Occupational exposure to hydrogen sulfide (H2S) and the medical management of H2S-associated toxicity remains a problem in the sour gas industry and some other industrial settings. The acute effects of exposure to H2S are well recognized, but accurate exposure-response data are limited to acutely lethal effects, even in animal studies. Odor followed by olfactory paralysis and keratoconjunctivitis are the characteristics effects of H2S at lower concentrations. H2S-induced acute central toxicity leading to reversible unconsciousness is a "knockdown"; it is controversial whether repeated or prolonged knockdowns are associated with chronic neurologic sequelae but the evidence is suggestive. Knockdowns can be acutely fatal as a consequence of respiratory paralysis and cellular anoxia. Pulmonary edema is also a well-recognized acute effect of H2S toxicity. Human studies of sublethal exposure with satisfactory exposure assessment are almost nonexistent. There are indications, poorly documented at present, of other chronic health problems associated with H2S exposure, including neurotoxicity, cardiac arrhythmia, and chronic eye irritation but apparently not cancer. Rigorous and comprehensive studies in the sour gas industry are difficult, in part because of confounding exposures and uncertain end points.