We monitored sulfur dioxide (SO2) and carbon monoxide (CO) concentrations in 14 suburban homes while modern kerosene heaters were operating. Mean concentrations were 7 parts per million (PPM) CO and 0.4 PPM SO2. Carboxyhemoglobin levels were significantly increased (p less than 0.0001), but performance on spirometric tests was not worsened by exposure to heater emissions in the 29 healthy residents of these homes. Our monitoring data agrees closely with predictions based on computer modeling and data from an environmental chamber study. Although we found no definite hazard for normal individuals, the concentration of either SO2 or CO exceeded standards for outdoor air quality in 8 of the 14 homes. Moreover, the SO2 concentrations we measured are sufficient to induce bronchospasm in some asthmatics. People with asthma should be informed of this risk.