To assess potential worker health hazards, the air in confinement structures on 21 randomly selected swine producing farms in Iowa was evaluated for selected gases. The gases measured (mean concentration) included ammonia (34 ppm), hydrogen sulfide (1.4 ppm), carbon monoxide (9.1 ppm) and carbon dioxide (1640 ppm). Ammonia (compared to other gases) most commonly exceeded the Threshold Limit Value (TLV); however, it was common to find buildings with several gases in excess of the TLV. Buildings housing younger animals were more likely to have hazardous gas levels than buildings housing older animals. This complex mixture of gases represents a potential health hazard to an estimated 400,000 individuals who work in swine confinement buildings. These data suggest that research is needed to control these exposures affecting a large number of workers.