Histologic and biochemical changes have been seen in the respiratory system after acute toxic exposures to H2S (for review, see ATSDR 1999a). The effects of lower concentrations of intermediate or chronic exposure are not as clear.
The Chemical Industry Institute of Toxicology (CIIT 1983a, 1983b, 1983c) exposed Fisher-344 rats, Sprague Dawley rats, and B6C3F mice to H2S at a time-weighted average concentration of 10.1, 30.5, or 80 ppm for 6 hr/day 5 days/week for 90 days. No treatment-related effects on the cardiovascular system, the gastrointestinal system, immune function, the renal system, the hepatic system, bone marrow or bones, or the hematologic system were seen. Body weight was decreased by 10% in female Sprague Dawley rats in the 80-ppm dose group, and inflammation of the nasal mucosa was seen in mice at that dose. CIIT also looked at behavior and neuropathology end points and reproductive effects; no effects on any of those end points were seen.
Curtis et al. (1975) exposed pigs to H2S at 8.5 ppm for 24 hr/day for 17 days; no effects on the gastrointestinal system were observed.
Hayden et al. (1990) saw an increase in parturition time and difficulty in delivery in 6 of 17 rats exposed to H2S at 20, 50, or 75 ppm but in only one of seven control animals; no statistical analyses were performed. Although few studies have been conducted, H2S does not appear to be genotoxic (ATSDR 1999a). No chronic bioassays have been conducted.
Researchers examined neurohistologic characteristics (Hannah and Roth 1991) and brain amino acid concentrations (Hannah et al. 1989) in rats exposed to H2S from gestational day 5 through postpartum day 21. Some alterations in the architecture and growth characteristics of Purkinje cell dendritic fields were seen after exposure to H2S at 20 ppm (Hannah and Roth 1991) and a decrease in brain amino acid concentrations at 75 but not 50 ppm (Hannah et al. 1989). Data from Skrajny et al. (1992) have also demonstrated altered neurotransmitter concentrations after exposure to H2S. A decrease in norepinephrine concentrations and an increase in serotonin concentrations were seen after exposure of rats to H2S at 20 ppm for 7 hr/day from gestational day 5 to postpartum day 21.
Taken together, the data on H2S do not demonstrate any consistent effects at concentrations that are below those which are acutely toxic.
SO2 is a highly soluble irritating gas that is quickly absorbed in the nose and upper airway and does not reach the lower parts of the respiratory system under resting conditions (ATSDR 1998). Lower parts of the respiratory system can become targets during exercise. Immediate responses have been seen after exposure to SO2 in many controlled human experiments at 5 ppm and above (Costa 2001). Asthmatics appear to be more sensitive to the effects of SO2.
SO2 has been demonstrated to decrease mucociliary clearance. Prolonged exposure of donkeys to SO2 at 102 µg/m3 for 1 hr/day 5 days/week for 6 months (Schlesinger et al. 1979) and rabbits at 250 µg/m3 for 1 hr/day 5 days/week for 4, 8, or 12 months (Gearhart and Schlesinger 1989) caused persistent decreases in mucociliary clearance. Observed effects might have been mediated by a change in the mucus pH, composition, or consistency. Inhalation of SO2 might also have altered mucus production by increasing the number of mucus-secreting cells.