IRIS Toxicological Review and Summary Documents for Hydrogen Sulfide (External Review Draft)
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Hydrogen sulfide (H2S) is a colorless gas with a strong odor of rotten eggs. Its primary uses include the production of elemental sulfur and sulfuric acid, the manufacture of heavy water and other chemicals. Occupational exposure occurs primarily from its presence in petroleum, natural gas, soil, sewer gas and as a byproduct of chemical reactions. Case reports involving accidental overexposure to H2S have resulted in neurotoxicity, pulmonary edema, respiratory paralysis and, in many instances, death. Levels associated with incapacitation and death may occur around 500 ppm (695 mg/m3) and above. Limited epidemiological studies have described cardiovascular, pulmonary, and ocular effects. The lack of adequate monitoring data preclude the identification of more precise cause-effect levels for acute or chronic exposure scenarios.
Subchronic inhalation of H2S has been shown to mainly affect the nasal mucosa in laboratory animals and been reported to increase seminiferous tubular degeneration and epididymal changes in rats exposed to 80 ppm (111 mg/m3) H2S. Three subchronic animal studies were considered for derivation of an inhalation RfC: the study by Brenneman et al. (2000) was considered to be the best study for derivation of an inhalation RfC. Nasal lesions were identified as the critical effects. Application of the RfC methodology yielded a NOAEL of 10 ppm (14 mg/m3) and a LOAEL of 30 ppm (42mg/m3). The LOAEL and NOAEL were then converted to continuous exposure by means of a dosimetric adjustment factor derived from the regional gas dose ratio (RGDR) for H2S in the extrathoracic (ET) region. The animal NOAEL is then multiplied by the RGDRET to yield the NOAELHEC. The RfC was derived by dividing the NOAELHEC for nasal effects by a total uncertainty factor of 300, yielding an inhalation RfC of 0.002 mg/m3 or 1 ppb. No data pertaining to the potential carcinogenicity of H2S were identified.
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