Wearing protective clothing (PC) such as firefighter bunker gear leads to increased heat strain. Bunker gear has multiple layers which creates a heat sink during the first several minutes after donning, potentially providing some amount of cooling of the micro-environment (micro-E) around the skin. Furthermore, predicting heat strain at various work rates while wearing PC is important for determining work tolerance limits. The purpose of these studies was to determine the effect of a COLD (< 0 °C) bunker gear on rectal temperature (T_re ) and micro-E at a steady-state exercise intensity and to begin creating a regression model to predict micro-E temperature and relative humidity under bunker gear based on work rate. Studies I and II utilized a repeated-measures randomized experimental design, which consists of 4 trials (LO, MOD, HI (HOT), and HI (COLD)). Human volunteers wore bunker gear for 30 min while oxygen uptake (V̇O_2), heart rate, perceptive measures (ratings of perceived exertion, thermal sensation, sweating sensation, and clothing comfort), T_re, skin temperature, and micro-E (temperature and relative humidity at the anterior and posterior portion of the jacket and pants) were measured. A repeated measures MANOVA with a Bonferroni post hoc analysis was used to evaluate: the physiological measures; while the Friedman's Test was performed with a Wilcoxon Signed Rank Test on all perceptual measures. The COLD bunker gear did not significantly alleviate heat strain when compared to HOT bunker gear. Additionally we low, moderate and high intensities of work in ~ 29 °C macro-E WBGT did not differentially affect micro-E WBGT under bunker gear, and these data were used to derive models of micro-E from macro-E. Further data is needed to cross-validate the derived micro-E WBGT model predicted from macro-E WBGT and work rate.