Maximal Skin Wettedness Across Extreme Environments and Metabolic Rates in Young and Older Adults (PSU HEAT Project)

Maximal skin wettedness (ω max ), the proportion of the skin covered in sweat at the boundary between compensable and uncompensable heat stress, is an important parameter for modeling human responses to heat stress. We previously determined ω max at two extreme environments during activities of daily living across the life span; however, ω max has yet to be quantified across a broader range of environmental conditions and at additional metabolic rates for young (Y) and older (O) adults during extreme heat events. Therefore, in the present study we determined ω max using partitional calorimetry across a wide range of environments (34-49°C dry-bulb temperature, T db ; 14-80% relative humidity, rh) in 51 Y (18-35; 29F) and 55 O (65-92; 33F) adults during minimal activity (MinAct; ~150 W), light ambulation (LightAmb; ~270 W; Y only), and at seated rest (O only) at previously determined upper limits of heat balance. ω max during MinAct was higher in Y compared to O across all environmental conditions (all P ≤ 0.01) and ranged from 0.52 to 0.78 (across environments from most dry to most humid) in Y and 0.39 to 0.52 in O. During LightAmb in Y, ω max ranged from 0.65 to 0.89 and was higher compared to Y MinAct only in hot-dry environments (P < 0.0001). During seated rest in O, ω max ranged from 0.28 to 0.48 and was lower compared to O MinAct only in the most humid conditions (34-36°C, T db ; 53-60%, rh) (P < 0.001). These findings indicate that compared to young adults, older adults have reduced ω max during minimal activity across a wide range of hot and humid environmental conditions. Additionally, for young adults, higher metabolic rates elicit greater ω max in hot-dry environments while in older adults, higher metabolic rates elicit greater ω max in humid environments only. ω max established in the present study for unacclimated young and older adults across a wide range of environmental conditions and various metabolic rates can be used for heat stress modeling in these populations and environments. Funding: T32 AG049676 and R01 AG067471 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.