Moisture transfer of the clothing–human body system during continuous sweating under radiant heat

Abstract

Mass transfer due to perspired moisture in a clothing system is critical for the understanding of thermo-physiology and thermal protection of a clothed body. Previous studies usually investigated moisture transfer without considering the effect of liquid sweating or external heat hazards. To understand the mechanisms of sweat evaporation, accumulation and dripping with continuous sweating under radiant heat, a multi-phase experiment was designed with a sweating Torso. The concept of clothed wettedness was proposed to understand sweat evaporation of the clothed body. Results showed that the evaporation rate of the clothed body increased with increasing perspiration rate and the rate increase can be explained by the material properties (e.g., material composition, hydrophilicity and evaporative resistance ( $R_{et}$ )), which affected the sweat accumulation ability. Results also demonstrated a dual relationship of $R_{et}$ with the evaporation rate of the clothed body. Firstly, the evaporation rate was increased for greater $R_{et}$ due to the higher moisture accumulation. Secondly, when $R_{et}$ exceeded a certain value, the evaporation rate decreased with greater $R_{et}$ due to the reduction in the mass transfer coefficient. For radiant heat exposure, evaporated sweat may condense on the skin surface, decreasing the evaporation rate and increasing the dripping rate. The sweat transfer process was also investigated in detail by the combined analysis of the sweat transfer rate and the evaporative cooling efficiency. This study provides insights into how continuous liquid sweat transfers and evaporates in the clothed body and its interaction with clothing material and environment radiant heat, contributing to the understanding of thermo-physiological burden and thermal protection of the clothed body with intensive activities.

Keywords

sweat evaporation clothed human body radiant heat steam burn thermo-physiology

Firefighters and industrial workers wearing thermal protective clothing often perform intensive physical activities under radiant heat exposure and, consequently, sweat profusely. The perspired moisture is not only closely related to thermo-physiological responses but also affects thermal protection of the clothed human body. On one hand, the perspired moisture provides evaporative cooling to the body, relieving heat strain.¹ On the other hand, the liquid moisture can increase thermal conductivity and heat capacity of the clothing material, affecting skin burns when exposed to hazardous conditions.^2,3 Therefore, the investigation of perspired moisture transfer of the clothed human body in radiant heat provides helpful inputs to understand the thermo-physiological burden and thermal protection of protective clothing systems.^4,5 The evaporation rate of the clothing–body system is an important indicator of the moisture transfer process, providing information regarding the thermal effects of perspired moisture. Previous research pointed out that the evaporation rate of perspired moisture depends on the air velocity, the vapor pressure gradient between the skin and ambient air and the vapor permeability of the clothing.^6,7

In early studies, the evaporation rate was usually quantified by human trials with the weight loss of the (clothed) human body.^8–11 The sweat evaporative efficiency (the ratio of evaporative sweat to total sweat production) was introduced to understand the relationship between the evaporation rate and the sweat rate.^7,12–14 Previous studies showed that sweat evaporative efficiency is related to skin wettedness. The efficiency could be unity when the skin wettedness is low and tends to decline when the wettedness increases.^7,12,15–19 Usually these studies investigated the evaporation rate and sweat evaporative efficiency merely on nude subjects^7,15,18 or subjects with swimming trunks.¹² Some studies adopted the clothed human body, but without considering the effect of clothing material properties on the evaporation process.^16,17

Recently, the effect of clothing/material on the evaporation process has usually been characterized by water vapor resistance (named evaporative resistance in ASTM standards) through the sweating hot plate and thermal manikin measurements.^20–24 With lower water vapor resistance, the clothed body has a greater evaporation rate.²⁵ The existing experimental and model investigations of sweat evaporation usually assume that the sweat evaporates on the skin and only sweat vapor transfers through the clothing system.^26,27 However, in practice, liquid sweat may transfer from the skin to the clothing layers and evaporate either from the skin or from the clothing.

Existing studies investigating the effect of liquid sweat usually adopted pre-wetted skin and clothing. A human trial showed that the evaporative mass loss was greater with higher water content in the clothing.⁹ The study conducted by Wang et al.²⁸ showed that the evaporation rate was significantly lower when all the sweat was transferred to the clothing layer. In Havenith et al.'s study,²⁵ they found that the evaporative mass loss reduced with decreased material vapor permeability, but showed no significant dependence on temperature nor any interaction of temperature and permeability, since the vapor pressure gradient between the skin and environment was held constant at different temperatures. Their follow-up study²⁹ showed a clear reduction in evaporative mass loss with increasing clothing thickness. Cain and McLellan³⁰ explored the effect of liquid sweat on the evaporation rate but without controlling the sweat rate. The results showed that the evaporation rate was greater when the fabric sample was in direct contact with the liquid surface. They hypothesized that the greater evaporation may result from a reduction of water vapor resistance of the material due to the wicking of liquid sweat through fabric layers. Besides, a human trial study³¹ showed that hydrophilic clothing exhibited more favorable thermal physiological performance than hydrophobic clothing, which indicates that material hydrophilicity may have an effect on sweat evaporation.

In addition, in the field of thermal protective performance, which considers external heat sources, the effect of liquid sweat was also usually simulated by pre-wetted materials. Through continuous recording of a balance, the evaporation rate of a clothed manikin with pre-wetted inner clothing was calculated.³² The results showed that radiant heat increased the evaporation rate of permeable clothing but had a negligible effect on impermeable clothing. By means of thermocouple measurement techniques, a study investigated the evaporation rate of pre-wetted materials under radiant heat exposure.³³ It was found that the moisture location in the materials influenced the evaporation rate by affecting the temperature of the wet surface and the diffusion length. Except for these, most studies could not provide information on the sweat transfer process due to technical limitations of measuring moisture transport,^33,34 and thus could not analyze how liquid and vaporous sweat transfer mechanisms affect heat transfer.

The aim of our study is to understand the sweat evaporation as well as the sweat accumulation and dripping of the clothed human body with continuous sweating when exposed to radiant heat. By means of a clothed Torso manikin system, we obtained the sweat evaporation rate, accumulation rate and dripping rate within a wide range of clothing materials. Three factors and their interaction influencing the evaporation process of the clothed human body were investigated: (I) the sweat rate; (II) the key material properties; (III) the external radiant heat. To further understand sweat transport within clothing, the dynamic sweat transfer process was analyzed in detail. The study contributes to the understanding of liquid sweat transfer and its thermal effects in the clothed human body, helping improve the thermal comfort and protection of the human body.

