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_max)-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 <i>days 1</i>, <i>3</i>, <i>5</i>, and <i>7</i> of seven consecutive days of heat acclimation. In each RAMP trial, participants cycled continuously at 275 W·m^-2 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^-1. An upward inflection in esophageal temperature (T_eso) signaled a transition to uncompensable heat stress with the critical water vapor pressure at that point used to calculate ω_max. In days between RAMP assessments, participants cycled for 90 min at 75% HR_max at 37°C, 60% RH. T_eso, whole body sweat rate (WBSR), local sweat rates on the back and forearm (LSR_back and LSR_arm, respectively), and activated sweat gland density (ASGD) were measured throughout. ω_max was progressively and significantly greater from <i>day 1</i> (0.68 ± 0.10) to <i>day 3</i> (0.75 ± 0.10; <i>P</i> = 0.002), to <i>day 5</i> (0.79 ± 0.10; <i>P</i> = 0.004), and to <i>day 7</i> (0.87 ± 0.06; <i>P</i> = 0.009). WBSR was higher on <i>days 5</i> (1.11 ± 0.30 L·h^-1; <i>P</i> = 0.01) and <i>7</i> (1.12 ± 0.19 L·h^-1; <i>P</i> < 0.001) compared with <i>day 1</i> (0.94 ± 0.21 L·h^-1). ASGD was higher on <i>day 5</i> (78 ± 15 glands·cm^-2; <i>P</i> < 0.001) and <i>day</i> 7 (81 ± 17 glands·cm^-2; <i>P</i> = 0.001) compared with <i>day 1</i> (65 ± 12 glands·cm^-2). There were no observed differences in sweat gland output (<i>P</i> = 0.21). In conclusion, ω_max significantly increased throughout 7 days of heat acclimation. These progressive increases in ω_max were predominantly mediated by an increase in the number of active sweat glands, not the output per gland.<b>NEW & NOTEWORTHY</b> Significant increases in ω_max 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_max with progressive heat acclimation and provide new information about ω_max levels between the standard "unacclimated" and "acclimated" states in existing heat stress models.