Evaluation of WBGT-based exposure limits

The evaluation of exposure limits based on the wet-bulb globe temperature (WBGT) has been under scrutiny for years, and more so with the increasing interest in heat stress associated with recent record-hot summers. The authors’ assessment of current research papers designed to evaluate WBGT-based exposure guidelines (such as in determining work/rest cycles) have two fundamental weaknesses: (1) the case definitions do not focus on the actual control goal for exposure limits and (2) the heat stress exposure classification does not account for an individual’s level of heat stress exposure. In particular, using a core temperature limit of 38 °C to classify heat stress exposures is problematic. This commentary offers a view on how to address these weaknesses. The recommended case definition centers on demonstrating a steady-state physiological response: specifically, a threshold change in Tc of 0.1 °C/hr. A second, candidate case definition is a steady heart rate response, which is a more protective threshold focused on populations that may have subclinical or clinical cardiovascular disease. Regarding exposure classifications, individual classifications of exposure using an ΔOEL metric as a continuous variable or a dichotomous variable where ΔOEL is either ≤ 0 or >0. An individually assigned ΔOEL accounts for differences in actual exposures among individuals. Adopting both of these recommendations is a rigorous approach to compare experimental results to WBGT-based exposure assessment.