Methods

Torso manikin

To accurately determine the sweat transfer³⁵ between the clothed human body and its surrounding environment, measurements were performed on a sweating thermal Torso (Figure 1) (Swiss Federal Laboratories for Materials Science and Technology, St. Gallen, Switzerland).^36,37 (As the water is the main component (≈99%) of the sweat³⁵ and other minor components may complicate the heat and moisture transfer analyses, we used distilled water to simulate sweat in the current research stage. To emphasize the direction of the moisture source (i.e., sweat), we used the terminology “sweat” to refer to the sweat simulated by the Torso system.) The Torso consists of a multi-layered main cylinder with the dimension of an adult human torso and two heated aluminum guards. The main cylinder can maintain a constant surface temperature by controlling the heating power, simulating heat dissipation of the human body. The two guards are used to prevent heat losses in the upward and downward directions. Fifty-four sweating outlets are evenly distributed over the surface of the main cylinder and are connected to internal, separately controlled valves to simulate various sweating levels. The balance system of the Torso, consisting of three weighing scales, was applied to measure the sweat transfer. Other detailed information regarding the Torso can be found in the literature.^38,39

Figure 1.

Schematic diagram of the sweating thermal Torso. PTFE: polytetrafluoroethylene.

Clothing materials

Twelve common material systems of thermal protective clothing with different physical properties (Table 1) were selected, including seven single-layered materials of station uniform (SU) and five multi-layered material systems of turnout gear (TG). As the research aim is to understand the transfer of liquid sweat, material hydrophilicity/hydrophobicity was included as a potential property influencing liquid sweat transfer. The hydrophilicity/hydrophobicity was defined by the contact angle of the innermost side of the tested material system. Each multi-layered material system was measured as a whole with the inside of the inner layer facing the water droplet. According to the measured contact angle, pho-SU2, SU5, TG9 and TG10 were categorized as hydrophobic materials, while the other samples were categorized as hydrophilic materials. It should be noted that although the inside of inner layer of TG9 and TG10 are hydrophobic, liquid water can still penetrate through the pores of the layer with contact pressure and accumulate within the material system. Besides, the radiation wavelength range of infrared lamps (SICCATHERM infrared lamps, OSRAM Licht AG, Germany) used to simulate the external radiant heat was 590–2500 nm. Thus, material emissivity (

ɛ

) was also measured and averaged over this wavelength range.

Table 1.

Physical properties of clothing material systems

Fabric systems	Fiber content	Physical properties*
Fabric systems	Fiber content	Color	Weight^a (g/m²)	Thickness^b (mm)	Contact angle^c (°)	Thermal resistance ( $R_{ct}$ )^d (10⁻³℃·m²/W)	Evaporative resistance ( $R_{et}$ )^e (Pa·m²/W)	Emissivity ( $ɛ$ )^f
SU (single-layered material)
SU1	50% meta-aramid/50% fire retardant (FR) viscose		197.0	0.4	0	10.8	2.1	0.837
phi-SU2	34% aramid/33% lyocell/31% modacrylic/2% anti-static fiber		241.6	0.6	0	13.4	2.7	0.648
pho-SU2	34% aramid/33% lyocell/31% modacrylic/2% anti-static fiber		253.1	0.7	128.6	13.2	2.7	0.682
SU3	93% meta-aramid/5% para-aramid/2% antistatic fiber		154.7	0.3	0	11.7	2.0	0.254
SU4	93% meta-aramid/5% para-aramid/2% antistatic fiber		229.8	0.4	0	12.7	3.2	0.278
SU5	55% FR modacrylic/45% FR Cotton		367.3	0.7	130.7	16.6	3.4	0.247
SU6	FR cotton		366.8	0.8	0	14.5	4.4	0.421
TG (multi-layered material system)
TG7	99% aramid/1% beltron (OL) + PTFE coated meta-aramid (ML) + fleece (ML) + 50% meta-aramid/50% viscose (IL)		635.2	3.2	0	82.7	15.6	0.924
TG8	Meta-aramid with thread on backside (OL) + PU liner on meta-aramid (ML) + aramid (IL)		592.1	3.8	0	95.4	23.9	0.933
TG9	64% FR viscose/35% meta-aramid/1% antistatic (OL, yellow) + PU Coated 50% meta-aramid/50% FR viscose (ML) + 65% FR viscose/35% meta-aramid (IL)		587.8	2.0	130.8	46.6	9.4	0.489
TG10	64% FR viscose/35% meta-aramid/1% antistatic (OL, blue) + PTFE coated 50% meta-aramid/50% FR viscose (ML) + 65% FR viscose/35% meta-aramid (IL)		599.8	2.2	130.2	49.3	10.0	0.834
TG11	Meta-aramid (OL) + PTFE coated 25% meta-aramid/25% para-aramid/50% basofil (ML) + non-woven meta-aramid quilted with 50% meta-aramid/50% FR viscose (IL)		493.0	2.3	0	71.0	16.8	0.901

For all turnout gear, the images from top to bottom are the materials from the outermost layer to the innermost layer. *Physical properties were measured according to ^aISO 3801:1977 (by the weighing balance of Mettler-Toledo, Switzerland), ^bISO 5084:1996 (by the thickness tester of Frank-PTI, Germany), ^cASTM D7334-08(2013). If the contact angle of a material is 0 °, it is categorized as hydrophilic. If the contact angle is greater than 90 °, the material is categorized as hydrophobic. ^d,eIntrinsic values, ISO 11092:2014 (by hot plate tester of Hohenstein Institute, Germany) and ^fby the extended Fourier transform infrared spectrometer VERTEX 80, Germany.^22,40,41 SU: station uniform; TG: turnout gear; OL: outer layer; ML: middle layer; IL: inner layer; PTFE: polytetrafluoroethylene; PU: polyurethane.

