Effects of metabolic rate and ambient vapour pressure on heat strain in protective clothing

Abstract

Studies have shown that variations in ambient water vapour pressure from 1.7 to 3.7 kPa have little effect on heat tolerance time at a metabolic rate above 450 W while wearing protective clothing. With lighter exercise, where tolerance times exceed 60 min, variations in vapour pressure have a significant impact on evaporative heat loss and, therefore, heat tolerance. The present study has examined whether these findings extend to conditions with more extreme variations in vapour pressure. Twelve males performed light (L, 350 W) and heavy (H, 500 W) exercise at 40°C in a dry (D, 1.1 kPa) and humid (H, 4.8 kPa) environment while wearing a semi-permeable nuclear, biological and chemical protective clothing ensemble (0.29 m²×°C⁻¹·W⁻¹ or 1.88 clo; Woodcock vapour permeability coefficient,i _m=0.33). Partitional calorimetry was used to determine the rate of heat storage ($\dot S$) with evaporative heat loss from the skin ($\dot E_{sk}$) calculated from changes in dressed mass or the physical properties of the clothing and the vapour pressure gradient between the skin and the environment. Skin vapour pressure was predicted from measurements of water vapour pressure above the skin surface and in the clothing with humidity sensors coupled with thermistors. Final mean skin temperature ($\bar T$ _sk) was higher for the humid trials and averaged 37.4 (0.3)°C, 38.9 (0.4)°C, 37.6 (0.5)°C and 38.5 (0.4)°C for LD, LH, HD and HH, respectively. Final rectal temperature (T _re ) was higher for D with respective values for LD, LH, HD and HH of 39.0 (0.4)°C, 38.7 (0.4)°C, 38.8 (0.4)°C and 38.5 (0.4)°C. Tolerance time was significantly different among the trials and averaged 120.3 (19.3) min, 54.8 (7.3) min, 63.5 (6.9) min and 36.8 (3.1) min for LD, LH, HD and HH, respectively.$\dot E_{sk}$ was overestimated and, therefore,$\dot S$ was underestimated when the changes in dressed mass were used to determine evaporative heat loss. When skin vapour pressure determined from the humidity sensor data was used to calculate$\dot E_{sk}$, heat storage was significantly different among the trials and averaged 15.0 (3.0), 13.0 (1.8), 14.2 (2.6) and 12.2 (1.9) kJ·kg⁻¹ for LD, LH, HD and HH, respectively. It was concluded that while wearing the protective clothing all indices of heat strain, including tolerance time, were significantly affected by the change in ambient water vapour pressure from 1.1 to 4.8 kPa during both light and heavy exercise.