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

Maximal skin wettedness (ωmax), 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 ωmax in two extreme environments during activities of daily living across the life span; however, ωmax 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 ωmax across a wide range of hot environments (34–49 °C dry-bulb temperature, Tdb; 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, ωmax 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, ωmax 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, ωmax ranged from 0.16 to 0.78 and was lower compared to MinAct only in a 53–60% rh condition (36 °C, Tdb) (P < 0.008). These findings indicate that ωmax varies with age, metabolic rate, and environment. ωmax 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.