Limited data was found for sulfurous acid. Two similar structures were evaluated: sulfuric acid (H2SO4, CASRN: 7664-93-9) and sulfur dioxide (SO2, CASRN: 7746-09-5). AEGL values are published for sulfuric acid (interim) and sulfur dioxide (final). According to reputable academic sources, sulfurous acid most often appears as its conjugate base in solution (HSO3-,) and is detectable in the gas phase. The anhydrous form of sulfurous acid is sulfur dioxide. In its aqueous form, this conjugate base has been detected to have the following equilibrium:
SO_2+H_2 O↔〖HSO〗_3^- +H^+
Given that sulfurous acid exists in equilibrium in solution and has been detected in the gas phase, it is most likely to be available as its decomposition product sulfur dioxide in an uncontrolled release event. The chemical basis above suggests a sulfur dioxide basis is justified, therefore published AEGL values for sulfur dioxide are recommended for TEEL values:
TEEL-1 = AEGL-1, 0.20 ppm (60-minute, sulfur dioxide)
TEEL-2 = AEGL-2, 0.75 ppm (60-minute, sulfur dioxide)
TEEL-3 = AEGL-3, 30 ppm (60-minute, sulfur dioxide)
The molecular weight of sulfurous acid was used for ppm calculations because it is the chemical of interest.
This review generated TEEL values inconsistent with those published in Rev. 29A due to a different basis and application of current TEEL methodology (Section 3.2.2), specifically that it is not appropriate to use compound adjustment factors on AEGLs. In Rev. 29A TEEL development, the values were molecular weight adjusted AEGLs for sulfuric acid. This review considered: the sulfurous acid liquid equilibrium products, the interim status of sulfuric acid AEGLs, and conservatism in developing the recommendation for a different basis for TEEL development.