Keeping older individuals cool in hot and moderately humid conditions: wetted clothing with and without an electric fan

Abstract

The present study evaluated whether wearing a water-soaked t-shirt, with or without electric fan use, mitigates thermal and cardiovascular strain in older individuals exposed to hot and moderately humid conditions. Nine healthy older individuals (68 ± 4 yr; five women) completed three 120-min heat exposures (42.4 ± 0.2°C, 34.2 ± 0.9% relative humidity) on separate days while wearing a dry t-shirt (CON), a t-shirt soaked with 500 ml of tap water (WET), or a t-shirt soaked with 500 ml of tap water while facing an electric fan (2.4 ± 0.4 m/s; WET+FAN). Measurements included core and skin temperatures, evaporative mass losses, heart rate, and blood pressure. In the WET condition, elevations in core temperature were attenuated compared with DRY from 30 to 120 min and compared with WET+FAN from 30 to 90 min (P < 0.05). Evaporative mass losses (inclusive of sweat and water losses from the shirt) were greatest in WET+FAN, followed by WET, and then DRY (P < 0.01). Sweat losses were lowest in WET, followed by DRY, and then WET+FAN (P < 0.01). Heart rate was lower only at 60 min in WET versus DRY (P = 0.01). No differences in mean arterial pressure were observed (P = 0.51). In conclusion, wearing a water-soaked t-shirt without, but not with, electric fan use is an effective heat management strategy to mitigate thermal strain and lower sweat losses in older individuals exposed to hot and moderately humid conditions.

NEW & NOTEWORTHY In older individuals exposed to hot and moderately humid environments, electric fan use coupled with a water-soaked t-shirt exacerbates sweat losses without mitigating heat strain compared with a dry t-shirt. However, wearing a water-soaked t-shirt without fan use reduces sweat losses and attenuates heat strain compared with a dry t-shirt and a fan/water-soaked t-shirt combination. These findings suggest wearing a water-soaked t-shirt is an effective heat-management strategy for older individuals during heat waves when air conditioning is inaccessible.

INTRODUCTION

As average global surface temperatures are projected to rise, and heat waves are expected to increase in frequency, duration, and intensity (24, 33), exposure to high levels of heat stress will occur with greater regularity. Older individuals are particularly vulnerable to heat stress, evidenced by greater hospitalizations and mortality among individuals aged >60 yr during heat waves (2, 12, 21, 50, 60). Indoor cooling with an air conditioner remains the most effective strategy to prevent the deleterious health effects of heat waves (21, 29, 38, 42, 51, 62). However, recent estimates suggest ~12% of American households in hot-dry or hot-humid climates do not have air conditioners (58). For individuals using air conditioners, rising electricity demands associated with residential cooling will increase household energy costs, potentially making air conditioning unaffordable for lower- and fixed-income individuals (22, 31). Higher energy demands will also exacerbate rising global temperatures through greater greenhouse gas emissions, which are linked to the rising incidence of heat waves (33). Finally, the risk of power outages during periods of peak electricity demand or following extreme weather events threatens access to air conditioning when it would be most protective (8, 59). Therefore, alternative cooling modalities with minimal power requirements are needed for older individuals unable to benefit from air conditioning.

The vulnerability of older individuals to environmental heat stress stems from the age-related decline in thermo-effector function. Specifically, sedentary older individuals can exhibit a delayed onset of sweating (11, 18, 20) and/or a diminished sweat sensitivity (10, 23, 53), resulting in an attenuated rate of whole body sweat production compared with younger individuals. The consequently lower rate of evaporative heat loss leads to greater body heat storage (28) and elevations in core temperature (7, 34, 48) that provoke higher levels of cardiovascular strain (12, 21, 27, 50, 60), culminating in a heightened risk of heat-related health problems. It follows that strategies aimed at correcting the attenuated rates of sweat secretion and evaporative heat loss among older individuals may alleviate thermal and cardiovascular strain, and, thereby, mitigate the risk of heat-related illness during heat waves.

