The heat stress experienced by waste abatement workers was examined. Seven healthy male subjects with experience in waste abatement work exercised on a treadmill while wearing a chemical protective suit and a self contained breathing apparatus. Temperatures inside the testing chamber were 32+/-0.2 (hot), 26+/-0.2 (warm), or 21+/-0.2 degrees-C (neutral). Testing ceased when the subject reached the threshold core temperature (TCT) or 90% of the heart rate (HR) threshold. In the hot environment, five subjects reached the TCT after a mean exercise duration of 41.7+/-10.9 minutes, and two stopped prior to reaching the TCT. After 5 and 20 minutes of exercise in the hot environment, the HR ranged from 95 to 130 beats per minute (bpm) and from 105 to 150bpm, respectively. The mean time required to return to the pretest core temperature was 62.8+/- 3.3 minutes. During the first 10 minutes of resting in a cool environment, core temperatures continued to rise by 0.2 to 0.3 degrees-C. The average weight loss rate was 0.74 kilograms per hour (kg/hr). In the warm environment, two subjects reached the TCT between 55 and 70 minutes. The time required to return to the pretest core temperature ranged from 85 to 90 minutes. The average HR was 40bpm below the HR threshold upon exercise termination. The average weight loss rate was 0.5kg/hr. In the neutral environment, subjects asked to stop after an average exercise duration of 57.1+/- 11.6 minutes. The average HR was 48bpm below the HR threshold upon exercise termination. The average weight loss rate was 0.46kg/hour. The authors conclude that wearing a totally encapsulating suit while working in hot environments is a risk factor for the development of heat stress.