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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.


OSHA's former 8-hour TWA limit for hexone (methyl isobutyl ketone), or MIBK, was 100 ppm. The ACGIH has established a TLV-TWA of 50 ppm and a 15-minute STEL of 75 ppm for this substance. NIOSH recommends a TWA of 50 ppm for MIBK, which is a clear liquid with a characteristic ketone odor. OSHA proposed a 50-ppm 8-hour TWA and a 75-ppm STEL, and the final rule establishes these limits. NIOSH (Ex. 8-47, Table N1) concurred with the Agency's selection of these limits.

A four-hour exposure to 4000 ppm MIBK killed all exposed rats, but a similar exposure to 2000 ppm was not fatal to these animals (Smyth, Carpenter, and Weil 1951/Ex. 1-439). Guinea pigs exposed to a MIBK concentration of 10,000 ppm immediately showed signs of irritation (Specht, Miller, Valaer, and Sayers 1940/Ex. 1-1179).

MacEwen, Vernot, and Haun (1971/Ex. 1-194) exposed rats, mice, dogs, and monkeys to 100 or 200 ppm MIBK for two weeks and noted no signs of intoxication; however, rats exposed to 100 ppm had heavier kidneys and higher kidney-to-body-weight ratios, and, at 200 ppm, livers were heavier as well. Postmortem examination revealed nephrosis of the proximal tubules.

The same authors (MacEwen, Vernot, and Haun 1971/ Ex. 1-194), exposed rhesus monkeys, dogs, and rats continuously for 90 days to MIBK concentrations of 100 ppm. These authors observed no significant changes in clinical chemistry or blood test results, although the rats had heavier kidneys and livers, reversible hyaline droplet degeneration of the proximal tubules of the kidneys, and some necrosis of the tubules.

Silverman, Schulte, and First (1946/Ex. 1-142) determined that the maximum dose of MIBK tolerable to human volunteers for eight hours was 100 ppm; at 200 ppm, these subjects found the odor of MIBK objectionable and the vapor irritating. Linari and co-workers (1964/Ex. 1-1159) reported that more than half of all workers exposed to 500 ppm of MIBK for 20 to 30 minutes daily, and perhaps to 80 ppm for the remainder of the shift, experienced weakness, loss of appetite, headache, burning eyes, nausea, vomiting, and sore throat; several of these workers also reported insomnia, somnolence, heartburn, and intestinal pain. Some workers had enlarged livers and others had colitis. Clinical test results on these workers were normal (Linari, Perrelli, and Varese 1964/Ex. 1-1159).

In a follow-up study on this same group of centrifuge operation workers, Armeli and co-workers (1968/Ex. 1-1028) determined that reduction of MIBK levels (during the 15 to 30 minutes of centrifuge operation) to 100 to 105 ppm, and (for the remainder of the shift) to 50 ppm had also significantly reduced the symptomatology reported earlier by these workers. However, liver enlargement persisted in two workers, and a few workers continued to report gastrointestinal and nervous system effects (Armeli, Linari, and Martorano 1968/Ex. 1-1028).

Elkins (1959f/Ex. 1-734) noted that exposure to 100 ppm during boot-waterproofing operations caused workers to develop headache and nausea; another similarly exposed group experienced only irritation at 100 ppm.

The AFL-CIO (Ex. 194) commented on MIBK. The AFL-CIO supports the limits OSHA has established for this substance in the final rule.

In the final rule, OSHA is establishing an 8-hour TWA of 50 ppm and a 15-minute STEL of 75 ppm for hexone. The Agency concludes that these limits will work together to protect workers from the significant risks of headache, nausea, and irritation, as well as the potential kidney and liver effects that constitute material health impairments that are associated with exposures to hexone above the revised PELs.