A recent news article covering heat-related mortality in vulnerable populations (56) reported on the use of a “swamp cooler” strategy—soaking clothing with water and then seeking a breezy location—to alleviate outdoor heat stress. During exposure to hot air temperatures, greater air velocity over the skin surface elevates the rate of evaporative heat dissipation, provided the skin surface is sufficiently covered with sweat (1, 30, 36, 46, 47). Decrements in sweat output with age make electric fan use by older individuals detrimental during extreme heat stress (18). However, electric fan use while wearing a water-soaked t-shirt may be beneficial, as moisture trapped in the shirt could supplement sweat production, possibly leading to greater evaporative heat loss and an attenuated level of heat strain. Importantly, this so-called “swamp cooler” strategy relies only on access to clothing, tap water, and an electric fan, making it a simple and relatively inexpensive cooling option for older people.

It is also possible that the combination of a water-soaked t-shirt and electric fan provides no benefit or is, in fact, detrimental. Although fan use enhances evaporative heat loss potential (1, 30, 46, 47), it also exacerbates the rate of convective heat gain in air temperatures exceeding mean skin temperature, which usually occurs above ~35°C. Therefore, by combining a water-soaked t-shirt with an electric fan, added convective heat gain may offset any improvement in evaporative heat loss (25, 36, 47). If this is the case, wearing a water-soaked t-shirt without fan use might be a better strategy to attenuate heat strain in older individuals than the water-soaked t-shirt/electric fan combination, as wearing a t-shirt soaked in tap water alone would increase conductive and evaporative cooling but avoid additional convective heat gain. In support, Song et al. (55) recently found that intermittently soaking a summer clothing ensemble attenuated the elevation in core temperature by ~0.2°C in young men during an extremely severe 90-min passive heat exposure (~43°C, 57% relative humidity). Additionally, Morris et al. (37) recently reported that self-dousing with water during a 120-min hot-humid exposure attenuated elevations in heart rate in young adults. However, to our knowledge, the effect of wearing a water-soaked t-shirt on heat strain among older individuals exposed to environmental conditions representative of very hot and moderately humid heat waves in the United States (e.g., Chicago in 1995 or Newark in 2001 and 2011) has not been investigated. Since this approach requires only clothing and tap water, wearing a water-soaked t-shirt would be an inexpensive and accessible strategy to mitigate heat strain, even during power outages.

The purpose of this study was to assess whether a water-soaked t-shirt, with or without electric fan use, improves evaporation and attenuates heat strain (i.e., the rise in core temperature) in older individuals during an exposure to hot and moderately-humid conditions. It was hypothesized that 1) wearing a water-soaked t-shirt in combination with electric fan use results in offsetting increases in evaporative heat loss and convective heat gain, leading to a similar level of heat strain; and 2) wearing a water-soaked t-shirt alone augments evaporation and attenuates heat strain compared with a wet-shirt/electric fan combination and a dry t-shirt.

METHODS

Participants.

Nine older individuals completed the study (five women and four men; age, 68 ± 4 yr; body mass, 70.4 ± 10.4 kg; height, 1.68 ± 0.09 m; body surface area, 1.79 ± 0.17 m²; and body mass index, 24.9 ± 2.9 kg/m²). All participants completed a detailed medical history questionnaire and underwent a 12-lead electrocardiogram and resting supine blood pressure measurements before testing. None of the participants reported a history of cardiovascular, neurological, respiratory, or metabolic disease. Self-reported physical activity levels varied among the participants: one individual was completely sedentary, four individuals participated in light-intensity exercise several times per week (e.g., golf, walking), and four individuals participated in moderate-intensity activities 2–5 times per week (e.g., running, cycling, weight-lifting, aerobics classes). A detailed explanation of the study procedures and potential risks was provided before written informed consent was obtained. The study protocol was approved by the Institutional Review Boards of the University of Texas Southwestern Medical Center and Texas Health Presbyterian Hospital Dallas (STU 092017-078). All procedures conformed to standards set forth in the Declaration of Helsinki.

Measurements.

