Body regional influences of l-menthol application on the alleviation of heat strain while wearing firefighter’s protective clothing

The purpose of this study was to investigate the influences of menthol application according to the amount of surface area on physiological and psychological heat strains, along with body regional influences. Male students underwent two stages of experiments: [Experiment 1] Cutaneous thermal threshold test at rest on eight body regions with/without a 0.8% menthol application at T _a 28°C and 50% RH; [Experiment 2] Six exercise tests with/without a 0.8% menthol spray at T _a 28°C and 40% RH, while wearing firefighter’s protective clothing (No menthol, PC_NO; Face and neck menthol, PC_FN; Upper body menthol, PC_UP; Whole body menthol application, PC_WB) or wearing normal clothing (No menthol, NC_NO; Upper body menthol, NC_UP). Experiment 1 showed that menthol caused no significant influence on cutaneous warm thresholds, while menthol applications evoked earlier detection of cool sensations, especially on the chest (P = 0.043). Experiment 2 revealed that NC_UP, PC_UP and PC_WB caused lower mean skin temperature, especially with higher peripheral vasoconstrictions on the extremities at rest. During exercise, NC_UP, PC_UP and PC_WB induced greater and earlier increases in rectal temperatures (T _re) and a delayed sweat response, but lessened psychological burdens (P < 0.05). Both physiological and psychological effects of PC_FN were insignificant. For a composite analysis, individual Menthol Sensitivity Index at cooling in Experiment 1 had significant relationships with the threshold for T _re increase and changes in heart rate in NC_UP of Experiment 2 (P < 0.05). Our results indicate that menthol’s topical influence is body region-dependent, as well as depending on the exposed body surface area.