A Protective Cap Does Not Alter Rectal or Skin Temperature Responses During Simulated Occupational Work in the Heat

Although helmets protect against head injuries, many do not consider the effects of wearing a helmet, or protective cap, on thermoregulation during work in the heat. Purpose: To examine the influence of a protective cap on rectal temperature (T REC ), skin temperature (T SK ), and thermal gradient [core-to-skin ratio (CSR)] compared to a baseball cap during simulated work in the heat. We hypothesized that there would be significant differences in T REC , T SK , and CSR in the protective cap compared to a regular cap. Methods: Twelve physically-active men and women (age:26.5±3.5 years; VO 2 peak:45.8±7.4 ml·kg -1 ·min -1 ) were asked to complete threerandomized controlled trials in a hot, humid environment (35°C, 50% rH). The three experimental trials included: 1) black protective layer cap (DARK); 2) light grey protective layer cap (LIGHT); and 3) a baseball cap (CON). Each experimental trial included two cycles of: 15 min of walk/run intervals on a treadmill (2% incline) and 5 min of box lifting (BL). A 3-min rest break (RB) followed each cycle. Participants then ran at 65% velocity of VO 2 peak until their T REC reached 39.5°C (RTT). A Two-Way Repeated Measures Analysis of Variance (RMANOVA) examined changes in T REC , T SK , and CSR (n=8) between trials and over time until the first RB. Bonferroni post-hoc corrections were used when appropriate. An Analysis of Covariance (ANCOVA) examined the effect of cap type (DARK, LIGHT, or CON) on T REC prior to RTT with baseline T REC (n=6) as a co-variate. ANCOVA results were described by Partial Eta effect size (ηpa) and 95% confidence interval (CI). A One-Way ANOVA examined changes in the rate of rise of T REC between trials. Sex differences were not assessed. Results: There were no statistically significant differences in T REC [DARK: 37.99±0.38°C, LIGHT: 37.96±0.38°C, CON: 38.03±0.35°C], T SK [DARK: 35.28±1.17°C, LIGHT: 35.51±1.12°C, CON: 35.30±1.04°C] or CSR[DARK: 1.05±0.01, LIGHT: 1.06±0.01, CON: 1.07±0.01] between trials from the beginning of the experimental trial until the end of the first RB. There was no statistically significant effect of cap type on T REC prior to the start of RTT (p=0.273, ηpa=0.169, large effect, 95%CI=38.81-39.10). There were no statistically significant differences in the rate of rise of T REC between trials [DARK: 0.035±0.006, LIGHT: 0.036±0.006, CON: 0.034±0.009]. Conclusion: The addition of a protective layer in a cap (DARK or LIGHT) does not alter T REC or T SK responses during simulated work in the heat. Therefore, utilizing a cap that includes this protective layer as personal protective equipment may reduce the likelihood of head injuries without contributing to thermal strain. This study was funded by Delta Air Lines. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.