Abstract
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.
Keywords: Aging, Sweat evaporation, Heat balance, Critical environmental limits, Thermoregulation
Introduction
The primary mechanism by which the human body is cooled during passive and exertional heat stress is the evaporation of sweat. Maximal evaporative heat loss is directly related to the proportion of the body covered in sweat, i.e., maximal skin wettedness (ωmax). Current heat strain standards, such as ISO 7933, use ωmax to calculate heat exchange between an individual and their environment, but rely on a previously determined ωmax of 1.00 for acclimated individuals and 0.85 for unacclimated individuals (Candas et al. 1979). However, these ωmax values are derived from of a small sample size of young adult men lying supine. More recently, Ravanelli et al. determined ωmax in unacclimated, untrained adults to be ~ 0.72 (Ravanelli et al. 2018). However, this is specific to a warm-humid environment and a metabolic heat production of 450 W, a much higher metabolic rate than in the present study.
Using partitional calorimetry in conjunction with a progressive heat stress protocol, our laboratory previously determined ωmax in young, middle-aged, and older adults in a single hot-dry and single -warm-humid environment at a low metabolic rate representative of activities of daily living (Fisher et al. 2024). Young unacclimated adults exhibited mean ωmax of 0.69 and 0.52 in the warm-humid and hot-dry environments, respectively. Older unacclimated adults exhibited significantly lower ωmax of 0.47 and 0.40 in the warm-humid and hot-dry environments. ωmax was further reduced when the older adults were resting, illustrating the variability present in ωmax as a function of age, environmental condition, and the intensity of the activity preformed.
Previous work in our laboratory empirically derived critical environmental limits representing the boundary between compensable and uncompensable heat stress across the adult age span from 18 to 92 yrs (Wolf et al. 2022, 2023; Cottle and Fisher 2024). These data represent a best-case scenario for prolonged exposure in an indoor setting during seated rest or at low metabolic rates representing activities of daily living in and around the home and illustrate the decline in critical environmental limits with advancing age. As ωmax represents the highest achievable skin wettedness at the critical environmental limit, establishing empirically derived ωmax for specific age cohorts, metabolic rates, and environmental conditions can aid in more accurate heat stress modeling and prediction.
Therefore, the purpose of the present study was to use partitional calorimetry to determine ωmax at the previously established critical environmental limits (Wolf et al. 2022, 2023) (i.e., the upper limit of compensable heat stress) for young and older adults across a wider spectrum of environmental conditions and at multiple low metabolic rates.
Methods
Subjects
All testing was conducted in controllable environmental chambers housed in Noll Laboratory at the Pennsylvania State University and all procedures were approved by the Pennsylvania State University Institutional Review Board and conformed to the Declaration of Helsinki’s stated guidelines. Prior to participation participants gave oral and written consent after being informed of all aspects of the experimental study. All experimental procedures are registered on ClinicalTrials.gov (NCT0428439).
Calculated values for ωmax presented herein utilized data collected at the upper limits of compensable heat stress that were previously determined for the present cohort of subjects (Wolf et al. 2022, 2023). Young and older groups were aged 18–29 yrs and 65–89 yrs, respectively. To increase generalizability of the data, subjects were recruited without regard to individual characteristics such as body size, aerobic fitness, blood pressure, etc. There was no attempt to control for menstrual status, contraceptive use, or heat acclimatization status. Before each experiment, subjects refrained from vigorous exercise for 24 h and caffeine for 12 h and provided a urine sample upon arrival to ensure euhydration (urine specific gravity ≤ 1.020) (PAL-S, Atago, Bellevue, WA, USA). All participants wore standard clothing during experiments of ~ 0.3 CLO, consisting of a short-sleeved t-shirt, a sports bra (women), athletic shorts, socks, and walking/running shoes.
Experimental procedures
All experimental procedures have been previously reported in detail (Wolf et al. 2022, 2023). The a prior power analysis for the previous research applies here (Wolf et al. 2023). All subjects performed light physical activity on a cycle ergometer against zero resistance at a cadence of 40–50 rpm to represent the metabolic cost associated with minimal activities of daily living (MinAct; ~150 W). Older subjects completed additional experimental trials during seated rest (~ 90 W). Young subjects completed additional trials walking continuously on a motor-driven treadmill at a speed of 2.2 mi/h and grade of 3% to achieve light ambulatory activity (LightAmb; ~250 W).
