In separate materials performance testing, the TPP and THL of the base layers were assessed individually and with turnout gear according to NFPA Standards. The G-XTREME turnout gear had a TPP rating of 42 cal/cm2 and a THL of 266 W/m2, which meet the NFPA 1971 performance requirements of 35 cal/cm2 and 205 W/m2 for TPP and THL, respectively. The TPP and THL ratings for the base layers were obtained in combination with turnout gear to indicate the overall thermal protection and heat dissipation afforded by the integrated firefighting clothing ensemble. Cotton had a higher TPP than the technical layers but was also a heavier material (we had tried to match the weight of all material, but we could not match them exactly).
Base layers added
When base layers were considered along with the turnout gear, there was an increase in TPP (ranging from 9 to 14 cal/cm2) for the ensembles compared with turnout gear alone. The cotton-plus-turnout gear ensemble had a significantly higher TPP than modacrylic plus turnout gear; otherwise there were no significant differences in TPP among the ensembles. It is important to note that the turnout gear alone exceeded TPP requirements and the addition of the base layer further increased TPP.
Among THL comparisons, the THL of modacrylic plus turnout gear was significantly lower than the THL of cotton plus turnout gear and wool plus turnout gear. When only the base layer was considered, there were large differences in THL ratings, with PCM showing the greatest heat dissipation. Yet, even with THL ratings indicating that modacrylic and wool were able to dissipate approximately 45% less heat than PCM, there were no differences in thermoregulatory responses (body temperature) during the recovery periods of the human performance testing.
The base layer that is worn next to the firefighter is the foundational part of an ensemble that protects the firefighter. Thus, it is important to give careful consideration to the base layer that is worn, considering such aspects as the protection it affords, its functional properties and its comfort. This study found that the type of base layer worn under structural firefighting gear had little influence on physiological responses measured during exercise or recovery. This information is consistent with other research that suggests that when heavy clothing is worn during muscular work, the metabolic heat production and sweat production may overwhelm the properties of technical layers.
In contrast, the base layers did have different materials performance results. Importantly the phase change material had the greatest breathability (higher THL) of all the technical layers. This property may provide improved comfort over cotton under conditions of everyday wear. This is especially important to consider since the base layer is typically worn during an entire duty shift.