OSHA formerly had no limit for methyl isopropyl ketone (MIPK). The ACGIH has a TLV-TWA of 200 ppm. The proposed PEL was 200 ppm as an 8-hour TWA; NIOSH (Ex. 8-47, Table N1) concurs, and the final rule establishes this limit. Methyl isopropyl ketone is a colorless, flammable liquid.
Animal studies have shown MIPK to have an acute toxicity somewhat greater than that of diethyl ketone and somewhat less than that of di-n-propyl ketone or methyl-n-propyl ketone (ACGIH 1986/Ex. 1-3, p. 405). Rats exposed for four hours at a concentration of 5700 ppm died (NIOSH 1977i, Ex. 1-1182). Other data concerning the inhalation toxicity of MIPK are lacking. Dr. Grace Ziem (Ex. 46) noted that respiratory irritation, headaches, and nausea have been demonstrated to occur in humans at low levels of MIPK exposure.
OSHA establishes in the final rule a limit of 200 ppm TWA for methyl isopropyl ketone. The Agency concludes that this limit will protect workers against the significant risk of irritation, a material health impairment that is associated with exposure to this ketone at levels above the new PEL.