Experiment design

Three-phase experiment

Experiments were performed in a climatic chamber with air temperature of 20.0 ± 0.5℃, relative humidity of 50 ± 2% and air velocity of 0.65 ± 0.10 m/s. Before the experiments, clothing materials were preconditioned in the same chamber for at least 24 hours. To understand liquid sweat transfer in the clothing system, the materials were wrapped on the Torso surface with direct contact.

The main experiment was designed to include three consecutive phases (Figure 2) to simulate the firefighting scenario: preparatory work without sweating followed by sweating and finally additional radiant heat exposure when entering the fire field. The constant surface temperature mode of 35℃ was used to simulate the thermal state of the human body according to ISO 18640.³⁶ In phase one (P1), the clothed Torso was kept in the dry thermal state for 1 hour. In phase two (P2), a pre-set sweat rate was applied for 2 hours. Three different sweat rates of 100, 175 and 250 g/h were chosen, denoted as SR100, SR175 and SR250, respectively. The three sweat rates correspond to the sweat rate of 415.2–1038.1 g/h of a firefighter with a low to moderate sweating level³ (assuming a body surface area of 1.8 m²).⁴² In phase three (P3), a 96 × 188 cm² radiant heat panel filled with infrared lamps was used to expose the front side of the Torso to external radiant heat for another 2 hours, while the sweat rate was kept at its previous level. To guarantee the normal working of the Torso, the radiation intensity on the Torso surface was determined as 1 kW/m², simulating the routine thermal environment for firefighters, in which the heat flux remains lower than 1.67 kW/m².⁴³ Pre-tests with a thermal sensor (Schmidt-Boelter heat flux sensor, Medtherm Corporation, USA) were performed to identify a distance of 137 cm between the radiant heat panel and the front side of the Torso, to achieve the target radiation intensity. The test duration for each phase was determined according to the required time to reach the steady state of heat and moisture transfer.

Figure 2.

Schematic diagram of the three-phase experiment.

Since the starting points of P2 and P3 of the main experiment contain different amounts of water in the clothing system, additional experiments were carried out similarly to the main experiment, but without radiant heat exposure during P3. This helped clarify that the different water content in the material had no significant effect on the moisture transfer results and the differences in moisture transfer observed between P2 and P3 can be attributed to the effect of external radiant heat.

Experimental measurement and calculation

Sweat transfer

To evaluate the sweat transfer of the clothed Torso, the real-time sweat amount ( $m_{sweat}$ ) was controlled with weighing scale I (accuracy: 0.1 g, Mettler-Toledo SB8001, Mettler-Toledo GmbH, Greifensee, Switzerland); the real-time sweat accumulation ( $m_{accu}$ ) in the clothed Torso was obtained by the weight change of the clothed Torso (i.e., $m_{accu} = Δ m_{clothedtorso}$ ), measured by weighing scale II (accuracy: 1 g, Mettler-Toledo KCC150, Mettler-Toledo GmbH, Greifensee, Switzerland); the real-time dripping sweat was collected in a bottle and its weight ( $m_{drip}$ ) was measured by weighing scale III (accuracy: 0.1 g, Mettler-Toledo XS6001S, Mettler-Toledo GmbH, Greifensee, Switzerland). According to the relationship among the component weights of sweat transfer, the amount of evaporative sweat ( $m_{evap}$ ) can be obtained as Equation (1)

m_{evap} = m_{sweat} - m_{accu} - m_{drip}

(1)

Sweat transfer rate

The sweat rate $({\overset{\cdot}{m}}_{sweat})$ , the evaporation rate $({\overset{\cdot}{m}}_{evap})$ , the accumulation rate $({\overset{\cdot}{m}}_{accu})$ and the dripping rate $({\overset{\cdot}{m}}_{drip})$ were calculated as the slope of weight versus time during the steady state. Sweat evaporative efficiency ( $η_{evap}$ ), which is the ratio of the evaporative sweat to the total sweat production,^7,12–14 was calculated according to Equation (2)

η_{evap} = \frac{{\overset{\cdot}{m}}_{evap}}{{\overset{\cdot}{m}}_{sweat}}

(2)

Sweat accumulation ability

For test cases with dripping sweat during the last 30 minutes of P2 and P3, the material tended to reach the maximum sweat accumulation amount. Thus, the sweat accumulation ability of the material can be calculated as the average value of the sweat accumulation amount in the clothed Torso during this period of time.

Vapor pressure gradient

To help understand the sweat transfer process, the vapor pressure gradient from the Torso surface to the environment was calculated based on the temperature and relative humidity measurements. At four sides (front, back, left- and right-hand side) of the clothed Torso, temperature and relative humidity underneath the clothing (on the Torso surface) and in the environment (next to the clothed Torso) were measured with MSR sensors (MSR 145, MSR Electronics GmbH, Seuzach, Switzerland). The sensors were placed 8 cm below the upper edge of the Torso main cylinder.

Steady-state criteria

Since the test condition was changed at the beginning of each phase, the heat and moisture transfer of the Torso would change abruptly before reaching the steady state. The steady state in each phase was considered only, as the transient periods depended on inherent regulation properties of the methodology and, thus, may only provide limited information about moisture transfer behaviors. The steady state was determined by the Torso surface temperature, the heating power and the evaporation process. During the last 30 minutes of each phase, the coefficient of variation (CV) of the mean Torso surface temperature was less than 0.7% and the CV of Torso heating power was less than 13.5%. Compared to a previous study,⁴⁴ we considered that the heat transfer of the clothed Torso during this period of time reached the steady state. For each sweat rate, at least three replications of the materials were tested on the Torso, guaranteeing the CV of the Torso heating power among repetition tests to be less than 8.5%. For all tests, the normalized root-mean-square deviation (NRMSD) of the evaporation rate ( ${\overset{\cdot}{m}}_{evap}$ ) during the last 30 minutes of P2 and P3 was lower than 1.1%; thus, we considered that ${\overset{\cdot}{m}}_{evap}$ was constant during this period of time and the evaporation process reached the steady state.