During critical water vapor pressure trials (Pcrit), dry-bulb temperature (Tdb) was held constant at either 34, 36, 38, or 40 °C. During critical temperature trials (Tcrit), water vapor pressure (Pa) was held constant at either 12, 16, or 20 mmHg. Following a 30-minute equilibration period in which ambient chamber conditions were held constant, either Tdb (Tcrit tests) or Pa (Pcrit tests) were increased in a stepwise fashion (1 °C or 1 mmHg every five minutes) until a clear upward inflection in Tc was detected. Experimental trials lasted approximately 90–120 min. The Tc inflection marked the upper limit of compensable heat stress for that trial. ωmax was calculated using environmental and physiological data at the Tc inflection point.
Measurements and partitional calorimetry calculations
Continuous measurements of Tc were recorded using gastrointestinal temperature telemetry capsules (VitalSense, Philips Respironics, Bend, OR, USA; BodyCap, Hérouville-Saint-Clair, France). Subjects ingested the capsules 1–2 h before reporting to the laboratory in accordance with previously published data demonstrating that ingestion times from 1 to 12 h before use do not influence the precision of Tc data (Notley et al. 2021). Skin temperature was measured continuously on each subject’s chest (Tchest), arm (Tarm), thigh (Tthigh), and lower leg (Tleg) and was used to calculate a weighted mean skin temperature (Tsk):
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To calculate thermal balance parameters and ωmax, partitional calorimetry was used. The rate of oxygen uptake (VO2) and respiratory exchange ratio (RER) were measured twice during experimental trials at 5 and 60 min using open-circuit spirometry (Parvo Medics TrueOne® 2400, Parvo, UT, USA). Average VO2 and RER values from the two time points (which did not differ significantly), were used to calculate metabolic rate. Metabolic rate [M; Watts (W)], normalized to body surface area, was calculated from V̇o2 and RER (Cramer and Jay 2019) as:
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where AD is the Dubois surface area (m2). Because minimal external work (W) was done by participants during MinAct and rest trials, M equaled net metabolic rate (Mnet). For treadmill walking trials, external work was calculated as
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where mb is the body mass (k), Vw is the walking velocity, and Fg is the fractional grade of the treadmill (Cramer and Jay 2019). For walking Mnet was calculated as M – W.
Heat balance calculations were performed at the final compensable combination of Tdb and Pa, i.e., the 5-min period immediately preceding the upward core temperature inflection. Dry heat exchange via radiation and convection (R + C; W∙m−2) was determined (Cramer and Jay 2019; Barker et al. 1999) as
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where 
is assumed to be 0.163 m2∙°C∙W−1 and is the total calculated insulation between the skin surface to the environment (Barker et al. 1999). In conditions of greater air or body movement 
should be adjusted to a resultant value via ISO9920 to reflect the air movement.
Maximal evaporative capacity of the environment (Emax; W∙m−2) was calculated as (Barker et al. 1999)
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where Ps, sk is the saturated vapor pressure at the skin surface (assuming 100% relative humidity at the given skin temperature) and 
the clothing total evaporative resistance, is 0.13 m2∙mmHg∙W−1 (Wang 2011).
The evaporative cooling required to maintain thermal balance (Ereq; W∙m−2) was calculated from the heat balance equation as:
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where Cres (the convective heat loss associated with respiration; <1 W∙m−2), Eres (evaporative heat loss associated with respiration; ≤2 W∙m−2), and S (heat storage) were considered to be negligible (Cottle et al. 2022).
Maximum skin wettedness (ωmax; unitless) was calculated from Ereq and Emax (Cramer and Jay 2019) as
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Total sweat rate (SR, g∙m−2∙h−1) and percent body mass loss (% BML) were determined during each experiment from the loss of nude body mass on a scale accurate to ± 10 g. Fluid intake was prohibited between the initial and final measurements of nude body mass.
