Lower maximal skin wettedness in both warm-humid and hot-dry environments with advanced age (PSU HEAT project)

Maximum skin wettedness (x_max) is the proportion of the body covered in sweat at the upper limit of compensable heat stress. It has yet to be determined how x_max changes with aging. We examined variability in x_max at the upper limit of compensable heat stress in warm-humid (WH) and hot-dry (HD) environments in young (Y, 18-29 yr), middle-aged (MA, 40-60 yr), and older (O, 65-89 yr) adults during minimal activity (MinAct; 1.8 METS) and in O subjects at rest. x_max was calculated using partitional calorimetry for 27 Y (13 F), 27 MA (16 F), and 32 O (18 F) at the previously determined upper limits of compensable heat stress in WH and HD environments. In WH environments, x_max was greater in Y (0.69 ± 0.12) and MA (0.64 ± 0.20) compared with O (0.47 ± 0.14; both P < 0.05), but not different between Y and MA (P ¼ 0.85). In HD environments, x_max was greater in Y (0.52 ± 0.05) compared with O adults (0.40 ± 0.07; P < 0.05), but not different between MA (0.48 ± 0.10) and Y or O (both P > 0.15). In O participants at rest, x_max was lower than MinAct in WH (P < 0.001) but not HD environments. These findings indicate that 1) x_max is lower with advanced age across environments and 2) is lower at rest than during light activity in O in humid conditions. x_max established herein for unacclimated adults during activities of daily living and older adults at rest may be used to model heat stress responses for these populations and environments.