OSHA's former Z tables had no limit for thionyl chloride. The ACGIH has established a ceiling limit of 1 ppm for this substance. The proposed ceiling was 1 ppm, and the final rule establishes this limit. NIOSH (Ex. 8-47, Table N1) concurs. Thionyl chloride is a colorless to pale yellow liquid with a suffocating odor.
Thionyl chloride vapors are skin, eye, and mucous membrane irritants, probably because they form sulfur dioxide and hydrogen chloride on contact with moisture (ACGIH 1986/Ex. 1-3, p. 572). An inhalation exposure of 17.5 ppm proved lethal to cats within 20 minutes (Sax 1979/Ex. 1-866).
The ACGIH's exposure limit for thionyl chloride is based on the exposure limits for the decomposition products (hydrogen chloride and sulfur dioxide) of thionyl chloride when mixed with water. The reaction of one mole of thionyl chloride with water produces two moles of hydrogen chloride and one of sulfur dioxide, so that 1 ppm of thionyl chloride can be shown to produce a total irritant gas concentration of 3 ppm. The exposure limit for hydrogen chloride is 5 ppm as a ceiling limit; for sulfur dioxide, the limit is a TWA of 2 ppm. Thus, "the...ceiling limit of 1 ppm for thionyl chloride should prevent the irritant effects of its reaction products" (ACGIH 1986/Ex. 1-3, p. 572). No comments, other than NIOSH's, were received on this substance.
In the final rule, the Agency is establishing a ceiling limit of 1 ppm for thionyl chloride on the basis of analogy to the irritation potential of hydrogen chloride and sulfur dioxide. OSHA concludes that this limit will protect workers from the significant risk of irritation of the eyes, skin, and mucous membranes, which constitutes a material health impairment that is associated with exposure to this substance at levels above the new PEL.