In February 2004, the City of Liberal, Kansas, asked the National Institute for Occupational Safety and Health (NIOSH) for help in evaluating carbon monoxide (CO) exposures of employees that operate two euthanasia chambers at the city-run animal shelter. The Kansas Animal Health Department suspended use of the unvented chambers during a licensing inspection. The City requested NIOSH assistance in measuring employee CO exposure during operation of the chambers and modifying the chambers so that they could be again be used in a manner that would be acceptable to the Kansas Animal Health Department. In March 2004, NIOSH responded to the request by using direct-reading monitors to measure CO near the two chambers and in the breathing zone of workers that operated the empty chambers specifically for this investigation. (Operation of the chamber for this investigation was authorized by the licensing organization.) Past and present employees were interviewed about methods used to operate the chambers and symptoms experienced when the chambers were operated. NIOSH also gathered the following information: (1) details about the death of a Tennessee animal shelter worker during operation of a similar chamber; (2) the extent of use of CO euthanasia chambers across the United States; (3) policies and guidelines of the American Veterinary Medical Association (AVMA), the Humane Society of the United States (HSUS), and the National Animal Control Association (NACA) related to the appropriate use of CO for euthanasia in animal shelters; and (4) classification of such chambers as a "confined space" by NIOSH and the Occupational Safety and Health Administration (OSHA). When CO was introduced into the chamber, CO concentrations near the chamber rapidly exceed the NIOSH ceiling limit of 200 parts per million (ppm). The NIOSH Immediately Dangerous to Life and Health value of 1,200 ppm was exceeded in one instance. Peak CO concentrations in the general area during CO introduction were 800 ppm to 950 ppm, and greater than 1,200 ppm (the maximum range of the sampling instruments). Slowing the delivery rate of CO resulted in much lower concentrations near the chamber. When the chambers were opened, CO concentrations in the general area of the chambers ranged from 400 ppm to >1,200 ppm for several minutes. CO concentrations inside the chambers remained above 1,200 for an undetermined length of time. Employee exposures during this investigation were impacted by the fact that normal tasks were not conducted, and also that employees appropriately removed themselves from exposure when their CO monitors alerted them to the severity of the hazard. During two occasions of chamber operation, employees were exposed to maximum CO concentrations of 380 ppm and 945 ppm before they were able to move to safer locations. NIOSH noted several other health and safety deficiencies. There were no confined space entry procedures. Respiratory protection was not available. Operating procedures were undefined. There was no hazard communication program. There was no program for employee training. There were no warning signs related to the CO hazard. There was no emergency action plan. The employer had not assessed the workplace to determine if hazards were present. The NIOSH investigator concluded that the use of homemade CO chambers, such as the one investigated here, presents an unacceptable health risk to animal shelter employees. Suggestions for reducing this risk are included in the Recommendations section of this report. Modification of the existing chambers was not an acceptable control method, and thus no recommendations were provided in that regard.