Concept of the equivalent wet bulb globe temperature index for indicating safe thermal occupational environments

We propose the concept of the equivalent wet bulb globe temperature (eWBGT) index, which requires no adjustment procedures to indicate the upper limit of safe thermal occupational environments in various conditions. We clarify that the original wet bulb globe temperature (WBGT) is an index that expresses the heat storage rate of the human body (S) in its standard condition for acclimatized workers, and demonstrate the relationship between S and WBGT in the standard condition. Based on these findings, we introduce the concept of the eWBGT, which enables conversion of the heat risk in a given condition into the WBGT heat risk scale in the standard condition. The eWBGT estimates heat strain of S under a given condition by using the heat balance equation for the human body. S was converted into the corresponding WBGT heat stress risk scale by referring to the relation between the WBGT and S in the standard condition. The eWBGT changes with the heat and vapor transfer properties of clothing, even when the air temperature, thermal radiation, water vapor pressure, and air velocity are constant. The eWBGT increases as the thermal insulation of clothing increases, and the influence of environmental water vapor pressure on the eWBGT decreases with decreasing vapor permeability index for the clothing layer. The eWBGT has the potential to provide a more accurate evaluation of the effects of heat and vapor transfer properties of clothing on occupational heat risk than the original WBGT.