Statistical analyses
All statistical analyses were performed using IBM SPSS Statistics, v. 28, IBM Corp., Armonk, NY. Independent samples t-tests were used to compare subject characteristics present in Table 1 between young and older adults. Independent samples t-tests were used to compare ωmax in each environmental condition between young and older adults during MinAct. Differences in ωmax due to metabolic rate were compared within young and older adults individually using pair-samples t-test. Graphical and Table 1 data are reported as means ± SD. Tables 2 and 3 data is presented as mean and 95% confidence intervals. The Bonferroni method was used to account for multiple comparisons between age groups and metabolic rates, significance was accepted at α = 0.008 for the age group comparison and older adults (6 comparisons) and α = 0.007 for young adults.
Table 1.
Subject characteristics
| Characteristic | Young | Older |
|---|---|---|
| n | 51 (29 F) | 55 (33 F) |
| Age Range (yr) | 18–33 | 65–92 |
| Height (m) | 1.7 ± 0.1 | 1.7 ± 0.1 |
| Weight (kg) | 75 ± 15 | 73 ± 17 |
| BMI (kg∙m−2) | 25 ± 4 | 26 ± 5 |
| AD (m2) | 1.9 ± 0.2 | 1.8 ± 0.2 |
| AD∙mass−1 (m2∙kg−1) | 0.026 ± 0.002 | 0.025 ± 0.003 |
| VO2max (ml∙kg−1∙min−1) | 46 ± 12 | 28 ± 9* |
*P < 0.001 compared to young. AD, DuBois body surface area; AD·kg−1, body surface area-to-mass ratio; Vo2max, maximal oxygen consumption
Table 2.
Environmental conditions, partitional calorimetry parameters, and ωmax for young adults during minact and lightamb in all environments
| MinAct | 34 °C | 36 °C | 38 °C | 40 °C | 12 mmHg | 16 mmHg | 20 mmHg |
| Tdb (°C) | 33.9 | 36.0 | 38.1 | 40.1 | 49.3 | 46.4 | 43.7 |
| (33.7, 34.2) | (35.9, 36.1) | (38.0, 38.3) | (39.9, 40.3) | (48.2, 50.4) | (45.5, 47.3) | (42.5, 44.9) | |
| rh (%) | 79.7 | 66.5 | 46.9 | 36.8 | 13.8 | 20.7 | 29.5 |
| (77.6, 81.7) | (64.1, 69.0) | (45.6, 48.2) | (34.4, 39.1) | (12.9, 14.7) | (19.9, 21.5) | (27.8, 31.2) | |
| Twb (°C) | 30.8 | 30.4 | 31.1 | 30.6 | 25.5 | 26.9 | 27.8 |
| (30.5, 31.9) | (29.9, 30.8) | (30.2, 32.0) | (30.0, 31.3) | (25.3, 25.8) | (26.6, 27.2) | (27.4, 28.2) | |
| Pa (mmHg) | 31.7 | 29.6 | 29.6 | 27.6 | 12.2 | 16.3 | 19.7 |
| (30.9, 32.5) | (28.6, 30.7) | (27.7, 31.5) | (26.4, 28.9) | (12.0, 12.4) | (16.0, 16.5) | (19.3, 20.1) | |
| SR (g∙m−2∙h−1) | 122 | 140 | 196 | 178 | 167 | 142 | 121 |
| (93, 150) | (97, 184) | (143, 249) | (141, 215) | (123, 210) | (109, 174) | (89, 152) | |
 (°C) |
36.2 | 36.3 | 36.5 | 37.7 | 38.5 | 38.0 | 37.5 |
| (36.1, 36.4) | (36.1, 36.4) | (36.2, 36.8) | (36.4, 37.1) | (38.3, 38.7) | (37.8, 38.2) | (37.1, 37.8) | |
| (R + C) (W∙m−2) | −14 | −2 | 10 | 21 | 66 | 52 | 38 |
| (−15, −13) | (−3, −1) | (8, 12) | (19, 23) | (60, 72) | (46, 57) | (34, 42) | |
