Total heat loss (THL) in the National Fire Protection Association (NFPA) 1971, Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting, standard is the index used to evaluate the heat loss capability of firefighters' turnout gear. The true evaporative resistance (Ret) covered in ASTM F1868, Test Method for Thermal and Evaporative Resistance of Clothing Materials Using a Sweating Hot Plate, Part B is the other index used to predict the garment's breathability, which is not included in NFPA 1971. To explore the better index among these two values for predicting the human physiological response under warm conditions. This paper reported a human subject study to measure the physiological responses of firefighters conducting a designed protocol while wearing three different types of turnout garments in a warm condition. These three garments had similar THL indexes but different Ret. The results showed that firefighters dropped out only when they wore garments with the highest Ret value. A significant difference in the maximum change in heart rate was found between firefighters wearing garments with different Ret values. An apparent difference in core temperature when all firefighters were present was observed when wearing the garment with the highest Ret value. No obvious difference was found between subjects wearing different garments. The work demonstrated the need to evaluate the Ret in addition to the THL for heat strain evaluations of protective clothing.