Theoretical analysis and concept of clothed wettedness

The evaporation rate from the nude skin of the human body can be expressed as Equation (3).⁶ ${\overset{\cdot}{m}}_{sk}$ is the evaporation rate from the nude skin, g/h; $A_{sk}$ is the skin area available for evaporation, m²; $w_{sk}$ is the fraction of the body surface covered by water (skin wettedness), dimensionless; $h_{m}$ is the mass transfer coefficient from the skin to the environment, g·m⁻²·h⁻¹·kPa⁻¹; $P_{sk, s}$ and $P_{a}$ are the saturated vapor pressure on the skin and the vapor pressure in the environment, kPa, respectively

{\overset{\cdot}{m}}_{sk} = A_{sk} w_{sk} h_{m} (P_{sk, s} - P_{a})

(3)

For the clothed human body, the evaporative mass loss from the body to the environment may occur both on the skin and within the clothing layers²⁹ (Figure 3). We assume that the clothing material is homogeneous, and the liquid water is distributed homogeneously in the material. We divided the clothing into n layers. Thus, for a certain layer k (k = 1,…,n), the evaporation rate ${\overset{\cdot}{m}}_{evap, k}$ can be written as Equation (4)

{\overset{\cdot}{m}}_{evap, k} = A_{clo, k} w_{k} h_{m, k} (P_{clo, s, k} - P_{a})

(4)

Figure 3.

Schematic diagram of evaporation of the liquid sweat in the clothed human body.

To simplify the expression of the subsequent formulae, the skin layer is denoted by layer k = 0. Thus, for the whole skin and clothing system, the evaporation rate ( ${\overset{\cdot}{m}}_{evap}$ ) can be written as Equation (5)

{\overset{\cdot}{m}}_{evap} = \sum_{k = 0}^{n} A_{clo, k} w_{k} h_{m, k} (P_{clo, s, k} - P_{a})

(5)

where

A_{clo, k}

(k = 0, …, n) represent the surface area of skin covered by clothing and the area of each clothing layer, respectively, m²; for a certain material system,

A_{clo, k}

is constant.

$w_{k}$ (k = 0, …, n) represent the wettedness of skin and each clothing layer, respectively, dimensionless. We also define $w_{clo}$ as the wettedness for the whole clothing system.

$h_{m, k}$ , (k = 0, …, n) represent the mass transfer coefficients from each layer to the environment, g·m⁻²·h⁻¹·kPa⁻¹, calculated as Equation (6). $R_{et}$ is the material intrinsic evaporative resistance, kPa·m²·W⁻¹. According to the literature,³⁰ due to wicking of liquid sweat through the material, there will be a reduction of evaporative resistance of the material. Based on this, we consider that the evaporative resistance of the wet material is the function of clothed wettedness $w_{clo}$ , denoted as $R_{et} (w_{clo})$ , and with greater $w_{clo}$ , $R_{et} (w_{clo})$ decreases. $R_{e, bl}$ is the evaporative resistance of the boundary layer, kPa·m²·W⁻¹. Since in our study the air velocity is constant, $R_{et, bl}$ is constant. $λ$ is the latent heat of sweat evaporation, 0.673 W·h·g⁻¹ at 35℃. Since $R_{et} (w_{clo})$ decreases with greater $w_{clo}$ and $R_{et, bl}$ is constant, $h_{m, k} (w_{clo})$ increases with greater $w_{clo}$

h_{m, k} (w_{clo}) = \frac{1}{(R_{et} (w_{clo}) \times \frac{n - k}{n} + R_{et, bl}) \times λ}

(6)

$P_{clo, s, k}$ (k = 0,…,n) represent the saturated vapor pressures at each layer, kPa, calculated as Equation (7). The temperature of each layer $T_{clo, k}$ can be estimated as Equation (8).⁴⁵ $T_{clo, 0}$ is the skin (Torso surface) temperature, 35℃. $T_{a}$ is the environment temperature, 20℃. $R_{ct}$ is the material intrinsic thermal resistance, ℃·m²·W⁻¹. $R_{ct, bl}$ is the thermal resistance of the boundary layer, ℃·m²·W⁻¹; due to the constant air velocity, $R_{ct, bl}$ is constant. With greater $w_{clo}$ , $R_{ct} (w_{clo})$ becomes lower, and thus $T_{clo, k}$ becomes greater. Thus, $P_{clo, s, k}$ ( $w_{clo}$ ) increases with greater $w_{clo}$ . $P_{a}$ is the vapor pressure in the environment, kPa

P_{clo, s, k} = \frac{e^{18.956 - 4030 / (T_{clo, k} + 235)}}{10}

(7)

\begin{matrix} T_{clo, k} = T_{clo, 0} - (T_{clo, 0} - T_{a}) \times \frac{\frac{k}{n} \times R_{ct} (w_{clo})}{R_{ct} (w_{clo}) + R_{ct, bl}} \end{matrix}

(8)

Therefore, the evaporation rate of the clothing system (Equation (5)) can be re-written as Equation (9). According to this equation, we hypothesized that ${\overset{\cdot}{m}}_{evap}$ is positively related with $w_{clo}$ . Similar to the concept of skin wettedness,⁴⁶ $w_{clo}$ is the wettedness of the clothed system (clothed wettedness). If a clothed human body contains a greater amount of water (i.e., it leads to a greater $w_{clo}$ ), the clothed body has a higher evaporation rate (Different environment conditions and an active human body may influence the result of ${\overset{\cdot}{m}}_{evap}$ . However, since these are not the independent variables in the current research, we do not discuss these factors in this paper.)

\begin{matrix} {\overset{\cdot}{m}}_{evap} = \sum_{k = 0}^{n} A_{clo, k} w_{k} h_{m, k} (w_{clo}) (P_{clo, s, k} (w_{clo}) - P_{a}) \end{matrix}

(9)

Statistical analyses

Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) version 23.0 (IBM, Armonk, NY, USA). The individual effect of sweat rate and radiant heat on sweat transfer rates were analyzed by a one-way analysis of variance (ANOVA) followed by Tukey's honestly significant difference (HSD, if the variance of the group was equal) or Tamhane's T2 (if the variance of the group was not equal) post-hoc tests for multiple comparison. The combined effects of the sweat rate and material properties on the evaporation rate were investigated with multiple linear regressions. The force enter method was chosen for predictor selection. The multicollinearity assessment, casewise diagnostics, assumptions of linearity and homoscedasticity and normality of the residuals were also investigated for all the multiple linear regression models. The same method of multiple linear regressions was also performed between ${\overset{\cdot}{m}}_{evap, P 3}$ and ${\overset{\cdot}{m}}_{evap, P 2}$ .