| Ereq (W∙m−2) | 70 | 74 | 144 | 156 | 155 | 137 | 122 |
| (66, 74) | (68, 79) | (138, 151) | (150, 163) | (147, 164) | (130, 144) | (115, 128) | |
| Emax (W∙m−2) | 101 | 115 | 169 | 184 | 297 | 259 | 221 |
| (94, 107) | (109, 122) | (158, 180) | (171, 198) | (294, 300) | (255, 263) | (216, 226) | |
| ωmax | 0.71 | 0.65 | 0.73 | 0.65 | 0.52 | 0.53 | 0.55 |
| (0.65, 0.77) | (0.59, 0.70) | (0.68, 0.93) | (0.61, 0.79) | (0.49, 0.55) | (0.50, 0.56) | (0.52, 0.58) | |
| LightAmb | 34 °C | 36 °C | 38 °C | 40 °C | 12 mmHg | 16 mmHg | 20 mmHg |
| Tdb (°C) | 33.8 | 35.9 | 38.1 | 40.0 | 44.5 | 42.5 | 39.1 |
| (33.7, 34.0) | (35.9, 36.0) | (37.8, 38.3) | (39.9, 40.1) | (43.4, 45.5) | (41.7, 43.2) | (38.5, 39.7) | |
| rh (%) | 60.8 | 54.6 | 60.9 | 51.0 | 18.6 | 25.6 | 37.9 |
| (57.3, 64.4) | (52.5, 56.7) | (56.3, 65.6) | (48.2, 53.8) | (17.1, 20.0) | (24.5, 26.6) | (36.9, 38.8) | |
| Twb (°C) | 27.4 | 28.0 | 28.2 | 27.2 | 24.1 | 25.7 | 26.8 |
| (26.7, 28.0) | (27.6, 28.4) | (27.9, 28.4) | (26.5, 27.8) | (23.8, 24.5) | (25.5, 25.9) | (26.5, 27.0) | |
| Pa (mmHg) | 24.0 | 24.2 | 23.0 | 20.9 | 12.1 | 16.0 | 20.0 |
| (22.7, 25.4) | (23.3, 25.2) | (22.4, 23.6) | (19.5, 22.2) | (11.9, 12.3) | (15.9, 16.2) | (19.7, 20.2) | |
| SR (g∙m−2∙h−1) | 179 | 205 | 269 | 291 | 223 | 194 | 197 |
| (150, 207) | (168, 242) | (220, 317) | (246, 335) | (188, 258) | (153, 235) | (170, 225) | |
 (°C) |
35.9 | 35.8 | 36.6 | 37.0 | 37.4 | 37.1 | 36.0 |
| (35.7, 36.1) | (35.6, 36.1) | (36.4, 36.8) | (36.8, 37.3) | (37.2, 37.6) | (36.9, 37.3) | (35.6, 36.4) | |
| (R + C) (W∙m−2) | −13 | 0 | 9 | 18 | 38 | 32 | 19 |
| (−15, −11) | (−1, 2) | (8, 10) | (17, 20) | (33, 43) | (28, 36) | (16, 22) | |
| Ereq (W∙m−2) | 125 | 132 | 88 | 99 | 180 | 163 | 152 |
| (117, 133) | (125, 140) | (83, 93) | (94, 103) | (171, 189) | (156, 170) | (144, 160) | |
| Emax (W∙m−2) | 155 | 151 | 117 | 141 | 279 | 241 | 188 |
| (145, 165) | (145, 156) | (102, 133) | (128, 154) | (273, 285) | (235, 247) | (181, 194) | |
| ωmax | 0.82 | 0.88 | 0.84 | 0.83 | 0.65 | 0.68 | 0.82 |
| (0.75, 0.88) | (0.82, 0.94) | (0.78, 0.98) | (0.78, 0.97) | (0.62, 0.68) | (0.64, 0.72) | (0.75, 0.88) |
Data are presented as MEAN (95% confidence intervals). Tdb, ambient dry-bulb temperature at critical environmental limit; rh, relative humidity at critical environmental limit; Twb, ambient wet-bulb temperature at critical environmental limit; Pa, ambient water vapor pressure at critical environmental limit; 
, mean skin temperature at critical environmental limit; SR, sweat rate; (R + C), radiative and convective heat exchange; Ereq, evaporative cooling required to maintain thermal balance; Emax, maximal evaporative heat loss; ωmax, maximal skin wettedness
Table 3.
