METRIBUZIN

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: 21087-64-9; Chemical Formula: C₈H₁₄N₄OS

OSHA has not formerly regulated exposure to metribuzin. The proposed PEL was 5 mg/m³. NIOSH (Ex. 8-47, Table N1) concurred with the proposal, and the final rule adopts this limit. The ACGIH has recommended a TLV-TWA of 5 mg/m³ for this substance. Metribuzin is a crystalline solid.

Metribuzin is a herbicide that has a low order of acutetoxicity; single exposures to high concentrations produce central nervous system depression, and repeated high doses affect the thyroid and liver function (Deutsche Forschungsgemeinschaft 1981, as cited in ACGIH 1986/Ex.1-3, p. 411). The oral LD(50) in rats has been reported to be 2000 mg/kg;in cats and rabbits, the LD(50) is as high as 500 mg/kg. A four-hour aerosol exposure at concentrations of between 860 and 892 mg/m³was tolerated by rats and mice; no skin or eye irritation was observed in rabbits. No sensitizing effects were seen in guinea pigs, and a skin application of the 70-percent wettable powder of 1000 mg/kg per day for three weeks produced no effects in rats (Deutsche Forschungsgemeinschaft 1981, as cited in ACGIH 1986/Ex. 1-3, p. 411)

Inhalation studies have shown no adverse effects in rats exposed to 31 mg/m³ of the aerosol for six hours/day, five days/week during a three-week period (Bayer 1981, as cited in ACGIH 1986/Ex. 1-3, p.411). No carcinogenic effects were observed in rats and mice fed 20, 800,or 3200 ppm for two years (Kimmerle 1982a, as cited in ACGIH 1986/Ex.1-3, p. 411). A no-effect level of 100 ppm was observed in a two-year dietary study of rats and dogs (Deutsche Forschungsgemeinschaft 1981, as cited in ACGIH 1986/Ex. 1-3, p. 411); these same investigators observed no teratogenic, embryotoxic, or reproductive effects in rats or rabbits. In Chinese hamsters and mice, no mutagenic activity was observed (Siebert and Lemperle 1974/Ex. 1-689).

No human poisonings caused by metribuzin have been reported. In oral long-term studies, the highest no-observed-effect levels (NOELs) were 2.5 to 5 mg/kg per day (ACGIH 1986/Ex. 1-3, p. 411). Singleand repeated patch tests in humans did not cause irritation orsensitization (Deutsche Forschungsgemeinschaft 1981, as cited in ACGIH 1986/Ex.1-3, p. 411). Except for NIOSH’s concurrence with this limit (Ex. 8-47,Table N1), no comments were received on metribuzin.

In the final rule, OSHA is establishing an 8-hour TWA PEL of5 mg/m³ TWA for metribuzin. The Agency concludes that this limit will protect workers against the significant risks of metabolic and central nervous system effects, which are material impairments of health that are potentially associated with workplace exposure to metribuzin at the levels permitted by the absence of any OSHA limit.