The high temperatures, which are often encountered by mine rescue teams during emergency situations, may cause severe physiological strain. For example, at the Somerset coal mine, temperatures of 49°C (120°F) with 100 percent relative humidity caused the normal rescue team recovery mission to be shortened from 2 h to only 25 min. In order to increase the team's effectiveness and comfort under such conditions, the U. S. Bureau of Mines (USBM) initiated a program with the National Aeronautics and Space Administration (NASA) to develop and test a personal cooling system based on technology developed for the thermal control of space-suited astronauts. The resulting system, which has 88 watt-h (300 Btu) heat absorption capacity, weighs approximately 5 kg (11 lb) and is compatible with existing rescue team breathing apparatus. Several different system configurations were tested under simulated mine rescue team operational conditions of 32.5°C (90°F) wet bulb globe temperature (WBGT) and 1.0 l O2 /min (1200 Btu/h) metabolic rate. The system, which consists of a liquid circulation garment and belt-worn heat sink unit, was found to be capable of reducing the physiological strain by nearly 50 percent under these conditions. A simple analytical heat balance model was used to predict the effects of the thermal control system for other environments and metabolic rates. It was found that in the most severe environments, WBGT greater than about 35-37°C, that the maximum endurance time could be further increased by providing an insulating overgarment to reduce the heat gained from the hot environment.