Threshold limit values for heat stress and strain are based on an upper limit wet bulb globe temperature (WBGT) for ordinary work clothes, with clothing adjustment factors (CAF) for other clothing ensembles. The purpose of this study was to determine the CAF for four clothing ensembles (Cotton Coveralls, Tyvek 1424 Coveralls, NexGen Coveralls, and Tychem QC Coveralls) against a baseline of cotton work clothes and to determine what effect relative humidity may have. A climatic chamber was used to slowly increase the level of heat stress by increasing air temperature at three levels of relative humidity (20%, 50%, and 70%). Study participants wore one of the five ensembles while walking on a treadmill at a moderate metabolic rate of 155 W m-2 (about 300 W). Physiological data and environmental data were collected. When the participant's core temperature reached a steady state, the dry bulb temperature was increased at constant relative humidity. The point at which the core temperature began to increase was defined as the inflection point. The environmental temperature recorded 5 min before the inflection point was used to calculate the critical WBGT for each ensemble. A three-way analysis of variance with ensemble by humidity protocol interactions and a multiple comparison test were used to make comparisons among the mean values. Only the vapor-barrier ensemble (Tychem QC) demonstrated an interaction with humidity level. The following CAFs are proposed: Cotton Coveralls (0 degrees C-WBGT), Tyvek 1424 Coveralls (+1), NexGen Coveralls (+2), and Tychem QC Coveralls (+10).