OSHA formerly had no limit for exposure to manganese cyclopentadienyl tricarbonyl (MCT). The ACGIH has a TLV-TWA of 0.1 mg/m3 (measured as manganese), with a skin notation. The Agency proposed, and the final rule establishes, a permissible exposure limit of 0.1 mg/m3 TWA (measured as manganese), with a skin notation, for this substance. NIOSH (Ex. 8-47, Table N1) concurs that these limits are appropriate.
A Russian study reported that a single two-hour exposure to MCT at 120 mg/m3 was fatal to 80 percent of albino rats, although rabbits, guinea pigs, and rats survived a single two-hour exposure at 20 to 40 mg/m3. Chronic exposure of rats for 11 months at levels averaging 1 mg/m3 for four hours daily showed delayed effects (seven months from onset of exposure) of neuromuscular excitability, evidence of kidney damage, and decreased resistance to infection (Arkhipova, Tolgskaya, and Kochetkova 1965/Ex. 1-1046). The tails of 10 white mice were dipped in a gasoline mixture containing 1 gram MCT per 100 ml; a second group of mice had their tails immersed in gasoline without MCT. An equal number of fatalities were observed in the gasoline plus MCT and gasoline only groups after four or five two-hour applications, and all tails exhibited necrosis. The authors concluded that these effects were caused by the gasoline and not by the MCT (Arkhipova, Tolgskaya, and Kochetkova 1965/Ex. 1-1046). Further studies in rabbits showed that MCT applied dermally as an oil emulsion caused irritation of the skin. These authors also investigated the dermal toxicity of MCT solutions in tetrahydrofuran versus solutions of tetrahydrofuran in oil. All animals whose tails had been dipped in the hydrofuran solution of MCT died within an hour, while animals whose tails had been dipped in pure tetrahydrofuran did not (Arkhipova, Tolgskaya, and Kochetkova 1965/Ex. 1-1046). The same authors concluded that MCT is toxic at low concentrations, has cumulative properties, affects the nervous system, is irritating to the skin, and causes early histological changes in the respiratory tract.
More recent reports describe MCT-induced pulmonary edema and convulsions in the rat (Penney, Hogberg, Traiger, and Hanzlik 1985/Ex. 1-431). The ED(50s) for convulsions were 32 mg/kg orally and 20 mg/kg intraperitoneally; LD(50s) were 24 mg/kg orally and 14 mg/kg intraperitoneally. Necrosis of the bronchiolar tissue and pulmonary parenchymal damage were seen in mice and rats given intraperitoneal doses (Haschek, Hakkinen, Witschi et al. 1982/Ex. 1-1083). No comments other than NIOSH's were received on MCT.
OSHA has concluded that occupational exposure to MCT poses a risk of neuropathic effects, kidney damage, skin irritation, pulmonary edema, and tissue damage, which together constitute material health impairments. The Agency is therefore establishing an 8-hour TWA PEL of 0.1 mg/m3 for manganese cyclopentadienyl tricarbonyl, with a skin notation, to protect workers against the significant risk of these effects, which have been shown to occur at levels above the new standard.