Results and discussion

Effect of the sweat rate on the evaporation rate

Overall, the evaporation rate ( ${\overset{\cdot}{m}}_{evap}$ ) increased with the sweat rate ( ${\overset{\cdot}{m}}_{sweat}$ ) for both hydrophilic and hydrophobic materials (Figures 4(a-1) and (a-2)). The results agree with previous studies on nude subjects and subjects wearing swimming suits.^7,47 The reason for the ${\overset{\cdot}{m}}_{evap}$ increment could be the increased mean vapor pressure within the clothed Torso, which can also be explained by the increased clothed wettedness (Equation (9)) caused by a higher sweat rate. The sweat evaporative efficiency ( $η_{evap}$ ) was inversely related to ${\overset{\cdot}{m}}_{sweat}$ (Figures 4(b-1) and (b-2)). This demonstrates that, with a higher sweat rate, less percentage of the sweat evaporates from the clothed system and the percentage of sweat accumulation and dripping increase. This could be explained by the relative permeability and saturation relationship.⁴⁸ With a higher sweat rate, that is, greater water saturation, the relative permeability of water vapor decreased, and the relative permeability of liquid water increased, which corresponds to the decreased percentage of sweat evaporation and higher percentage of sweat accumulation and dripping. The different slopes between the materials (Figure 4) demonstrate that the effects of the sweat rate on the evaporation rate and on the sweat evaporative efficiency also depend on the material properties.

Figure 4.

Relationship between the evaporation rate and sweat rate for hydrophilic (a-1) and hydrophobic (a-2) materials. Relationship between the sweat evaporative efficiency and sweat rate for hydrophilic (b-1) and hydrophobic (b-2) materials. SU: station uniform; TG: turnout gear.

Effect of material properties on the evaporation rate

Table 2 presents the multiple linear regressions between the evaporation rate (P2:

{\overset{\cdot}{m}}_{evap, P 2}

; P3:

{\overset{\cdot}{m}}_{evap, P 3}

) and material properties. For SUs, the material hydrophilicity/hydrophobicity had a significant effect on

{\overset{\cdot}{m}}_{evap, P 2}

; for TG, in addition to the significant effect of material hydrophilicity/hydrophobicity, material weight, thickness and evaporative resistance (

R_{et}

) were also significant factors on

{\overset{\cdot}{m}}_{evap, P 2}

. These material property effects are further discussed in the Relationship between the evaporation rate and sweat accumulation ability and Relationship between the evaporation rate and evaporative resistance sections.

Table 2.

Multiple linear regression between the evaporation rate (P2: ${\overset{\cdot}{m}}_{evap, P 2}$ ; P3: ${\overset{\cdot}{m}}_{evap, P 3}$ ) and material properties for station uniform (SU) and turnout gear (TG)

Material	R ²	Factors	Unstandardized coefficient (a1–a7, b)	Standardized coefficient	Material	R ²	Factors	Unstandardized coefficient (a1–a7, b)	Standardized coefficient
SU	${\overset{\cdot}{m}}_{evap, P 2}$				TG	${\overset{\cdot}{m}}_{evap, P 2}$
	0.87	Constant	62.39			0.74	Constant	−761.62
		sweat rate**	0.36	0.42			sweat rate**	0.27	0.52
		Hydrophilicity^a**	−103.88	−0.89			Hydrophilicity**	−61.97	−0.96
							Weight*	1.94	2.9
							Thickness*	−287.7	−6.2
							R_et ^b*	33.59	5.59
	${\overset{\cdot}{m}}_{evap, P 3}$					${\overset{\cdot}{m}}_{evap, P 3}$
	0.89	Constant	−1.21			0.80	Constant	135.56
		sweat rate**	0.73	0.6			sweat rate**	0.28	0.63
		Hydrophilicity**	−126.69	−0.77			Hydrophilicity**	−63.95	−1.14
							Weight*	0.11	0.19
							Emissivity**	−99.97	−0.61

Note: the form of the regression equation is as follows: y = a1 * sweat rate + a2 * hydrophilicity + a3 * weight + a4 * thickness + a5* R_ct + a6 * R_et + a7 * emissivity + b, y is ${\overset{\cdot}{m}}_{evap, P 2}$ and ${\overset{\cdot}{m}}_{evap, P 3}$ , respectively. ^aIf the material is hydrophilic, the value was labeled as 0; if the material is hydrophobic, the value was labeled as 1. ^bR_ct: thermal resistance, R_et: evaporative resistance. *p < 0.05, **p < 0.001.

In addition to the similar effects of material properties in P2, material emissivity showed a significant negative effect on ${\overset{\cdot}{m}}_{evap, P 3}$ for TG. The reason could be that the low emissivity of the material decreased the heat absorption from the external radiant heat source. The Torso itself thus had to provide higher heating power to maintain the constant surface temperature,⁴⁹ which led to the higher evaporation rate.

Relationship between the evaporation rate and sweat accumulation ability

Figure 5 demonstrates the significant positive relationship between the evaporation rate and the sweat accumulation ability (R² > 0.9, p < 0.05). This is in line with a previous human trial that demonstrated an increased evaporative mass loss with higher water content of clothing.⁹ This could be explained by Equation (9). The greater sweat accumulation, that is, the higher clothed wettedness, leads to a greater evaporation rate.^7,50

Figure 5.

Relationship between the evaporation rate and sweat accumulation ability. SR100, SR175 and SR250: sweat rates of 100, 175 and 250 g/h, respectively.

We further investigated properties that potentially affect sweat accumulation ability (Figure 6). Figure 6(a) shows that hydrophilic materials accumulated more sweat than hydrophobic materials; multi-layered materials (TG) accumulated more sweat than single-layered materials (SU). Figure 6(b) presents the positive relationship between the evaporative resistance and the sweat accumulation ability. The measurement of material evaporative resistance considered that the sweat evaporates on the skin and then the evaporated sweat transfers through the material. The greater evaporative resistance would decrease the sweat vapor transfer to the environment, leading to a higher amount of sweat accumulation within the clothing system. In addition, the measurement of evaporative resistance was conducted in the isothermal condition, that is, there was no temperature gradient between the environmental and skin temperature (35℃). In our case, a lower environmental temperature (20℃) compared to skin temperature (35℃) possibly induces some condensation of vapor within the outer layer and, thus further increasing the sweat accumulation amount.

