INTRODUCTION Firefighters who become lost, disoriented, or trapped in a burning building may die after running out of air in their self-contained breathing apparatus (SCBA). An emergency escape device has been developed that attaches to the firefighter's mask in place of the SCBA regulator. The device filters out particulate matter and a number of hazardous components of smoke (but does not provide oxygen), providing additional time to escape after the firefighter runs out of SCBA air. OBJECTIVE To field-test the device under realistic fire conditions to 1) ascertain whether it provides adequate protection from carbon monoxide (CO) and 2) examine firefighters' impressions of the device and its use. METHODS A wood-frame house was fitted with atmospheric monitors, and levels of CO, oxygen, and hydrogen cyanide were continuously recorded. After informed consent was obtained, firefighters wearing the escape device instead of their usual SCBA regulators entered the burning structure and spent 10 minutes breathing through the device. A breath CO analyzer was used to estimate (+ or - 3 ppm) each subject's carboxyhemoglobin level immediately upon exiting the building, vital signs and pulse oximetry were assessed, and each firefighter was asked for general impressions of the device. RESULTS Thirteen subjects were enrolled (all male, mean age 42.5 years, mean weight 94 kg). The mean peak CO level at the floor in the rooms where the subjects were located was 546 ppm, and ceiling CO measurements ranged from 679 ppm to the meters' maximum of 1,000 ppm, indicating substantial CO exposure. The firefighters' mean carboxyhemoglobin level was 1.15% (range 0.8%-2.1%) immediately after exit. All pulse oximetry readings were 95% or greater. No subject reported problems or concerns regarding the device, no symptoms suggestive of smoke inhalation or toxicity were reported, and all subjects expressed interest in carrying the device while on duty. CONCLUSION The emergency escape device provides excellent protection from CO in realistic fire scenarios with substantial exposure to toxic gases, and the firefighters studied had a positive impression of the device and its use.