Environmental conditions, partitional calorimetry parameters, and ωmax for older adults during rest and minact in all environments
| Rest | 34 °C | 36 °C | 38 °C | 40 °C | 12 mmHg | 16 mmHg |
| Tdb (°C) | 33.9 | 36.2 | 38.1 | 40.1 | 46.3 | 43.4 |
| (33.7, 34.2) | (35.9, 36.4) | (37.9, 38.3) | (40.0, 40.1) | (44.3, 48.2) | (41.1, 45.6) | |
| rh (%) | 59.2 | 60.0 | 53.4 | 37.7 | 15.6 | 25 |
| (55.0, 63.4) | (55.6, 64.5) | (48.3, 58.5) | (33.5, 41.9) | (13.7, 17.4) | (22.2, 27.8) | |
| Twb (°C) | 26.9 | 29.2 | 29.5 | 27.4 | 24.4 | 25.9 |
| (26.0, 27.7) | (28.3, 30.1) | (28.4, 30.6) | (26.3, 28.5) | (23.9, 24.9) | (25.3, 26.6) | |
| Pa (mmHg) | 23.5 | 26.9 | 26.7 | 21.6 | 11.7 | 16.1 |
| (21.8, 25.2) | (24.9, 28.9) | (24.2, 29.3) | (19.3, 23.9) | (11.2, 12.2) | (15.7, 16.5) | |
| SR (g∙m−2∙h−1) | 48 | 51 | 91 | 75 | 58 | 49 |
| (35, 61) | (33, 70) | (27, 155) | (40, 111) | (34, 81) | (35, 63) | |
 (°C) |
35.5 | 36.2 | 36.4 | 36.7 | 37.3 | 36.8 |
| (35.2, 35.8) | (36.0, 36.4) | (36.2, 36.6) | (36.4, 37.0) | (36.9, 37.6) | (36.3, 37.3) | |
| (R + C) (W∙m−2) | −10 | 0 | 11 | 20 | 48 | 37 |
| (−12, −8) | (−2, 1) | (9, 12) | (18, 23) | (37, 60) | (24, 49) | |
| Ereq (W∙m−2) | 39 | 46 | 60 | 72 | 93 | 88 |
| (33, 45) | (42, 50) | (54, 66) | (67, 77) | (78, 108) | (73, 102) | |
| Emax (W∙m−2) | 151 | 133 | 137 | 187 | 286 | 232 |
| (133, 168) | (121, 145) | (118, 156) | (165, 210) | (255, 318) | (220, 244) | |
| ωmax | 0.28 | 0.36 | 0.48 | 0.40 | 0.33 | 0.37 |
| (0.21, 0.35) | (0.31, 0.41) | (0.38, 0.58) | (0.34, 0.46) | (0.28, 0.37) | (0.32, 0.43) | |
| MinAct | 34 °C | 36 °C | 38 °C | 40 °C | 12 mmHg | 16 mmHg |
| Tdb (°C) | 33.8 | 36.2 | 38.0 | 40.2 | 42.9 | 40.4 |
| (33.6, 34.1) | (36, 36.4) | (37.8, 38.3) | (40.0, 40.3) | (40.9, 44.8) | (38.5, 42.2) | |
| rh (%) | 58.4 | 52.8 | 46.1 | 33.7 | 18.8 | 31.5 |
| (48.9, 68.0) | (47.2, 58.4) | (41.7, 50.6) | (28.9, 38.5) | (17.0, 20.5) | (27.6, 35.4) | |
| Twb (°C) | 26.8 | 27.8 | 27.8 | 26.4 | 23.5 | 24.5 |
| (24.9, 28.7) | (26.5, 29.0) | (26.7, 29.0) | (25.1, 27.7) | (22.9, 24.2) | (23.8, 25.1) | |
| Pa (mmHg) | 23.1 | 24.9 | 23.0 | 19.5 | 11.9 | 15.8 |
| (20.0, 26.3) | (22.4, 27.3) | (20.6, 25.5) | (16.8, 22.2) | (11.6, 12.2) | (15.5, 16.0) | |
| SR (g∙m−2∙h−1) | 78 | 87 | 117 | 90 | 91 | 85 |
| (52, 103) | (58, 116) | (52, 182) | (62, 118) | (59, 122) | (51, 119) | |
 (°C) |
35.6 | 36.3 | 36.6 | 37.2 | 37.4 | 36.4 |
| (35.4, 35.9) | (36.1, 36.6) | (36.3, 36.9) | (37.0, 37.4) | (37.0, 37.8) | (35.8, 37.0) | |
| (R + C) (W∙m−2) | −11 | −1 | 9 | 18 | 34 | 24 |
| (−9, −13) | (−3, 1) | (7, 11) | (16, 20) | (24, 44) | (17, 31) | |
| Ereq (W∙m−2) | 66 | 75 | 88 | 103 | 111 | 98 |
| (61, 70) | (69, 81) | (82, 95) | (95, 111) | (99, 124) | (84, 111) | |
| Emax (W∙m−2) | 154 | 152 | 171 | 219 | 281 | 227 |
| (127, 180) | (134, 170) | (152, 190) | (195, 242) | (273, 290) | (219, 235) | |
| ωmax | 0.47 | 0.52 | 0.52 | 0.49 | 0.39 | 0.43 |
| (0.37, 0.57) | (0.45, 0.59) | (0.47, 0.62) | (0.42, 0.55) | (0.36, 0.43) | (0.37, 0.48) |
Data are presented as MEAN (95% confidence intervals). Tdb, ambient dry-bulb temperature at critical environmental limit; rh, relative humidity at critical environmental limit; Twb, ambient wet-bulb temperature at critical environmental limit; Pa, ambient water vapor pressure at critical environmental limit; 
