The performance characteristics of a personal data logger for carbon- monoxide (630080) (CO) were evaluated. The monitor evaluated was the Draeger 190 data logger CO monitor which had been widely used by NIOSH and OSHA. It had dimensions of 124 by 60 by 25 millimeters and weighed approximately 200 grams. It had a digital liquid crystal display which cycled between current CO concentration, the time weighted average (TWA) concentration, and the peak concentration for any measurement cycle. CO concentrations were displayed in parts per million (ppm). Data could be stored, as 1 minute averaged CO concentrations, for up to 12 hours. The monitor had to be reset manually to start a measurement and data storage cycle. After a measurement cycle was completed, the stored data could be transferred to an IBM compatible computer. The monitor was tested in an instrument exposure chamber containing O to 950ppm CO. Temperatures and relative humidity (RH) in the chamber were 0, 25, and 40 degrees-C and 0, 50, and 100%. The monitor responded linearly to O to 950ppm CO. When exposed to 20 to 100ppm in repeated cycles, the monitor showed less than 3% variation in its responses. When it was similarly exposed to 57ppm CO for 3 months, the relative coefficient of variation in the readings was only 9%. When tested against 25 or 250ppm CO, the monitor reached 90% of its final response within 20 seconds. The responses of the monitor were not affected by changes in RH. Its responses increased with increasing temperature, the increased responsiveness being greater at higher CO concentrations. Exposure to potential interferents such as nitrogen oxides, hydrogen-sulfide, sulfur-dioxide, acetylene, ethylene, hydrogen, and hair spray showed that the effects of nitrogen oxides, hydrogen-sulfide, and sulfur-dioxide could be removed by using appropriate filters. Interference from the hydrocarbons and hydrogen could only be partially removed. Hair spray caused a large interference when sprayed directly on the monitor. The authors conclude that the monitor can provide warning of high CO exposures as well as measure TWA exposures over a work shift. It is subject to some interferences including hair spray which should not be used when the monitor is used.