Wet bulb globe temperature (WBGT) is a widely used heat stress index (Parsons, 1995). It is used to predict the physiological consequences of heat stress (i.e. heat strain), and to set WBGT threshold limit values to minimise heat illness risk, in many occupational and athletic settings. Implicit in its use is that it is valid in all environmental conditions. This study aimed to establish if heat strain is equivalent in humid and dry heat matched for WBGT, as previous work on this topic is sparse and equivocal (e.g. Keatisuwan et al. 1996).
Following local ethics committee approval, ten euhydrated men (mean (1 S.D.): age, 27.4 (3.6) years; height, 1.80 (0.06) m; body mass, 75.2 (9.3) kg; peak oxygen uptake rate, 55.4 (3.8) ml min-1 kg-1) completed 60 min of continuous treadmill walking (speed, 1.53 m s-1; grade, 0 %) in two simulated hot environments – warm-humid and hot-dry. Dry-bulb temperature, globe temperature, relative humidity, and air speed were 33.4 °C, 34.1 °C, 88 % and 1.2 m s-1, respectively (WBGT 32.1 °C) for warm-humid, and 45.6 °C, 46.3 °C, 20 % and 1.3 m s-1, respectively (WBGT 32.3 °C), for hot-dry. Subjects wore lightweight clothing (intrinsic clothing insulation, 0.63 clo; Woodcock moisture vapour permeability index, 0.55), and were encouraged to drink 6 ml (kg body mass)-1 of water every 30 min. Changes in rectal temperature (ΔTre), mean skin temperature (Tsk) and heart rate (HR) were measured every 5 min. Total (mwl) and evaporative (me) water losses were calculated from changes in nude and clothed body mass, and corrected for water consumed. Metabolic rate was measured (Douglas bags) at 25 min.
ANOVA showed that in warm-humid, ΔTre (P < 0.01), HR (P < 0.01) and mwl (P < 0.05) were higher, and Tsk (P < 0.01) and me (P < 0.01) were lower, than in hot-dry (Table 1). Metabolic rate was the same in both environments (195 (13) and 192 (15) W m-2).
Heat strain was greater in the warm-humid than in the hot-dry environment of equal WBGT. Therefore, in the context of this study, the applicability of WBGT as a predictor of heat strain is questioned and warrants further investigation.
This work was funded by the Chemical, Biological and Human Science Domain of the UK Ministry of Defence Corporate Research Programme.