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

Maximum skin wettedness (ω_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 ω_max changes with aging. We examined variability in ω_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. ω_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, ω_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 = 0.85). In HD environments, ω_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, ω_max was lower than MinAct in WH (P < 0.001) but not HD environments. These findings indicate that 1) ω_max is lower with advanced age across environments and 2) is lower at rest than during light activity in O in humid conditions. ω_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.NEW & NOTEWORTHY This study is the first to identify 1) maximal skin wettedness values for unacclimated adults across the adult age span of 18 to 89 yr at a metabolic rate representative of minimal activities of daily living and 2) establish specific ω_max values for older adults during rest and activities of daily living. These findings provide empirical data for the modeling of physiological responses to heat stress across the adult age span.