Figure 6.

(a) Sweat accumulation ability of different material systems. Example: SR250 in P3. Since hydrophilic station uniforms (SU) did not reach moisture saturation, their sweat accumulation ability cannot be obtained. (b) Relationship between the material evaporative resistance and sweat accumulation ability. (c) Relationship between the material weight and sweat accumulation ability. (d) Relationship between the material thickness and sweat accumulation ability. TG: turnout gear.

Figures 6(c) and (d) show the positive relationship between material weight and thickness and the sweat accumulation ability. It may be interesting to note that the weight and the thickness of TG showed the opposite role in ${\overset{\cdot}{m}}_{evap, P 2}$ (Table 2). The reason could be that even though greater weight and thickness may both lead to greater sweat accumulation, the greater thickness may hinder the evaporation due to the greater evaporation distance from the Torso surface to the environment,²⁹ thus showing a negative effect on the evaporation rate.

Relationship between the evaporation rate and evaporative resistance

For hydrophilic materials, ${\overset{\cdot}{m}}_{evap}$ of SUs was higher than that of TG (Figure 4(a-1)). The explanation is that the evaporative resistance ( $R_{et}$ ) of SUs is lower than that of TG, leading to a higher mass transfer coefficient (Equation (9)) and thus a higher evaporation rate. However, for hydrophobic materials, SUs with lower $R_{et}$ presented a lower ${\overset{\cdot}{m}}_{evap}$ than TG. To clarify the exact effect of evaporative resistance on the evaporation rate, the relationship between $R_{et}$ and ${\overset{\cdot}{m}}_{evap}$ was investigated (Figure 7). The hydrophilic SUs (data points in dashed circles) evaporated all the sweat due to their high sweat accumulation ability. Except for these cases, material $R_{et}$ seems to show a dual relationship with the evaporation rate. When $R_{et}$ was low, it showed a positive relationship with ${\overset{\cdot}{m}}_{evap}$ , which is opposite to the general understanding of evaporative resistance. The reason could be that the higher evaporative resistance corresponded to a higher sweat accumulation in the material (Figure 6(b)), leading to a greater evaporation rate. However, when $R_{et}$ exceeded a certain value (≈20 m²·Pa/W in our case), ${\overset{\cdot}{m}}_{evap}$ tended to decrease with higher $R_{et}$ . This may be due to the fact that evaporative resistance began to exert its negative effect in the evaporation process. For materials with high $R_{et}$ , the material reached sweat saturation (i.e., the clothed wettedness was constant), and thus the benefit of the sweat accumulation ability/clothed wettedness on the evaporation rate remained constant. However, higher $R_{et}$ decreased the mass transfer coefficient $h_{m, clo}$ , thus leading to a lower evaporation rate. This demonstrates the dual relationship of evaporative resistance with the evaporation process of the clothed system, different from the single negative impact that is commonly assumed.

Figure 7.

Relationship between the evaporation rate and evaporative resistance. Data points in black circle: hydrophilic station uniforms that can evaporate all the sweat (i.e., ${\overset{\cdot}{m}}_{evap}$ = ${\overset{\cdot}{m}}_{sweat}$ ). The polynomial regression was performed for materials that cannot evaporate all the sweat, including hydrophobic station uniforms and all turnout gear. SR100, SR175 and SR250: sweat rates of 100, 175 and 250 g/h, respectively.

Effect of radiant heat on the evaporation rate

Figure 8 presents the comparison between the evaporation rate in P2 ( ${\overset{\cdot}{m}}_{evap, P 2}$ ) and the evaporation rate in P3 ( ${\overset{\cdot}{m}}_{evap, P 3}$ ). Table 3 presents the regression model of ${\overset{\cdot}{m}}_{evap, P 3}$ predicted by ${\overset{\cdot}{m}}_{evap, P 2}$ with R² = 0.824–0.999, showing that ${\overset{\cdot}{m}}_{evap, P 2}$ can account for more than 80% of the variation in ${\overset{\cdot}{m}}_{evap, P 3}$ . The lower than 20% of the variation that cannot be explained by ${\overset{\cdot}{m}}_{evap, P 2}$ demonstrates the difference between ${\overset{\cdot}{m}}_{evap, P 2}$ and ${\overset{\cdot}{m}}_{evap, P 3}$ .

Figure 8.

Comparison between the evaporation rate in P2 and P3 for all materials. Solid line: hydrophilic materials; dashed line: hydrophobic materials; dotted line: Ref., the reference line on which the evaporation rate in P2 equals that in P3. SU: station uniform; TG: turnout gear.

Table 3.

Regression model of the evaporation rate in P3 ( ${\overset{\cdot}{m}}_{evap, P 3}$ ) predicted by the evaporation rate in P2 ( ${\overset{\cdot}{m}}_{evap, P 2}$ )

Materials	Regression model	R ²	Sig.
SU	${\overset{\cdot}{m}}_{evap, P 3} = 0.990 * {\overset{\cdot}{m}}_{evap, P 2} + 3.836$	0.999	p < 0.001
TG	${\overset{\cdot}{m}}_{evap, P 3} = 0.653 * {\overset{\cdot}{m}}_{evap, P 2} + 41.765$	0.824	p < 0.001
SU and TG	${\overset{\cdot}{m}}_{evap, P 3} = 0.941 * {\overset{\cdot}{m}}_{evap, P 2} + 8.542$	0.965	p < 0.001

SU: station uniform; TG: turnout gear.

