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

Water vapour resistance of thermal insulating protective clothing impedes sweat evaporation, the most substantial pathway of heat dissipation for working humans. Consequently, 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.

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 (5 × 20 min work respectively rest each; work: treadmill, 3 km/h, 5% incline; handling of loads) without cooling and with insufflating conditioned air (30 °C, < 3% RH, 600 l/min) into an air-diffusing undergarment during rest periods (interval cooling).

With interval cooling, all participants tolerated the whole exposure time never exceeding a mean core temperature of 38.0 °C. In contrast, without cooling 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). All parameters indicated lower heat stress and distinctive heat dissipation with interval cooling: e.g., HR between both conditions differed significantly (p ≤ 0.05; p ≤ 0.01), as were the skin temperatures (p ≤ 0.001). HR decline was 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). Without cooling 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.

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.