Body mass was measured using a precision balance with ±10 g accuracy (Mettler Toledo, OH). Height was measured using a stadiometer (Detecto, Webb City, MO). Body surface area was estimated using the equation of DuBois and DuBois (9). Urine specific gravity (USG) was measured with a hand-held refractometer (Atago, Bellevue, WA). Core temperature was measured from the gastrointestinal tract using a telemetric pill ingested ~2 h before experimentation (HQ, Palmetto, FL). Skin temperatures were measured at six sites on the right side of the body using thermocouples, with mean skin temperature calculated as the weighted average of chest, upper back, lower back, abdomen, anterior thigh, and calf temperatures (57). Skin blood flow was indexed from laser-Doppler flux (LDF), which was measured using an integrated laser-Doppler probe placed on the midline of the anterior right thigh (Perimed, Sweden). Heart rate was recorded from an electrocardiogram (Solar 8000M; GE Medical Systems, Madison, WI). Arterial blood pressures were measured using an electrosphygmomanometer (Tango, SunTech Medical Instruments, Raleigh, NC), with mean arterial pressure subsequently calculated as 1/3·(pulse pressure) + diastolic blood pressure.

Experimental protocol.

Participants visited the laboratory on three occasions to complete a 120-min heat exposure while wearing a dry t-shirt (DRY), a t-shirt soaked with 500 ml of tap water while facing an electric fan (WET+FAN), and a t-shirt soaked with 500 ml of tap water without a fan (WET). The same large-sized cotton t-shirt was worn by each participant in all trials. In general, the t-shirt was loose-fitting in the DRY condition but adhered more tightly to the body surface in the WET and WET+FAN trials. The order of the experimental conditions was randomized. Participants arrived at the laboratory having consumed a light meal and ~500 ml of water 2 h before their visit, and having avoided anti-inflammatory medications for 36 h, alcohol and strenuous exercise for 24 h, and caffeine for 12 h before their visit. Hydration was determined upon arrival via USG, with euhydration accepted at a value of 1.025 or less (26). A nude body mass measurement was then taken, after which participants put on a standard preweighed clothing ensemble consisting of a cotton t-shirt and cotton athletic shorts (plus a sports bra for female participants) and were instrumented. Following 30 min of supine rest in a thermoneutral environment (23.2 ± 0.6°C and 41 ± 8% relative humidity), baseline blood pressures were measured. In the WET and WET+FAN trials, participants then donned a cotton t-shirt soaked with 500 g of tap water. This amount of water was chosen to saturate the t-shirt without dripping. Approximately 1 h before experimentation, the t-shirt was placed in a Ziploc bag and onto the scale. Room temperature tap water was then poured evenly across the entire t-shirt until 500 g had been added. The t-shirt was then left in the thermoneutral laboratory to soak. Upon removal from the Ziploc bag, the t-shirt was visually inspected to ensure no dry spots were evident. Participants then entered the environmental chamber and, after a brief transition to complete instrumentation in the chamber (3–4 min), the 2-h heat exposure commenced. Air temperature and relative humidity were 42.4 ± 0.2°C and 34.2 ± 0.9%, respectively, throughout the exposures. In the WET+FAN trial, an electric fan (16′′; Dayton, Niles, IL) connected to a variable autotransformer was positioned 1.2 m in front of the participant to supply an air velocity of 2.4 ± 0.4 m/s. The 1.2-m distance was measured between the center of the fan’s protective guard and the participant’s umbilicus. Throughout the heat exposure, participants remained still in a supine posture on a reclining chair, consisting of a steel frame and a breathable mesh fabric (Lafuma, Annency-Le-Vieux, France), positioned on the aforementioned platform scale. Using this experimental setup, we measured the participant’s mass (inclusive of the clothing and instrumentation) in triplicate every 5 min. Water was not reapplied to the t-shirt during heat stress in any trial. Drinking water was maintained at body temperature in a water bath and provided ad libitum. Arterial blood pressure measurements were taken every 15 min. Upon completion of the protocol, subjects removed their clothing and placed the items in preweighed Ziploc bags. The Ziploc bags with the clothing were then weighed.

