Quantifying the time course of changes in maximal skin wettedness with 7 days of heat acclimation.

The aim of the present study was to quantify the time course of changes in maximum skin wettedness (ω)-that is, the proportion of skin surface area covered in sweat at the point of uncompensable heat stress, throughout seven consecutive days of heat acclimation. Nine adults (6 M, 3 F) completed a humidity-ramp protocol (RAMP) on , , , and of seven consecutive days of heat acclimation. In each RAMP trial, participants cycled continuously at 275 W·m for 120 min at 37°C: 60 min at a vapor pressure of 2.05 kPa followed by 60 min with vapor pressure increased by 0.045 kPa·min. An upward inflection in esophageal temperature (T) signaled a transition to uncompensable heat stress with the critical water vapor pressure at that point used to calculate ω. In days between RAMP assessments, participants cycled for 90 min at 75% HR at 37°C, 60% RH. T, whole body sweat rate (WBSR), local sweat rates on the back and forearm (LSR and LSR, respectively), and activated sweat gland density (ASGD) were measured throughout. ω was progressively and significantly greater from (0.68 ± 0.10) to (0.75 ± 0.10; = 0.002), to (0.79 ± 0.10; = 0.004), and to (0.87 ± 0.06; = 0.009). WBSR was higher on (1.11 ± 0.30 L·h; = 0.01) and (1.12 ± 0.19 L·h; < 0.001) compared with (0.94 ± 0.21 L·h). ASGD was higher on (78 ± 15 glands·cm; < 0.001) and 7 (81 ± 17 glands·cm; = 0.001) compared with (65 ± 12 glands·cm). There were no observed differences in sweat gland output ( = 0.21). In conclusion, ω significantly increased throughout 7 days of heat acclimation. These progressive increases in ω were predominantly mediated by an increase in the number of active sweat glands, not the output per gland. Significant increases in ω were observed as early as 3 days into a 7-day heat acclimation protocol. These data are the first to report dynamic changes in ω with progressive heat acclimation and provide new information about ω levels between the standard "unacclimated" and "acclimated" states in existing heat stress models.