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 environments (HD) in young (Y, 18-29 yrs), middle-aged (MA, 40-60 yrs) and older (O, 65-89 yrs) adults during minimal activity (MinAct; ~1.8 METS) and in O subjects at rest. ω_max was calculated using partitional calorimetry for 27 Y (13F), 27 MA (16F), and 32 O (18F) 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 to 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 to 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.