Calculations.

Evaporative mass loss in grams was estimated based on the change in total mass, accounting for fluid intake. Total evaporative mass loss was partitioned into evaporated moisture from the shirt in the WET and WET+FAN trials, and evaporated sweat. The following calculations were used: Evap_total = Mass₁₂₀ –Mass₀ – Fluid_in – Mass_resp,met (g), Evap_shirt = Clothing₀ – Clothing₁₂₀ – Trapped Moisture (g), and Evap_sweat = Evap_total – Evap_shirt (g).

Evap_total is the total amount of moisture evaporated; Mass₀ and Mass₁₂₀ represent clothed body mass values (including the moisture content of the clothing and instrumentation) at the start (0 min) and end (120 min) of heat stress, respectively. Fluid_in is the amount of water consumed during heat stress. Mass_resp,met represents respiratory and metabolic mass losses, which were estimated using the equation of Mitchell et al. (35), based on an assumed average V̇o₂ value of 4.5 ml·kg^–1·min^–1 during passive heat stress (4). Clothing₀ and Clothing₁₂₀ represent clothing mass values at the start (0 min) and end (120 min) of heat stress, respectively. Trapped moisture is the amount of moisture remaining in the reclining chair after heat stress. Evap_sweat represents sweat evaporation.

Data and statistical analyses.

Core temperature, mean skin temperature, and heart rate data were acquired at a sampling frequency of 25 Hz (Biopac MP150, Santa Barbara, CA) and are depicted as averages over a 2-min period at each time point.

All data are reported as means ± SD. Baseline USG, baseline mean arterial pressure, baseline core temperature, and evaporative mass losses from each source (sweat, shirt, total) were compared between conditions using a one-way repeated-measures ANOVA. Time-dependent variables (change in core temperature, mean skin temperature, changes in mass, heart rate, and mean arterial pressure) were analyzed using a two-way repeated-measures ANOVA with the repeated factors of time (five levels: 0, 30, 60, 90, and 120 min) and experimental condition (three levels: DRY, WET, and WET+FAN). In the event of significant time-by-condition interaction, differences at each time point were assessed via paired t tests with a Holm-Bonferroni correction for multiple comparisons. Commercially available statistical software (GraphPad Prism version 8, La Jolla, CA) was used for all statistical analyses and for data visualization. Alpha was set at the 0.05 level.

RESULTS

Baseline measurements.

All participants presented with USG values below 1.025. No differences in baseline USG were observed between conditions (DRY: 1.019 ± 0.007; WET: 1.018 ± 0.006; WET+FAN: 1.018 ± 0.006; P = 0.92). Baseline mean arterial pressure did not differ between conditions (DRY: 92 ± 11 mmHg; WET: 93 ± 11 mmHg; WET+FAN: 93 ± 11 mmHg; P = 0.91). Similarly, core temperature at the onset of heat stress was not different between experimental conditions, averaging 36.7 ± 0.2°C in all conditions (P = 0.80).

Thermal responses.

Core temperature responses to the 120-min heat stress are presented in Fig. 1. A significant time-by-condition interaction was found for the change in core temperature (P < 0.01). Differences in the change in core temperature were observed at the 30-, 90-, and 120-min time points in DRY versus WET (P ≤ 0.02), from 30 to 90 min in WET versus WET+FAN (P ≤ 0.01), and at the 30-min time point in DRY versus WET+FAN (P = 0.048).

Fig. 1.

The change in gastrointestinal temperature throughout 120 min of heat stress while wearing a dry cotton t-shirt (DRY), a cotton t-shirt soaked with 500 ml of water while facing an electric fan (WET+FAN), and a cotton t-shirt soaked with 500 ml of water without a fan (WET). *Significantly different in DRY vs. WET, P < 0.05. †Significantly different WET vs. WET+FAN, P ≤ 0.01. ‡Significantly different in DRY vs. WET+FAN, P < 0.05. Data are expressed as means ± SD for nine participants.

