The use of isolation helmets has gained popularity as a method of possible protection of the operating-room personnel from diseases that can be transmitted during operative procedures. However, the use of these systems has been associated with a variety of symptoms, including fatigue, diaphoresis, nausea, headache, and irritability. These symptoms have often been attributed to the mental stress of the operative procedure or the physical discomfort of the helmet. As far as we know, no manufacturers include the measured levels of carbon dioxide or the rate of air exchange of their helmet system. A possible common cause of discomfort with helmet systems is the level of carbon dioxide to which the person wearing the device is exposed. We measured the levels of carbon dioxide in four helmet systems from three different manufacturers during light exercise designed to approximate the exertion during an orthopaedic operation. All but one unit failed to meet the exposure limits recommended by the National Institute for Occupational Safety and Health and the Occupational Safety and Health Administration regarding exposure to carbon dioxide. One unit, the Stackhouse Freedom Aire self-contained system, did meet these standards, but the levels of carbon dioxide in this helmet were more than 1000 per cent greater than the ambient levels in air (440 parts per million compared with 4939 parts per million). Isolation systems must be evaluated carefully not only for comfort but also for the physiological effects caused by exposure to elevated levels of carbon dioxide. Operating-room personnel who use such systems should be aware that many of the physical symptoms that they experience may be associated with elevated levels of carbon dioxide.