Hydrogen sulfide (H2S) is a toxic gas generated in significant quantities from a variety of industrial processes, including wastewater treatment. Acute exposures to high levels of H2S (> 1000 ppm) have been fatal in many instances. Conflicting and sometimes inconclusive studies regarding the health effects associated with low level (<20 ppm) chronic exposure to H2S have led to increased interest in H2S both as an occupational and an ambient air pollutant. Very limited quantitative data exist on the longer-term low-level H2S exposures likely to be encountered everyday by workers in the wastewater treatment industry. By characterizing H2S exposures during specific tasks in four large and four smaller waste treatment facilities over an extended period, the study will collect information needed to conduct future risk assessments or epidemiological studies in order to characterize the health risks, if any, that may be involved in exposure to low concentrations of hydrogen sulfide. The study will also identify factors associated with potential high-level exposures.