Figure 2 shows the mean skin temperature responses. The time-dependent mean skin temperature response was significantly influenced by condition (P < 0.01), with differences in mean skin temperature observed from 30 to 90 min in DRY versus WET (P ≤ 0.01) and DRY versus WET+FAN conditions (P < 0.01). No differences in mean skin temperature were found between WET and WET+FAN conditions (P ≥ 0.053).

Fig. 2.

Mean skin temperatures throughout 120 min of heat stress while wearing a dry cotton t-shirt (DRY), a cotton t-shirt soaked with 500 ml of water while facing an electric fan (WET+FAN), a cotton t-shirt soaked with 500 ml of water without a fan (WET). *Significantly different in DRY vs. WET, P ≤ 0.01. †Significantly different WET vs. WET+FAN, P < 0.01. ‡Significantly different in DRY vs. WET+FAN, P < 0.01. Data represent means ± SD for nine participants.

Evaporative mass losses.

Changes in body mass over time, which reflect the rates of total evaporative mass losses (inclusive of the shirt), are presented in Fig. 3. In the DRY condition, no appreciable change in body mass was evident before 30 min, after which body mass began to decrease gradually. In contrast, body mass began to decrease sooner (~15 min of heat stress) in WET and WET+DRY conditions, and continued to decrease linearly afterward. Overall, body mass decreased most rapidly in WET+FAN, followed by WET, and then DRY (P < 0.01), with the rates of change in body mass averaging 7.0 ± 1.8 g/min, 3.9 ± 0.8 g/min, and 2.6 ± 1.5 g/min in WET+FAN, WET, and DRY, respectively.

Fig. 3.

Changes in clothed body mass throughout 120-min of heat stress while wearing a dry cotton t-shirt (DRY), a cotton t-shirt soaked with 500 ml of water while facing an electric fan (WET+FAN), and a cotton t-shirt soaked with 500 ml of water without a fan (WET). *Significantly different in DRY vs. WET, P < 0.01. †Significantly different WET vs. WET+FAN, P < 0.01. ‡Significantly different in DRY vs. WET+FAN, P < 0.01. Data represent means ± SD for nine participants.

Figure 4 shows evaporative mass losses from sweat, from the water-soaked t-shirt (in the WET and WET+FAN trials), and total evaporative mass losses. Total evaporative mass losses were highest in WET+FAN, which stemmed from greater evaporative mass losses from both sweat and the water-soaked t-shirt compared with the other trials (P < 0.01). Total evaporative mass losses were greater in WET compared with DRY (P < 0.01). This occurred despite lower evaporative mass losses from sweat in WET versus DRY due to the contribution of evaporative mass losses from the water-soaked t-shirt.

Fig. 4.

Individual data representing the source-specific mass of evaporated moisture during 120 min of heat stress while wearing a dry cotton t-shirt (DRY), a cotton t-shirt soaked with 500 ml of water while facing an electric fan (WET+FAN), and a cotton t-shirt soaked with 500 ml of water without a fan (WET). *Significantly different in DRY vs. WET, P < 0.01. †Significantly different WET vs. WET+FAN, P < 0.01. ‡Significantly different in DRY vs. WET+FAN, P < 0.01. Error bars indicate means ± SD for nine participants.

Cardiovascular responses.

Heart rate and mean arterial blood pressure are presented in Fig. 5. Heart rate demonstrated a significant time-by-condition interaction (P < 0.01), with greater heart rate values observed in DRY versus WET at the 60-min time point only (P = 0.01). No differences in mean arterial pressure responses were found (P = 0.51).

Fig. 5.

Heart rate (top) and mean arterial pressure (bottom) during 120 min of heat stress while wearing a dry cotton t-shirt (DRY), a cotton t-shirt soaked with 500 ml of water while facing an electric fan (WET+FAN), and a cotton t-shirt soaked with 500 ml of water without a fan (WET). *Significantly different in DRY vs. WET, P < 0.05. Data represent means ± SD for nine participants.

