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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: 5124-30-1; Chemical Formula: C15H22N2O2

OSHA had no former limit for methylene bis-(4-cyclo-hexylisocyanate). Prior to 1988, the ACGIH had a TLV ceiling of 0.01 ppm for this alicyclic diisocyanate compound. OSHA proposed a ceiling of 0.01 ppm, and NIOSH (Ex. 8-47, Table N1) supported the proposal. The final rule establishes that limit. OSHA notes that ACGIH adopted a new limit for this substance in 1988 of 0.005 ppm TWA. The NIOSH RELs for methylene bis-(4-cyclohexylisocyanate) are a 0.005-ppm 10-hour TWA and a 0.02-ppm 10-minute ceiling.

Methylene bis-(4-cyclohexylisocyanate) is a pulmonary, skin, and eye irritant. The oral LD(50) in rats is 9.9 g/kg. A 5-percent solution applied to the skin of guinea pigs produced strong erythema and edema, and rabbits treated with 0.1 mg showed severe skin reactions (Younger Laboratories 1965, as cited in ACGIH 1986/Ex. 1-3, p. 392).

Rats inhaling a lethal concentration of 20 ppm for five hours exhibited marked respiratory irritation, tremors, and convulsions during exposure, and their lungs revealed severe congestion and edema after death (E.I. du Pont de Nemours and Co. Inc. 1976, as cited in ACGIH 1986/Ex. 1-3, p. 392). Repeated inhalation exposure at 0.4 ppm produced initial weight loss in rats; exposure at 1.2 ppm caused respiratory irritation and decreased growth (E.I. du Pont de Nemours and Co. Inc. 1978, as cited in ACGIH 1986/Ex. 1-3, p. 392). Guinea pigs exposed to 0.12 ppm and mice exposed to 0.65 ppm did not exhibit dermal sensitivity (Stadler and Karol 1984/Ex. 1-612). Unlike toluene diisocyanate, which is a sensory irritant, methylene bis(4-cyclohexylisocyanate) depresses respiration by producing pulmonary irritation; for example, an exposed mouse showed a 50-percent decrease in respiration rate, along with lung irritation, when exposed to 3.7 ppm of this substance (Weyel and Schaffer 1985/Ex. 1-581).

Human exposures to this compound have resulted in skin sensitization but only infrequently in pulmonary sensitization (Emmett 1976/Ex. 1-552; Israeli, Smirnov, and Sculsky et al. 1981/Ex. 1-701).

NIOSH (Ex. 150, Comments on Methylene Bis-(4-Cyclohexyliso-cyanate)) notes that both the REL and TLV for this substance have been based on the toxicological properties of toluene diisocyanate (TDI) and that "a recent study by NTP (1986a) of chronic effects in animals has produced evidence that cancer is associated with exposure to commercial grade TDI...and to a TDI hydrolysis product, 2,4-TDA...treatment of rats and mice of both sexes by gavage to commercial grade TDI resulted in tumor induction, primarily in the pancreas and liver in male and female rats, and in female mice. The tumorigenic responses observed in both rats and mice treated with TDI meet the criteria of the OSHA cancer policy (29 CFR 1990) for classifying a substance as a potential occupational carcinogen." NIOSH suggests that the recommended RELs (0.005 ppm TWA and 0.02 ppm 10-minute ceiling) be considered as an interim level to be applied to methylene bis-(4-cyclohexylisocyanate) until adequate testing information is available. The AFL-CIO (Ex. 194) supported OSHA's proposed ceiling limit for this substance.

OSHA believes that a ceiling limit of 0.01 ppm is as protective as a 0.005-ppm TWA; the Agency therefore is establishing a ceiling limit of 0.01 ppm for methylene bis-(4-cyclohexylisocyanate). The Agency concludes that this limit will protect workers against the significant risk of eye, skin, and pulmonary irritation potentially associated with occupational exposures to this substance at the levels formerly permitted by the absence of an OSHA limit. The Agency considers these irritant effects caused by exposure to methylene bis-(4-cyclohexylisocyanate) to be material impairments of health.


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