The effects of operating unvented appl i ances and opening windows on indoor pollutant levels and air exchange rates are being studied under the sponsorship of the Gas Research Institute. The study is being conducted i n an instrumented, well-characterized bilevel house located near Washington, D.C. Air leakage due to window openings is characterized by pressurization measurements and the air exchange increment is characterized through tracer gas measurements. Two unvented space heaters, one radiant and the other convective, are operated singly and in combination with a gas cooking range. When an upstairs heater is operating, there is rapid horizontal mixing of pollutants but little penetration to the downstairs area. When a downstairs heater is operating, there is rapid mixing throughout the entire indoor space. Results to date also indicate that effect of opening windows on pollutant levels and air exchange is quite variable. Simultaneous operation of two or more unvented combustion appliances results in peak concentrations that are greater than what would be expected by summing the peaks for each single-source case.