Summary The heat stress associated with wearing heat- and flame-resistant personal protective clothing (PPC) is a serious problem for the health and safety of firefighters. The materials used in the manufacture of PPC often have the disadvantage of being heavy and rigid and of preventing evacuation of the heat generated by physical activity during an intervention. Two new material technologies have been developed by Groupe CTT (a technology transfer centre specialized in research and the design of textile materials) and its industrial partners in order to reduce the physiological effects suffered by workers exposed to heat stress. One of these technologies consists of a heat absorption system, while the other is a new material used in the manufacture of PPC. The aim of this study, which was carried out in two parts, was to evaluate the efficacy of these two technologies by analyzing the physiological response of subjects exposed to controlled environmental conditions. Male subjects (eight for each part of the study) performed stress tests on a treadmill with a workload equal to 250 watts, in a climatic chamber where the temperature and humidity were controlled and set at 35°C and 50% respectively. The experimental protocol used was based on the ASTM F2300-10 standardized test method. The thermophysiological responses elicited by the various experimental conditions were compared using the following variables: oxygen consumption, heart rate, water loss, skin and internal body temperatures, humidity and temperature levels inside the PPC, as well as the subjects’ psychophysical perceptions. The efficacy of a heat absorption system, composed of new phase-change materials (PCM), was evaluated in the first part of the study. A nylon mesh vest (prototype 1) equipped with pouches containing the PCMs, was worn over a cotton T-shirt inside a standard PPC outfit. The effects of the PCMs were evaluated under three experimental conditions: (1) standard PPC only, (2) standard PPC and a vest containing the PCMs, and (3) standard PPC and a vest containing a control substance equal in weight to the PCMs. The results showed that the PCMs reduced skin temperature and slowed the rise in the internal body temperature. However, no significant effects on heart rate and dehydration could be established, possibly due to the facts that the vest was not in direct contact with the body and that the cotton T-shirt may have impeded sweat evaporation. The vest’s weight did not appear to have a significant effect on the variables measured with respect to the third condition, compared to the other two conditions. A new prototype vest was designed in light of these results. In the second part of the study, a new double-density integrated material for the manufacture of PPC was evaluated and compared to the materials used in standard firefighter clothing. The use of PCMs in a new vest enhanced on the basis of the results of the first part of the study, was also evaluated. Replacing the cotton T-shirt, this vest (prototype 2), made from a conductive stretch fabric worn directly against the skin under the PPC, allowed a better heat exchange with the skin. Four experimental conditions were compared to cast light on the effects of the new double-density material and of the new vest containing the PCMs: (1) standard PPC and T-shirt, (2) standard PPC and PCMs, (3) double-density PPC and T-shirt, and (4) double-density PPC and PCMs. The results showed that inserting PCMs into the PPC (standard and double-density) significantly reduced the thermophysiological effects, with the vest allowing optimal heat exchange with the user’s skin, as was the case with prototype 2. The double-density PPC appears to have a bigger insulation effect than that of standard PPC. This insulation property appears to negatively affect user comfort because it keeps the heat inside the PPC when the body’s internal temperature is raised. However, it appears advantageous when the vest is worn under the PPC because it keeps the cooling effect of the PCMs inside the PPC, thus maintaining a lower internal body temperature. This study demonstrated the relevance of research and the development of new materials to the aim of improving the efficacy of thermoregulation during work in hot environments. The findings open the doorway to optimizing the system and the efficacy of the PCMs in PPC.