Derivation and analysis of the indoor Wet Bulb Globe Temperature index (WBGT) with a human thermal engineering approach ? Part 1. Properties of the WBGT formula for indoor conditions with no solar radiation.

Based on a heat balance equation between humans and the environment, the authors developed a theoretical derivation of the indoor WBGT formula, which was originally established empirically by Yaglou and Minard. We demonstrated that the coefficients of wet bulb temperature Tw and globe temperature Tg (= air temperature Ta), in the indoor WBGT formula contain variables such as the metabolic activity, the clothing insulation and the wind velocity, and that these three coefficients do not remain strictly constant. We also considered the thermophysiological significance of the WBGT based on the derived formula. In addition, we performed a numeric examination and indicated irrational results of the WBGT.
The newly derived formula is WBGT = 0.85Tw + 0.20Tg (= Ta) or WBGT = 0.80Tw + 0.20Tg (= Ta) which differs from Yaglou et als original formula WBGT = 0.7Tw + 0.3Tg (= Ta) in the coefficients.