Table 4 presents significant changes of

{\overset{\cdot}{m}}_{evap}

as well as

{\overset{\cdot}{m}}_{accu}

and

{\overset{\cdot}{m}}_{drip}

from P2 to P3 for different kinds of materials. Table 5 presents the magnitude of significant changes of

{\overset{\cdot}{m}}_{evap}

. For hydrophilic SUs, all the sweat evaporated in both P2 and P3 (i.e.,

{\overset{\cdot}{m}}_{evap}

{\overset{\cdot}{m}}_{sweat}

{\overset{\cdot}{m}}_{accu}

= {\overset{\cdot}{m}}_{drip} = 0

). For hydrophilic TG, at the low sweat rate of 100 g/h, the radiant heat (P3) increased

{\overset{\cdot}{m}}_{evap}

(5.0–7.6 g/h, 5.2–8.2%) and decreased

{\overset{\cdot}{m}}_{accu}

; at higher sweat rates of 175 and 250 g/h, the radiant heat (P3) decreased

{\overset{\cdot}{m}}_{evap}

(16.3–46.4 g/h, 10.3–24.0%) and increased

{\overset{\cdot}{m}}_{drip}

. For hydrophobic SUs and TGs, the radiant heat (P3) increased

{\overset{\cdot}{m}}_{evap}

(2.9–23.6 g/h, 6.1–20.3%) and decreased

{\overset{\cdot}{m}}_{drip}

{\overset{\cdot}{m}}_{accu}

Table 4.

Change of sweat transfer rates from P2 to P3 for station uniforms (SU) and turnout gear (TG)

Sweat transfer rate	Hydrophilic SUs	Hydrophilic TG			Hydrophobic SUs/TG10	Hydrophobic TG9
Sweat transfer rate	Hydrophilic SUs	Low sweat rate (SR100)	High sweat rate (SR175/250)		Hydrophobic SUs/TG10	Hydrophobic TG9
${\overset{\cdot}{m}}_{evap}$	−	↑*	↓**		↑*	↑*
${\overset{\cdot}{m}}_{accu}$	0	↓*	175g/h: −; 250g/h: ↓*	↑**	0	↓*
${\overset{\cdot}{m}}_{drip}$	0	0	↑*	↑**	↓*	−

Note. “−”: there was no significant difference in the sweat transfer rate between P2 and P3; “↑”: the sweat transfer rate in P3 was significantly greater than that in P2; “↓”: the sweat transfer rate in P3 was significantly lower than that in P2; “0”: there was no significant difference in the sweat transfer rate from zero for both P2 and P3. *p < 0.05, **p < 0.01. SR100, SR175 and SR250: sweat rates of 100, 175 and 250 g/h, respectively.

Table 5.

Change in magnitude of evaporation rate from P2 to P3 ( $Δ {\overset{\cdot}{m}}_{evap}$ = ${\overset{\cdot}{m}}_{evap, P 3}$ – ${\overset{\cdot}{m}}_{evap, P 2}$ ) for station uniforms (SU) and turnout gear (TG)

Materials	SR100	SR175	SR250
Hydrophilic materials
TG7	7.6 (8.2%)**	−1.5 (–1.0%)	−2.9 (–1.6%)
TG8	−2.1 (–2.2%)	−16.3 (–10.3%)**	−23.7 (–13.7%)**
TG11	5.0 (5.2%)*	−19.1 (–11.9%)**	−46.4 (–24.0%)**
Hydrophobic materials
pho-SU2	2.9 (6.9%)*	3.2 (7.2%)	3.8 (7.8%)*
SU5	4 (7.4%)	4.5 (7.0%)*	6.9 (9.9%)*
TG9	11.2 (11.8%)*	18.6 (16.7%)*	23.6 (20.3%)**
TG10	6.5 (7.7%)	6.6 (7.8%)	5.3 (6.1%)*

Note: the percentage in the parentheses is the variation in percentage from P2 to P3, $({\overset{\cdot}{m}}_{evap, P 3} - {\overset{\cdot}{m}}_{evap, P 2}) / {\overset{\cdot}{m}}_{evap, P 2}$ . *p < 0.05, **p < 0.01. SR100, SR175 and SR250: sweat rates of 100, 175 and 250 g/h, respectively.

The MSR sensor measurements demonstrate that the radiant heat can increase the vapor pressure gradient from the Torso surface to the environment (p < 0.001). This increased vapor pressure gradient could be the reason that the radiant heat increased the evaporation rate in most cases. On the other hand, the possible explanation for the lower evaporation rate of hydrophilic TG8 and TG11 at higher sweat rates (175 and 250 g/h) is that when the temperature of the outer layers of TG is higher than that of the Torso surface, a larger amount of evaporated moisture will condense on the Torso surface and within the clothing layers, decreasing the evaporation rate. This hypothesis can be confirmed by our observations: (I) condensation on the Torso surface for TG8 and TG11 at higher sweat rates (Figure 9(a)); (II) condensation pattern how liquid water distributed on the Torso surface following the inner side structure of the material (Figure 9(b)). The inner sides of TG8 and TG11 have special structures that caused irregular contact between the material and the Torso surface and the condensation water thus was formed at the position where the air space formed between the material and the Torso surface. The irregular air space may cause a higher temperature gradient between the Torso surface and the environment due to its high thermal resistance, facilitating the condensation. We thus infer that the special inner structures made TG8 and TG11 behave differently from other TGs. These observations give the picture of how sweat transfers from the human body through the clothing system. The sweat firstly evaporates on the skin surface, and part of the evaporated sweat may then condensate on the skin due to its lower temperature compared with the radiation environment. This condensation releases the heat to the skin and decreases the evaporation rate. It also accelerates the sweat accumulation and saturation of the clothed body, increasing the dripping rate. Furthermore, this evaporation–re-condensation process might be a steam burn mechanism caused by perspired moisture by condensation of sweat vapor and heat release.

Figure 9.

(a) Condensation pattern on the Torso surface in the environment with radiant heat. (b) Material inner side: special structure that caused irregular contact (air space) between the material and the Torso surface. Example: TG8.