, mean skin temperature at critical environmental limit; SR; sweat rate; (R + C), radiative and convective heat exchange; Ereq, evaporative cooling required to maintain thermal balance; Emax, maximal evaporative heat loss; ωmax, maximal skin wettedness
Results
Subject characteristics are shown in Table 1. V̇O2max was significantly lower in older compared to young adults (P < 0.001), but there were no differences in height, weight, body surface area (AD), or body surface area-to-mass ratio (AD·kg−1) between young and older adults (all P ≥ 0.05).
Tables 2 and 3 present environmental conditions, partitional calorimetry parameters, and ωmax for young and older adults, respectively, at each metabolic rate for all environmental conditions. There was a main effect of the skin temperature and sweat rate for older compared to young adults during MinAct such that skin temperature and sweat rate are lower in the older cohort (both P < 0.05).
Older adults exhibited lower ωmax compared to young adults in all environments during MinAct (all P ≤ 0.008; Fig. 1). Young adults had lower ωmax during MinAct compared to LightAmb in all Tcrit trials (all P ≤ 0.00002; Fig. 2) and the Pcrit trial in which Tdb was held constant at 36 °C (P = 0.00007; Fig. 2). Older adults exhibited lower ωmax at rest compared to Y during MinAct only in the Pcrit trial in which Tdb was held constant at 36 °C (P = 0.0008; Fig. 2).
Fig. 1.
Comparisons of maximal skin wettedness (ωmax) between young and older adults during minimal activities of daily living. Across all environmental conditions, ωmax was lower in older adults compared to their young counterparts (all P ≤ 0.008). Filled bars represent means and error bars represent standard deviations. The range of dry-bulb temperature and relative humidity in each environment is placed under the label for each environment. See Table 2 for additional psychometric parameters associated with x-axis values
Fig. 2.
Comparison of maximal skin wettedness (ωmax) between metabolic rates in young (A) and older (B) adults. Young adults had lower ωmax during minimal activities of daily living (MinAct) compared to light ambulation (LightAmb) in all critical temperature trials (Tcrit; all P < 0.007) and the critical water vapor pressure trial (Pcrit) in which temperature was held constant at 36 °C (P < 0.007). Older adults had lower ωmax at rest compared to MinAct only in the Pcrit trial in which temperature was held constant at 36 °C (P < 0.008). Filled bars represent means and error bars represent standard deviations. The range of dry-bulb temperature and relative humidity in each environment is placed under the label for each environment. See Table 2 for additional psychometric parameters associated with x-axis values
Discussion
The overall aim of this study was to characterize ωmax in unacclimated young and older adults across a wide range of environments and at various low metabolic rates. These data illustrate that ωmax in adults over 65 yrs is lower for a given metabolic rate across a wide spectrum of hot and humid environments compared to their young counterparts. Additionally, this study established ωmax during light ambulatory activity in young adults and resting older adults in environments ranging from 34 to 49 °C Tdb and 14–80% rh. These values can be used to model responses to heat stress in these unacclimated populations at a low metabolic rate and potentially aid in the prediction of risk of heat strain.
