• Wool/rayon blend – Wool maintains a dry air space next to the skin, which keeps the wearer cooler in hot temperatures, and has excellent moisture absorption properties
• Phase change material (PCM) – Absorbs, stores and releases heat and has the potential to interact with the skin’s temperature to provide a buffer against temperature swings
The 110-minute protocol simulated a work/recovery cycle that may be experienced by a firefighter. The protocol included three 20-minute work bouts, during which participants walked on a treadmill at a moderate intensity level (approximately 70% of age-predicted maximum heart rate) while wearing PPE, which included G-XTREME turnout coat and pants from Globe Manufacturing Co., LLC, boots, flash hood, helmet, gloves and self-contained breathing apparatus (SCBA). Work bouts were followed by seated recovery periods of 10, 20 and 20 minutes, respectively. During recovery periods, fluid was provided, gear was doffed and a fan was used to actively cool participants as is often recommended during incident rehabilitation. Testing was performed in a thermoneutral laboratory (70 degrees Fahrenheit, 58.6% relative humidity).
Throughout the protocol, heart rate (measured with a heart-rate monitor) and body temperature (measured with an ingestible pill) were recorded as measures of physiological strain. Thermal sensations were described on an eight-point scale from unbearably cold (0) to unbearably hot (8). Individual sensations, such as comfort, chafing and coolness/hotness, to the different base layers were also evaluated following the third exercise bout. Weights of the participant and articles of clothing were obtained before and after the protocol and sweat loss and sweat absorbed by the clothing were calculated.
As expected, all measures of strain increased during exercise and decreased during recovery in the physiological trials. However, the type of base layer had no effect on changes in heart rate, body temperature or thermal sensations. Based on previous research, it is not surprising that the technical base layers failed to provide an advantage when worn under turnout gear during exercise. Studies investigating the effect of different base layers worn under military PPE while performing moderate-intensity exercise in hot conditions have also reported no differences in physiological and perceptual responses. It appears that the high heat production during exercise when wearing protective clothing creates a condition in which the technical properties of the material are overwhelmed and fail to provide the proclaimed benefits.
In contrast to previous studies that investigated base layers worn under military PPE, most or all firefighting turnout gear was removed during the recovery periods in this study in order to be consistent with fire service practice during structured rehabilitation periods. Despite the removal of the turnout gear, which exposed the base layer to the ambient conditions, no differences in measures of physiological or perceptual strain were noted during recovery among the different base layers.
There were no differences in sweat loss while wearing different base layers; however, cotton and PCM absorbed significantly more sweat than modacrylic and wool. Differences in sweat retention may have influenced perceptions of clothing sensations. Immediately following exercise, participants indicated that compared with cotton, wool did not stick as much to the skin, was cooler and had a lower humidity sensation. Although both wool and modacrylic absorbed less sweat than cotton, no significant differences were found for any other sensations such as wetness or sweating sensation.
Materials performance testing
Materials performance testing of turnout gear is stipulated in NFPA 1971 and includes TPP and THL. TPP reflects the tolerance time of human skin to second-degree burn. The time (in seconds) to a second-degree burn is obtained by halving the TPP rating. The THL test measures the thermal and evaporative resistance of the clothing and combines these measures to yield a single number, the THL, to indicate the capability of the sample to dissipate heat.