Determination of body heat storage in clothing: calorimetry versus thermometry

Two methods of estimating body heat storage were compared under differing conditions of clothing, training, and acclimation to heat. Six male subjects underwent 8 weeks of physical training [60-80% of maximal aerobic power (VO2max) for 30-45 min.day-1, 3-4 days.week-1 at < 25 degrees C dry bulb (db)] followed by 6 consecutive days of heat acclimation (45-55% VO2max for 60 min.day-1 at 40 degrees C db, 30% relative humidity)]. Nine other male subjects underwent corresponding periods of control observation followed by heat acclimation. Before and after each treatment, subjects walked continuously on a treadmill (1.34 m.s-1, 2% grade) in a climatic chamber (40 degrees C db, 30% relative humidity) for an average of 118 min (range 92-120 min) when wearing normal light combat clothing and for an average of 50 min (range 32-68 min) when wearing protective clothing resistant to nuclear, biological, and chemical agents. The heat storage was determined calorimetrically (by the balance of heat gains and losses) and thermometrically [by the conventional equations, using one or two set(s) of relative weightings for the rectal temperature (Tre) to mean skin temperature (Tsk) of 4:1 and 4:1, 2:1 and 4:1, or 2:1 and 9:1 in thermoneutral and hot environments, respectively]. Tsk was calculated from 12-site measurements, weighted according to the regional distribution of body surface area and the first eigenvectors of principal component analysis. There were only minor differences (< 5%) between the heat storage values calculated by given weighting factors for Tre and Tsk, whether the individual coefficients were derived from estimates of regional surface area or principal component methodologies. When wearing normal clothing, no significant differences were found between the two estimates of heat storage (calorimetry vs thermometry with an invariant relative weighting of 4:1) in any experimental condition, with one specific exception: when wearing protective clothing, thermometry underestimated the heat storage by 24-31%. This under-estimation was attenuated by using two sets of relative weightings of 2:1 and 4:1 or 2:1 and 9:1. The results suggest that when subjects wearing protective clothing are transferred from thermoneutral to hot environments, the accuracy of thermometric estimates of heat storage can be improved by using two sets of weighting factors for Tre and Tsk.