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: 1317-35-7; Chemical Formula: Mn3O4

OSHA previously had no exposure limit for manganese tetroxide (compound and fume). The ACGIH recommends a TLV-TWA of 1 mg/m3, measured as manganese, for this brownish-black powder and its dust and fume. The Agency proposed a PEL of 1 mg/m3 TWA for manganese tetroxide, measured as Mn, and the final rule establishes this limit. Ferromanganese fume has been determined by X-ray diffraction analysis to consist primarily of manganese tetroxide.

Findings from a Russian study indicated that intratracheal suspensions of manganese oxide, manganese dioxide, and manganese tetroxide particles (particle size less than 3 um) produced pneumonitis and other similar pulmonary effects in rats (Levina and Robachevskiau 1955/Ex. 1-1041). These investigators also determined that manganese tetroxide has a greater toxicity than do the lower oxides of manganese and that freshly prepared oxides were more potent than those stored for six months to one year.

Two cases of manganese fume poisoning were reported in a plant where concentrations were between 2.7 and 4.7 mg/m3 (Whitlock, Amuso, and Bittenbender 1966/Ex. 1-455), but other investigators have questioned these air sampling results and believe that exposures to manganese tetroxide concentrations of 5 mg/m3 or less cause no harmful effects (Whitman and Brandt 1966/Ex. 1-1103). In a seven-year study, Smyth and co-workers (1973/Ex. 1-990) investigated chronic manganese poisoning in workers exposed to both ferromanganese fumes and dust. Five of 71 employees suffered from chronic manganism; of these five cases, three resulted from fume exposure and two from dust exposure. Two of the three fume-exposure victims were exposed over a five-year period to an estimated average ferromanganese concentration of 13.3 mg/m3; however, the third victim worked in an operation where air concentrations of manganese were less than 1 mg/m3, which suggests that certain individuals may be hypersusceptible to manganese poisoning. The dust-exposed victims worked in areas where air concentrations were in the range of 30 to 50 mg/m3 throughout the study period (Smyth, Ruhf, Whitman, and Dugan 1973/Ex. 1-990).

Martonik (1976, as cited in ACGIH 1986/Ex. 1-13, p. 357) reported that the fume of manganese has greater toxicity than does the dust. During a two-year period, at least one case of acute manganese poisoning was documented at a fume concentration level of 7.5 mg/m3, and another case at the same welding operation may also have involved manganism.

OSHA received two comments on this substance, one from NIOSH (Ex. 8-47; Tr. p. 3-86), and one from Chemetals, a manganese manufacturer (Ex. 3-189). NIOSH (Ex. 8-47, Table N2) does not concur with the limits being established by OSHA. NIOSH (Ex. 8-47, Table N2) notes that, based on the results of the Smyth and co-workers study (1973/Ex. 1-990), the 1-mg/m3 PEL being established by OSHA “may not be protective, especially to the potentially sensitive individual.” In response to this NIOSH comment, OSHA states that the Agency intends to monitor the literature on manganese tetroxide closely in the future to determine whether the new limit for this substance is adequately protective.

Chemetals (Ex. 3-189) asked OSHA to promulgate separate limits for the dust and fume of manganese tetroxide based on the relative toxicities of these two particulate forms. OSHA recognizes that some information in the literature (including some discussed above) points to the greater toxicity of the fume and that fumes are generally the more toxic form of particulate. However, the Agency notes that intratracheal suspensions of manganese tetroxide dust caused pneumonitis and other pulmonary effects in Russian workers (Levina and Robachevskiau 1955/Ex. 1-1041) and that several cases of manganism have been caused by dust exposure (Smyth, Ruhf, Whitman, and Anger 1973/Ex. 1-990). The Agency believes it prudent not to distinguish at this time between the dust and the fume but to set the TWA PEL at a level that will protect against the effects of exposure to both forms of particulate.

OSHA is establishing a 1-mg/m3 8-hour TWA for manganese tetroxide (compound and fume). The Agency concludes that this limit will provide protection against the significant risks of material health impairment in the form of chronic manganese poisoning, pneumonitis, and other respiratory effects that are associated with exposure to manganese tetroxide at levels above 1 mg/m3.

Page last reviewed: September 28, 2011