METHYL MERCAPTAN

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: 74-93-1; Chemical Formula: CH₃SH

OSHA previously had a ceiling limit of 10 ppm for methyl mercaptan. Based on the ACGIH recommendation, the Agency proposed revising this limit to an 8-hour TWA of 0.5 ppm, and OSHA is establishing this limit in the final rule. Methyl mercaptan is a flammable, water-soluble gas with a disagreeable odor like that of rotten cabbage.

Methyl mercaptan acts on the respiratory center, producing death by respiratory paralysis. DeRekowski (1893, as cited in ACGIH 1986/Ex. 1-3, p. 405) and Frankel (1927/Ex. 1-1033) have reported that the acute toxicity of methyl mercaptan is similar to but somewhat lower than that of hydrogen sulfide; however, Ljunggren and Norberg (1943/Ex. 1-916) have concluded that the two substances exhibit toxicities of the same magnitude. Pulmonary edema results from exposures to lower, less acute concentrations of methyl mercaptan (Fairchild, personal communication, as cited in ACGIH 1986/Ex. 1-3, p. 405).

Inhalation of (an unspecified concentration of) methyl mercaptan produced coma and death in one worker; acute hemolytic anemia and methemoglobinemia developed after this exposure (Schultz, Fountain, and Lynch 1970, as cited in ACGIH 1986/Ex. 1-3, p. 405). A 1918 report by Pickler (as cited by E.E. Sandmeyer in Clayton and Clayton 1981) describes the accidental exposure (for several hours) of 28 students to a concentration of methyl mercaptan estimated at 4 ppm. The individuals had headache and nausea, and one student showed some liver involvement, demonstrated by the appearance of epithelial cells, protein, and erythrocytes, in the excretion fluid. This condition subsided in six weeks (Sandmeyer 1981).

John L. Festa, Director of Chemical Control and Health Programs at the American Paper Institute, Inc. (Ex. 3-685) objected to OSHA’s proposal for methyl mercaptan, stating that the basis for the ACGIH TLV, from which the OSHA proposal was derived, was not to reduce irritant effects but to limit odor intensity. He commented further that regulating substances on the basis of “unpleasant sensory stimuli…embarks upon a new precedent” (Ex. 3-685, p. 3). Mr. Festa reported that the odor of methyl mercaptan is relatively faint at 0.05 ppm, although the substance may be mildly irritating over long periods of exposure to concentrations of 4 to 5 ppm.

OSHA does not agree with the suggestion made by Mr. Festa that the effects associated with exposure to methyl mercaptan below 10 ppm (the previous OSHA limit) do not warrant attention. First, Mr. Festa acknowledges that prolonged exposure to 4 to 5 ppm causes irritation; as discussed earlier in this section, OSHA has determined that sensory irritation constitutes material impairment of health. Furthermore, a single inhalation exposure to 7.9 ppm has been reported to result in nauseating odor (NIOSH 1978b, as cited in ACGIH 1986/Ex. 1-3 p. 405); clearly, this effect adversely affects the performance and functional capacity of employees. OSHA is also concerned about the possible liver effects that were reported from a single exposure to approximately 4 ppm methyl mercaptan (Pickler 1918, as cited by E.E. Sandmeyer in Clayton and Clayton 1981). Although this report is dated, OSHA has found no evidence that comprehensive studies have been undertaken in humans to examine the potential for liver or other organ damage as a result of long-term exposure to low levels of methyl mercaptan. Liver and other organ defects have been reported to occur in animals exposed to 50 ppm for only 90 days. Because of these considerations, OSHA concludes that a significant risk of acute sensory effects, as well as possible organ damage, exists at the former 10-ppm ceiling, and that a 0.5-ppm limit is necessary to ensure that these significant risks are adequately reduced. NIOSH (Ex. 8-47, Table N7) recommends a ceiling limit at the same 0.5 ppm level. OSHA is revising its limit for methyl mercaptan to 0.5 ppm as an 8-hour TWA, and this limit is promulgated in today’s rule.