were an 8-hour TWA of 0.2 mg/m3 and a 15-minute STEL of 0.6 mg/m3, with a skin notation; NIOSH (Ex. 8-47, Table N1) concurs with these limits. The 0.2 mg/m3 8-hour TWA and a skin notation are established in the final rule, but the proposed STEL is not retained.
Chlorpyrifos has an acute oral LD(50) of 135 mg/kg for female rats and 163 for male rats (Windholz 1983b, pp. 309-310, as cited in ACGIH 1986/Ex. 1-3, p. 138). Other sources have reported the acute oral LD(50) as 82 mg/kg in rats and the dermal LD(50) as about 2000 mg/kg for rabbits (Gray 1965/Ex. 1-1151; Gaines 1969/Ex. 1-320).
Chlorpyrifos is an active inhibitor of plasma cholinesterase but has only moderate capacity to reduce red blood cell cholinesterase or to cause cholinergic symptoms and systemic injury (ACGIH 1986/Ex. 1-3, p. 138). Particle inhalation has been shown to cause mild plasma cholinesterase depression in dogs exposed for four hours at the upper end of a 140- to 280-mg/m3 range (Spencer 1968, as cited in ACGIH 1986/Ex. 1-3, p. 138).
Dogs and rats fed 3.0 mg/kg of chlorpyrifos daily for two years showed no adverse effects (FAO/WHO (Food and Agriculture Organization/World Health Organization) 1972, as cited in ACGIH 1986/Ex. 1-3, p. 138). Male and female rats showed no teratogenic or reproductive effects when fed 1.0 mg/kg per day (Dow Chemical Company 1972a, as cited in ACGIH 1986/Ex. 1-3, p. 138).
Workers applying chlorpyrifos as a spray were exposed to 0.5 percent chlorpyrifos emulsion and exhibited a marked decrease in plasma and red cell cholinesterase levels (Eliason, Cranmer, von Windeguth et al. 1969/Ex. 1-633). In five of seven exposed sprayers, this reduction was greater than 50 percent. However, another study showed no ill effects on cholinesterase metabolism when human volunteers were exposed to an ultra-low-volume spray (0.8 um/m3 for three to eight minutes) (Ludwig, Kilian, Dishburger, and Edwards 1970/Ex. 1-563). Human cholinesterase levels appear to be less affected by dermal exposure than do those of rabbits (ACGIH 1986/Ex. 1-3, p. 138). However, human volunteers, administered four repeated dermal doses of 25 mg/kg, applied for 12 hours each, did exhibit depressed plasma cholinesterase levels. Human subjects ingesting 0.03 mg/kg for three weeks showed no cholinesterase effects, but subjects ingesting 0.1 mg/kg demonstrated plasma cholinesterase depression (Dow Chemical Company 1973f, as cited in ACGIH 1986/Ex. 1-3, p. 138).
The American Industrial Hygiene Association (Ex. 8-16; Tr. III, p. 307) urged OSHA to delete the proposed 0.6 mg/m3 STEL for chlorpyrifos because the ACGIH has now deleted the STEL for this substance. OSHA has carefully reviewed the health evidence for a STEL for this substance and has determined, in accordance with the Agency's policy (see Section VI.C.17 of this preamble), that it is appropriate not to include a short-term limit in the final rule.
In the final rule, OSHA is establishing a PEL of 0.2 mg/m3 as an 8-hour TWA, with a skin notation; these limits for chlorpyrifos will protect workers against the significant risk of cholinesterase inhibition caused by exposure to this previously unregulated substance. The skin notation is included in the final rule to prevent the systemic effects that have been demonstrated to occur in humans dermally exposed to chlorpyrifos. OSHA finds that the cholinesterase inhibition and systemic effects associated with exposure to chlorpyrifos constitute material impairments of health.