Maximal skin wettedness as a function of environment and metabolic rate in unacclimated young and older adults (PSU HEAT Project)

Maximal skin wettedness (ω_max), the proportion of the skin covered in sweat at the upper limits of compensable heat stress, is an important parameter for modeling human heat stress responses. We previously determined ω_max in 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 environments and metabolic rates for young (Y) and older (O) adults in extreme heat. The present study used partitional calorimetry to determine ω_max across a wide range of hot environments (34–49 °C dry-bulb temperature, T_db; 14–80% relative humidity, rh) in 51 Y (18–35 yrs; 29 F) and 55 O (65–92 yrs; 33 F) during minimal activity (MinAct; ~150 W), light ambulation (LightAmb; ~250 W; Y only), and rest (O only; ~90 W). During MinAct, ω_max was higher in Y compared to O across environments (all P ≤ 0.008) and ranged from 0.43 to 0.99 as humidity increased in Y and 0.21 to 0.83 in O. During LightAmb in Y, ω_max ranged from 0.53 to 1.10 but was higher compared to MinAct only in hot-dry environments (P < 0.0001). At rest in O, ω_max ranged from 0.16 to 0.78 and was lower compared to MinAct only in a 53–60% rh condition (36 °C, T_db) (P < 0.008). These findings indicate that ω_max varies with age, metabolic rate, and environment. ω_max established herein for unacclimated young and older adults across environments and relatively low metabolic rates can be used for heat stress modeling in these populations and environments.