Evaporative microclimate interval cooling impressively reduces heat stress in thermal insulating protective clothing

Background Thermal insulating protective clothing impedes sweat evaporation, the most substantial way of heat dissipation for working humans. As a consequence, metabolic heat can increase body core temperature rapidly, which can result in life-threatening heat illnesses. The evaporative microclimate cooling method Dry Air Comfort (DAC) has been shown to be very effective for reducing the risk of heat induced illnesses. Aim of this study was to determine whether DAC can also prevent heat stress when used in intervals. Methods Using a balanced within-subject design twelve men in protective overalls were randomly assigned (max. 205 min, 25 °C, 50 % relative humidity/RH, 0.2 m/s wind speed) to a work-rest schedule (5x20 min work respectively rest each; work: treadmill, 3 km/h, 5 % incline; handling of loads) without (permanently warm condition) and with insufflating conditioned air (30 °C, << 3 % RH, 600 l/min) into an air-diffusing undergarment during rest periods (interval cooling condition). Results In the interval cooling condition all participants tolerated the whole exposure time never exceeding a core temperature of 38.0 °C. In contrast, in the permanently warm condition eight subjects had to terminate work because of high core temperatures (39 °C, n = 5), subjective exhaustion or reaching 90 % of the maximum individual heart rate (HR; n = 3). HR decline was significantly more pronounced during rest periods during interval cooling, indicating significant recovery: e.g., 105.1±17.0 b/min vs. 149.7±15.5 b/min (rest 4, 165. min vs. work 4, 145. min, n = 12). During permanently warm HR showed only minor recovery: e.g., 138.4±15.0 b/min vs. 158.7±12.3 b/min (rest 4, 165. min vs. work 4, 145. min, n = 8) and exceeded the level of HR in rest periods of interval cooling. All other parameters (core/skin temperatures, ratings of perceived exertion, heat burden) indicated significantly lower heat stress and distinctive heat dissipation during rests in the interval cooling. Conclusions Interval heat evaporation during rests periods with the microclimate cooling method DAC was shown to be effective in preventing body heat accumulation during alternating work-rest cycles. The method can improve occupational health and safety of men working in thermal protective clothing.