DISCUSSION

The current study assessed whether wearing a water-soaked t-shirt, with or without electric fan use, mitigates heat strain in older individuals exposed to heat wave conditions. In accordance with our hypotheses, wearing a water-soaked t-shirt while facing an electric fan (WET+FAN) led to greater total evaporation, but did not alter the rise in core temperature compared with wearing a dry t-shirt alone (DRY) beyond 30 min of heat stress. However, wearing a water-soaked t-shirt without concurrent electric fan use (WET) attenuated the rise in core temperature compared with DRY throughout 120-min of heat stress and compared with WET+FAN up to 90 min of heat stress. Taken together, these findings suggest that wearing a water-soaked t-shirt in heat wave conditions can attenuate heat strain, while electric fan use in combination with a water-soaked t-shirt has no effect on heat strain compared with wearing a dry t-shirt alone.

Thermal responses and evaporative mass losses.

In recent studies from our laboratory, electric fan use among older individuals exposed to a 42°C environment elevated core and mean skin temperatures compared with a no-fan condition (17, 18). These findings were explained by the fact that the sweat response to heat stress, which is attenuated in older individuals (10, 11, 18, 20, 23, 28, 53), must be quickly initiated and maintained at a high rate if evaporative cooling and core temperature control are to be improved with fan use in extreme heat (15, 46). The current study examined whether electric fan use by older individuals could become effective in mitigating heat strain if supplemental moisture is applied to the body surface using a water-soaked t-shirt. Our findings show that heat strain—indicated by the change in core temperature—was not different in WET+FAN compared with DRY throughout the heat exposure (Fig. 1), which suggests that fan use with supplemental skin wetting was neither beneficial nor detrimental in mitigating heat strain in older individuals. In contrast, wearing a water-soaked t-shirt without electric fan use in the WET trial significantly attenuated the rise in heat strain throughout most of the 2-h heat exposure in WET compared with the DRY and WET+FAN conditions (Fig. 1). The magnitude of the difference in core temperature between WET and the other trials (~0.2°C) is consistent with that reported by Song et al. (55), who examined thermal strain in young individuals while soaking a summer clothing ensemble with water every 30 min during a 90-min heat exposure more severe than that used in the current study (43°C and 57% RH). To our knowledge, this is the first study to demonstrate the effectiveness of using a water-soaked t-shirt to alleviate heat strain in older individuals exposed to very hot and moderately humid conditions that typify extreme American heat waves.

The lack of a difference in core temperature between WET+FAN and DRY likely reflected offsetting elevations in convective heat gain and evaporative heat loss (25, 30, 46). Convective heat exchange increases with the skin-air temperature gradient and the convective heat transfer coefficient, the latter of which is directly influenced by air velocity (39). Evaporative heat loss increases with a wider skin-air vapor pressure gradient, the degree of skin wettedness, and the evaporative heat transfer coefficient, which is also influenced by air velocity through the convective heat transfer coefficient (14, 39). In the present study, convective heat gain would have been higher in WET+FAN versus DRY due to a lower mean skin temperature (and, thus, a wider skin-air temperature gradient; Fig. 2) and greater air velocity. Meanwhile, saturating the upper arms and torso with water and facing a fan would have accelerated evaporative heat loss, which is evidenced by greater total evaporative mass losses in WET+FAN versus DRY (Figs. 3 and 4). Accordingly, on the basis of the ambient conditions, mean skin temperatures (Fig. 2), an assumed clothing insulation of 0.20 clo (43), observed changes in body mass (Fig. 3), and rates of convective heat gain and evaporative heat loss were calculated to have been higher in WET+FAN (147 W and 280 W, respectively) compared with DRY (33 W and 102 W, respectively) (5).

