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Measures of physiological and perceptual strain were assessed during both work and recovery. Heart rate, body temperature (measured with an ingestible pill) and chest skin temperature were measured throughout the protocol to characterize physiological strain. Additionally, participants rated their thermal sensations (“unbearably cold” to “unbearably hot”) and perceived exertion level (“not tired at all” to “so tired, I can’t go anymore”) using standardized scales. Participants also rated clothing sensations such as comfort, coolness/hotness and wetness following the third exercise bout. Sweat loss and sweat absorbed by the clothing over the course of the protocol were calculated.
Clothing ensembles were compared by examining the differences in measures of strain over the individual exercise bouts and recovery periods. For example, if heart rate was 70 beats/minute at the start of exercise and 140 beats/minute at the end of exercise, the difference of 70 beats/minute was the value used for comparison purposes.
As expected, all measures of strain increased during exercise and decreased during recovery. Heart rate (Figure 2), body temperature (Figure 3), chest skin temperature (Figure 4) and rating of perceived exertion (Figure 5) reached similar levels during exercise when the station shirt was worn and when it was not. The change in heart rate during Exercise 2 was greater when wearing only the cotton T-shirt under turnout gear, but this was due to starting at a lower heart rate at the beginning of the exercise session rather than reaching a higher peak heart rate. The change in thermal sensations was slightly higher during the first exercise bout when the station uniform shirt was worn (Figure 6).
During all three recovery periods, chest skin temperature dropped more (7.13-11.93 degrees F vs. 3.10-5.15 degrees F) when the station uniform shirt was not worn. However, the difference in chest skin temperature was not maintained, as chest skin temperature reached the same levels for both ensembles by the end of the subsequent exercise bouts. Additionally, there were no differences in sweat loss or sweat retained by the clothing between ensembles for either measure. Similarly, no differences in clothing sensations between the ensembles were reported.
Materials performance testing
The second component of the research study, materials performance testing, was performed in accordance with NFPA standards. For each configuration examined, three tests were run. The TPP rating of 42.3 cal/cm2 for the G-XTREME turnout gear alone is the TPP rating reported in standard TPP tests of firefighting turnout gear. Both ensembles that included a clothing layer under the turnout gear provided greater thermal protection than turnout gear alone (56.4 cal/cm2 and 64.8 cal/cm2 for the two- and three-layer ensembles, respectively; Figure 7). When assessing THL, both ensembles that included clothing layers worn under the turnout gear had lower THL ratings than turnout gear alone (Figure 8). However, the ensembles that included clothing worn under the turnout gear did not differ from each other (TOG: 266 W/m2; two-layer: 235 W/m2; three-layer: 241 W/m2), suggesting that the addition of the station uniform did not interfere with THL in the material performance test.
This study shows that the levels of cardiovascular, thermoregulatory and perceptual strain experienced during intermittent moderate-intensity work/recovery work bouts were similar whether wearing a cotton T-shirt alone or in combination with a station uniform shirt under structural turnout gear. In contrast, chest skin temperature during recovery periods was lower when not wearing the station uniform shirt in our study, which used a fan to actively cool firefighters during our protocol.
Materials performance testing indicated that the addition of a station uniform shirt increased thermal protection without imposing greater thermal strain (THL was similar) compared to just wearing a cotton T-shirt under the turnout gear.
In summary, considering the layers of clothing worn under structural firefighting gear is important to understand the thermal protection provided and the physiological burden imposed by the total clothing ensemble. We conclude that during intermittent work (with active cooling in a thermoneutral environment) there is no detrimental effect of wearing a station uniform shirt and that there may be an increase in thermal protection around the upper body when a station uniform is worn. These findings, along with other concerns such as tradition and professionalism, should be considered when decisions are made about the use of station uniform shirts.