Dynamic sweat transfer process: combined analysis with evaporative cooling efficiency

The above results and discussion provide the knowledge of sweat transfer rates of the clothed human body. Through combined analyses of the transfer rates and the apparent evaporative cooling efficiency (

η_{cooling}

),²⁵ we can also get indications about the dynamic moisture transfer process. (Refer to Havenith et al.²⁵ for the concept of apparent evaporative cooling efficiency (

η_{cooling}

η_{cooling} = E_{app} / ({\overset{\cdot}{m}}_{evap} * λ)

E_{app}

: sweat cooling power (apparent evaporative heat loss),

{\overset{\cdot}{m}}_{evap}

: evaporation rate, λ: latent heat,

{\overset{\cdot}{m}}_{evap} * λ

: mass-based evaporative heat (evaporative cooling potential). Usually, if

η_{cooling} \geq 1

, all the sweat may evaporate on the skin; if

η_{cooling} < 1

, there may be sweat evaporating from the clothing layer.) (I) As shown in a previous study,⁵¹ 4 out of 15 hydrophilic SU cases (five SUs at three sweat rates) with

η_{cooling}

lower than 1 (p < 0.05), which demonstrates that part of the sweat possibly evaporated from the clothing material. Meanwhile,

{\overset{\cdot}{m}}_{accu}

was zero for these cases. This demonstrates that the material absorbed the sweat and meanwhile evaporated the absorbed sweat under the same rate. (II) For the other 11 SU cases and TG8 at the sweat rate of 100g/h,

{\overset{\cdot}{m}}_{evap}

was equal to

{\overset{\cdot}{m}}_{sweat}

and

η_{cooling}

was equal to 1. This demonstrates that all the sweat evaporated and all the evaporation possibly occurred on the Torso surface. (III) For TG8 at the sweat rates of 175 and 250 g/h and TG11 at 250 g/h,

η_{cooling}

was lower than 1 (p < 0.05). Meanwhile, the sweat content in the material kept increasing (

{\overset{\cdot}{m}}_{accu}

> 0, p < 0.01), and thus the sweat possibly evaporated within the material and the evaporation rate was lower than the absorption rate. (IV) For TG7 at three sweat rates and TG11 at the sweat rates of 100 and 175g/h,

η_{cooling}

was not lower than 1 (p < 0.05) and

{\overset{\cdot}{m}}_{accu}

> 0 (p < 0.05). This demonstrates that part of the sweat may evaporate on the Torso surface and part may be transferred to the material. The hypotheses of dynamic sweat transfer for hydrophilic SUs and TG are summarized in Table 6.

Table 6.

Dynamic sweat transfer process based on analyses of the apparent evaporative cooling efficiency and sweat transfer rate

Category	Material cases	$η_{cooling}$	Sweat transfer rate	Dynamic process
I	4/15 SU cases	<1	${\overset{\cdot}{m}}_{evap} = {\overset{\cdot}{m}}_{sweat}, {\overset{\cdot}{m}}_{accu} = 0$ , ${\overset{\cdot}{m}}_{drip} = 0$	The material absorbed all or part of the sweat and the absorbed sweat evaporated at the same rate.
II	11/15 SU cases; TG8, SR100	=1	${\overset{\cdot}{m}}_{evap} = {\overset{\cdot}{m}}_{sweat}, {\overset{\cdot}{m}}_{accu} = 0$ , ${\overset{\cdot}{m}}_{drip} = 0$	The material did not absorb the sweat, and all the sweat evaporated from the Torso surface.
III	TG8 (SR175)	<1	${\overset{\cdot}{m}}_{evap} < {\overset{\cdot}{m}}_{sweat}$ , ${\overset{\cdot}{m}}_{accu} > 0$ , ${\overset{\cdot}{m}}_{drip} = 0$	Sweat evaporated within the material and the evaporation rate is lower than the absorption rate.
III	TG8 (SR250); TG11 (SR250)	<1	${\overset{\cdot}{m}}_{evap} < {\overset{\cdot}{m}}_{sweat}$ , ${\overset{\cdot}{m}}_{accu} > 0$ , ${\overset{\cdot}{m}}_{drip} > 0$
IV	TG7 (SR100/175); TG11 (SR100/175)	>1	${\overset{\cdot}{m}}_{evap} < {\overset{\cdot}{m}}_{sweat}$ , ${\overset{\cdot}{m}}_{accu} > 0$ , ${\overset{\cdot}{m}}_{drip} = 0$	Part of the sweat evaporated on the Torso surface and part transferred to the material.
	TG7 (SR250)	=1	${\overset{\cdot}{m}}_{evap} < {\overset{\cdot}{m}}_{sweat}, {\overset{\cdot}{m}}_{accu} > 0$ , ${\overset{\cdot}{m}}_{drip} > 0$

Note: in the schematic diagrams, Eva, Abs and Dri refer to sweat evaporation, absorption and dripping, respectively. SR100, SR175 and SR250: sweat rates of 100, 175 and 250 g/h, respectively.

SU: station uniform; TG: turnout gear.

Conclusion

This study investigated the moisture transfer of the clothed human body with continuous sweating under radiant heat exposure. Based on the novel findings, the concept of wettedness of the clothed human body (clothed wettedness), in accordance with the concept of skin wettedness for the nude human body, was proposed to better understand the evaporation process. The results showed that the evaporation rate of the clothed body was increased with the sweat rate. The increment in the evaporation rate was influenced by material properties (e.g., material composition, hydrophilicity and evaporative resistance ( $R_{et}$ )), which affects the sweat accumulation ability. Material evaporative resistance ( $R_{et}$ ) showed a dual relationship with the evaporation rate of the clothed body. The evaporation rate was increased for greater $R_{et}$ due to the higher moisture accumulation; when $R_{et}$ exceeded a certain value, the evaporation rate decreased with greater R_et due to the reduction in the mass transfer coefficient. A re-condensation of the evaporated sweat on the skin surface was also observed in radiant heat, leading to a reduction in the evaporation rate and an increase in the dripping rate. This evaporation–re-condensation phenomenon might lead to steam burn injuries in more hazardous conditions. The dynamic sweat transfer process was also investigated in detail by combined analyses of the sweat transfer rate and apparent evaporative cooling efficiency. The findings from this study provide information for an in-depth understanding of sweat evaporation and its thermal effects for the clothed human body in intensive physical exercise and radiant heat exposure. Further study on an exercising manikin/human body considering the effect of movement as well as wind and air gap variation underneath the clothing may provide more realistic information on the sweat transfer process.

Footnotes

Acknowledgments

The authors would like to thank DuPont, Switzerland, and Trans–Textil, Germany, for supplying the materials for this study and they appreciate the technical support from Max Aeberhard, Ivo Rechsteiner and Shelley Kemp during the laboratory tests.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Nature Science Foundation (Grant Number 51576038), Shanghai Municipal Natural Science Foundation (Grant Number 17ZR1400500), the National Key Research and Development Plan (Grant Number 2017YFB0309100) and the China Scholarship Council.

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