Differences in the core temperature responses between WET and both WET+FAN and DRY conditions indicate that the water-soaked t-shirt alone was better able to mitigate body heat gain compared with the other experimental conditions. Specifically, saturation of the upper arms and torso with water led to greater total evaporative mass losses in WET versus DRY (Figs. 3 and 4), while convective heat gain was likely higher due to a lower mean skin temperature and, thus, a wider skin-air thermal gradient (Fig. 2). In support of this, calculated mean rates of convective heat gain were marginally higher in WET (46 W) relative to DRY (33 W), while mean rates of evaporative heat loss were estimated to have been greater in WET (152 W) relative to DRY (102 W) (5). In comparison to the WET+FAN trial, evaporative mass losses were lower in WET, but the absence of fan-forced air velocity in WET limited the rate of convective heat gain, which ultimately helped attenuate the rise in core temperature (Fig. 1).

While total evaporation was greater in WET vs. DRY, the contribution of sweat to total evaporation was lowered by 51% in WET versus DRY due to water evaporation from the t-shirt (Fig. 4). These findings contrast with those of Song et al. (55), who observed attenuated sweat production during the first 60 min of a 90-min exposure between their wet-clothing and dry-clothing conditions, but no difference in the mass of evaporated sweat between conditions. The discrepancy may relate to the volume of water trapped in the clothing. In our study, the amount of water used to soak the t-shirt was chosen to maximize moisture saturation without dripping, so that alterations in evaporative efficiency would not confound our estimations of total evaporative mass losses. Nevertheless, the observation that wearing a water-soaked t-shirt can mitigate heat gain in a hot environment while reducing the requirement for sweat production has important implications. Older individuals have a reduced thirst sensitivity (32) and are more likely to suffer from conditions that increase fluid losses (61). Therefore, strategies that can minimize sweat losses during prolonged heat stress may help older individuals limit exacerbations in thermal and cardiovascular strain associated with dehydration (6).

Although no differences in the core temperature responses were observed between WET+FAN and DRY beyond 30 min of heat stress, sweat evaporation was significantly greater in WET+FAN (Fig. 4). This finding is explained by a greater evaporative requirement for heat balance associated with a higher rate of convective heat gain (16) and suggests that even without any detrimental effect on heat strain, the use of an electric fan in hot conditions will substantially increase the volume of fluid intake required to stay hydrated.

Cardiovascular strain.

The time-dependent heart rate response to heat stress was influenced by the different study conditions, but heart rate was significantly attenuated only in WET versus DRY at 60 min (Fig. 5). Additionally, no effect of the various experimental conditions on mean arterial pressure was evident (Fig. 5). Previous studies have demonstrated that external wetting of the skin, with or without supplemental fan-forced airflow, can effectively lower heart rate in younger individuals. For instance, use of a mist fan attenuates the rise in heart rate during exercise (49) and accelerates the fall in heart rate during intermittent work rest periods and postexercise recovery under hot conditions, concomitant with rapid reductions in core and skin temperature (49, 54). In a recent study by Morris et al. (37) conducted with younger individuals, self-dousing with 22°C tap water attenuated heart rate by 5 beats/min following 120 min of a hot-humid exposure compared with a no-wetting control condition. Although differences in heart rate between WET and DRY conditions in the present study were not statistically significant throughout most of the heat exposure, it is noteworthy that the magnitude of the difference in heart rate between WET and DRY conditions (4–5 beats/min during the first 60 min) was similar to the differences reported by Morris et al. (37). Had we periodically resoaked the t-shirt with cool water in the current study, perhaps heart rate could have been further attenuated, but such an effect remains to be investigated.

Considerations.

While all participants were older (≥62 yr), they were healthy individuals with no history of cardiovascular disease and were not taking any medications at the time of participation. Older individuals with cardiovascular disease are disproportionately affected by heat stress, evidenced by higher rates of hospitalization and mortality during heat waves (2, 3, 12, 21, 50, 60). Future studies should consider investigating whether strategies to supplement sweat production and augment evaporative heat dissipation can effectively mitigate thermal and cardiovascular strain in older individuals with overt cardiovascular disease. Such strategies could be further evaluated in populations with injuries (e.g., spinal cord injury patients, burn survivors) or autonomic dysfunction (e.g., multiple sclerosis), which impair sweating capacity.