We have previously reported that ωmax decreases with advancing age from 18 to 89 yrs in both a warm-humid and a hot-dry environment during minimal activity (Fisher et al. 2024). Similarly, in the present study across a much larger range of environments from very hot-humid to very hot-dry, ωmax was lower in older adults compared to young adults during minimal activities of daily living (Fig. 1). Skin wettedness depends on both the capacity for sweat production as well as the environmental capacity for evaporation (Ravanelli et al. 2018; Mochida et al. 1997). Therefore, lower sweating output coupled with high ambient humidity at the critical environmental limits in older adults may result in a lower proportion of the skin covered with sweat (Tables 2 and 3). Further, higher ambient temperature and/or lower ambient water vapor pressure resulted in lower ωmax (Mochida et al. 1997; Atmaca and Yigit 2006), illustrating the dependance of ωmax on sweating output as well as environmental conditions.
Further, these data demonstrate variability in ωmax as a function of metabolic rate, even the rest-to-low metabolic rates tested herein. For example, in the hot-dry environmental conditions, young adults exhibit higher ωmax during LightAmb compared to MinAct (Fig. 2). Higher exercise intensity and therefore metabolic heat production elicits greater sweat rates leading to higher ωmax (Ravanelli et al. 2018; Shapiro et al. 1982). Conversely, the elevation in ωmax at the higher relative metabolic rate was less evident in the older cohort (Fig. 2). This may be due to a basement effect, given the already low ωmax during MinAct in the older cohort.
If known, ωmax can be useful in calculating and/or predicting heat balance and related variables. Incorporating ωmax values established herein and elsewhere (Ravanelli et al. 2018; Fisher et al. 2024), appropriate for the specific environment, metabolic rate, and cohort, may result in more accurate representation of evaporative heat loss and heat balance. For example, using the following equation evaporative heat loss from the skin in W·m−2 can be calculated based on ω and vapor pressure differences between the body and the surrounding air (Atmaca and Yigit 2006).
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8 |
Current models using ωmax to calculate and/or predict heat exchange between an individual and his/her environment rely on ωmax values obtained from seminal work by Candas et al. who reported that highest achievable ωmax for unacclimated individuals was 0.85 and 1.00 for heat-acclimated individuals (Candas et al. 1979). During minimal activity in the present study, in some cases ωmax was as low as ~ 0.42 and ~ 0.23 and as high as ~ 0.99 and ~ 0.83, in young and older adults respectively, depending on age and environmental conditions. Therefore, sweating capacity, which changes with aging, and environmental temperature and absolute humidity greatly impact ωmax; single static values do not accurately represent that variability in ωmax.
The ωmax values established herein may be incorporated into heat stress models to more accurately represent responses in these age cohorts across a wide range of environments and at various metabolic rates, assuming individuals are at their limit of compensable/uncompensable heat stress. However, these ωmax values are specific to the cohort, environment, and metabolic rate they were determined in. Additionally, as these calculations are based on previously published data, we are limited in the activities performed by age cohorts during the experiments. Therefore, future research is needed to further determine ωmax, such as at different activities, higher metabolic rates, and following training and acclimation in older adults.
Acknowledgements
This work was supported by National Institutes of Health Grant R01 AG067471 (WLK) and National Institutes of Aging Grant T32 AG049676 to (KGF and RMC).
Author contributions
K.G.F. and W.L.K. conceived and designed research; K.G.F, O.K.L and R.M.C. performed experiments; K.G.F. analyzed data; All authors interpreted results of experiments; K.G.F. prepared figures; K.G.F. drafted manuscript; All authors edited and revised manuscript; All authors approved final version of manuscript.
Funding
This work was supported by NIH Grant R01 AG067471 (WLK) and NIA Grant T32 AG049676 to (RMC, and KGF).
Data availability
Source data supporting the conclusions in this paper are publicly available and can be found at 10.5281/zenodo.14592713.
Declarations
Ethics approval
All procedures were approved by the Pennsylvania State University Institutional Review Board and conformed to the Declaration of Helsinki’s stated guidelines.
Consent to participate
Prior to participation participants gave oral and written consent after being informed of all aspects of the experimental study.
Conflict of interest
No conflicts of interest, financial or otherwise, are declared by the authors.
Footnotes
Publisher’s note
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Data Availability Statement
Source data supporting the conclusions in this paper are publicly available and can be found at 10.5281/zenodo.14592713.