The present findings are limited to the environmental conditions tested. If air temperature is equal to mean skin temperature, electric fan use with supplemental skin wetting would enhance evaporative heat loss, but convective heat exchange would be zero. Alternatively, if air temperature is below mean skin temperature, electric fan use coupled with skin wetting would enhance both evaporative and convective heat losses. Both scenarios would mitigate thermal and cardiovascular strain (49, 54), which could benefit individuals living in climates that do not generally necessitate air conditioning, but have experienced heat waves, resulting in remarkable morbidity and mortality [e.g., Montreal, Canada (44) or Moscow, Russia (52)]. Whether supplemental skin wetting without fan use would effectively mitigate heat strain among older individuals in air temperatures above the 42°C temperature tested herein is presently unknown, but should be explored in future research as numerous populations across the globe experience extreme heat events exceeding 42°C (40, 41). Additionally, it would be reasonable to expect that the efficacy of wearing a water-soaked shirt at high air temperatures would diminish as ambient humidity (i.e., vapor pressure) rises, as a narrower shirt-to-air vapor pressure gradient would slow the rate of evaporative heat loss. Nonetheless, the beneficial effects of wearing a repeatedly soaked t-shirt by young individuals in more severe conditions of 43°C and 57% relative humidity (i.e., an ambient vapor pressure of 4.92 kPa versus 2.83 kPa in the current study) (55) suggests this strategy could still be effective at the extremes of humidity. This remains to be demonstrated in older individuals.

The water-soaked t-shirt in the WET condition covered the upper arms and torso, which amounts to ~40% coverage of the total body surface. Had a greater surface area been saturated with moisture, it is possible that the rise in core temperature would have been further attenuated in the WET trial. It is also possible that had we reapplied moisture to the t-shirt at regular intervals (e.g., 30–60 min), the benefits of wearing a water-soaked shirt alone could have been even greater. Future studies should explore the optimal frequency with which tap water should be reapplied to the skin to maximize evaporative heat loss and best attenuate heat strain during heat waves.

It is well documented that garment fit alters heat and mass transfer through its effects on air layer thickness and, thereby, insulation and water vapor resistance (13, 19, 45). In the present study, no attempt was made to tailor the fit of the t-shirt to each participant; a standard (large) size t-shirt was worn by all participants. Whether using a standardized t-shirt had any impact on the observed results is unknown. It is possible that the water-soaked t-shirt worn in WET and WET+FAN, which adhered closely to the skin, would have impeded dry and evaporative heat loss to a lesser extent than the t-shirt in DRY (12). However, it is important to note that under the conditions tested (i.e., high air temperature exceeding mean skin temperature), the water-soaked t-shirt plays a large role in preventing body heat gain from the environment and not just facilitating metabolic heat dissipation, making any effect of moisture on water vapor resistance through a fabric less of an issue.

Conclusions.

For older individuals exposed to extremely hot/moderately humid conditions, wearing a water-soaked t-shirt mitigates the rise in core temperature and reduces whole body sweat losses. Combining a water-soaked t-shirt with electric fan use is neither harmful nor beneficial in mitigating heat strain, but exacerbates sweat losses compared with wearing a dry t-shirt or water-soaked t-shirt without fan use. This information should be incorporated into public health messaging regarding effective cooling strategies to reduce the risk of heat-related health problems during extreme heat events.

GRANTS

This work was supported by the National Institutes of Health (R01GM068865 to C.G.C.) and the Department of Defense (W81XWH-15-1-0647 to C.G.C.), as well as a Natural Sciences and Engineering Research Council of Canada Postdoctoral Fellowship (to M.N.C.).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

M.N.C., M.H., and C.G.C. conceived and designed research; M.N.C., M.H., and G.M. collected data; M.N.C. and C.G.C. analyzed data; M.N.C., M.H., G.M., and C.G.C. interpreted results of experiments; M.N.C. prepared figures; M.N.C. drafted manuscript; M.N.C., M.H., G.M., and C.G.C. edited and revised manuscript; M.N.C., M.H., G.M., and C.G.C. approved final version of manuscript.