Effect of perspired moisture and material properties on evaporative cooling and thermal protection of the clothed human body exposed to radiant heat

Abstract

Perspired moisture plays a crucial role in the thermal physiology and protection of the human body wearing thermal protective clothing. Until now, the role of continuous sweating on heat transfer, when simultaneously considering internal and external heat sources, has not been well-investigated. To bridge this gap, a sweating torso manikin with 12 thermal protective fabric systems and a radiant heat panel were applied to mimic firefighting. The results demonstrated how the effect of radiant heat on heat dissipation interacted with amount of perspired moisture and material properties. A dual effect of perspired moisture was demonstrated. For hydrophilic materials, sweating induced evaporative cooling but also increased radiant heat gain. For hydrophilic station uniforms, the increment of radiant heat gain due to perspired moisture was about 11% of the increase of heat dissipation. On the other hand, perspired moisture can increase evaporative cooling and decrease radiant heat gain for hydrophobic materials. In addition to fabric thermal resistance (R_ct) and evaporative resistance (R_et), material hydrophilicity and hydrophobicity, emissivity and thickness are important when assessing metabolic heat dissipation and radiant heat gain with profuse sweating under radiant heat. The results provide experimental evidence that R_ct and R_et, the general indicators of the clothing thermo-physiological effect, have limitations in characterizing thermal comfort and heat strain during active liquid sweating in radiant heat. This paper offers a more complete insight into clothing thermal characteristics and human thermal behaviors under radiant heat, contributing to the accurate evaluation of thermal stress for occupational and general individuals.

Keywords

sweating thermal physiology thermal protection evaporative cooling radiant heat gain clothed human body

Firefighters and industrial workers wear thermal protective clothing for protection against environmental and work-related thermal hazards. However, the garment also impedes heat dissipation from the human body, causing heat strain.^1,2 It is thus generally accepted that thermo-physiological effect (the real thermo-physiological effect of clothing is complex and influenced by a variety of factors, e.g., clothing thermal characteristics, clothing weight and bulkiness; in this study, the definition of “thermo-physiological effect” refers to the heat and moisture transfer properties of clothing according to ISO 11092: 2014)³ and thermal protective performance are the two crucial aspects of thermal protective clothing. The effective assessment of these two properties has therefore been extensively studied.^4–8 In the development of evaluation methodologies, the combination and balancing of heat dissipation and thermal protection aspects of thermal protective clothing has become apparent.^5,7,9 Standard measurements of clothing physiological effect were compared to the response of human subjects exercising in 250 W/m² radiant heat.⁵ The thermal protective performance as well as the physiological burden of protective materials were analyzed in combination.⁷ However, the interaction between the environment, clothing and the human body is complex. Due to this complexity, a convincing methodology enabling the combined investigation of heat strain and thermal protection has as yet not been developed. There seem to be two main limitations in the existing assessment methods: firstly, the inaccurate simulation of human sweating with laboratory tests and, secondly, the independent investigation of thermal physiology and thermal protection.

Firstly, the current evaluation methods do not consider liquid perspiration to its true extent, as encountered during intensive physical activities and in hot environments. When predicting the physiological impact of protective clothing, which is generally defined based on the reduction of heat dissipation from the human body due to the clothing, the thermal resistance (R_ct), the evaporative resistance (R_et) and the total heat loss (THL) are commonly investigated. These measures are obtained by the sweating hot plate^3,10–12 and sweating manikins.^13–15 However, in such tests, researchers typically use semi-permeable membranes or wet fabrics as the “skin layer.” Thus, only sweat vapor transferred from the skin is considered and the influence of liquid sweat transfer on heat dissipation is not studied. On the other hand, in spite of the fact that the majority of burns are caused by steam condensation, thermal protective performance is usually evaluated with dry skin models according to thermal protection standards (e.g., ISO 13506, ASTM 1930 and ASTM 2700).^8,16–20 To investigate the effect of perspired moisture on thermal protective performance, researchers have adapted these methods by using wet fabric systems^21–28 or a set-up with an adjustable microclimate relative humidity between the fabric specimen and the copper calorimeter (skin model).^29,30 Within these studies, there is no clear conclusion about the effect of moisture on thermal protective performance. Some studies^7,27,28 showed that the internal moisture enhanced the thermal protective performance, while other researchers^22,31–33 demonstrated the opposing effect. The effect of moisture on thermal protection seems to be influenced by moisture amount and material properties. The previous studies also indicate that it is crucial to realistically simulate human perspiration, which may give a reasonable and accurate evaluation of perspired moisture effects on heat transfer.^19,28

The second possible limitation of the existing evaluation approaches is that even though physiological impact and protective performance of clothing simultaneously affect the human body, they are usually addressed separately and, thus, only provide one-sided information on clothing thermal characteristics. Studies in the thermal protection field focus on heat transfer from the environment to the human body, paying less attention to the possible effect of metabolic heat production from the human body on personal safety. On the other hand, the standard evaluations of clothing physiological effects^3,10,13–15 focus on heat transfer from the human body to the environment without considering the effect of external heat sources on heat dissipation through clothing. The environmental conditions in these current standards usually require the mean radiant temperature to be no more than 1.0℃ different from the mean air temperature, which may be inappropriate for thermal protective clothing, which is specifically used for hot environments. Also, in the current heat stress assessment of radiant heat environments, the clothing thermal characteristics are estimated based on the above standards.^3,34 Some researchers⁴ have recognized this disadvantage for heat stress assessments by using these standard measurements and pointed out that the clothing characteristics in radiant heat have not been considered appropriately. Thus, a series of manikin and modeling studies considering external radiant heat and heat production of the human body simultaneously were conducted to study the heat dissipation and heat gain in radiant heat in the frame of the European Union project THERMPROTECT.^4,35 The shortcoming of these studies is that the effect of sweating was simulated by only moistening the underwear and the moisture amount was not controlled. Following that, some researchers⁶ quantified the effect of moisture on heat loss in radiant heat. However, only one type of turnout gear was used in their study and the effect of material properties cannot be explored.

The aim of this study is to comprehensively investigate thermal behaviors of a clothed (thermal protective clothing) human body in continuous sweating conditions under radiant heat exposure, and explore the factors influencing the thermal characteristics of protective clothing, specifically, both the thermo-physiological impact in radiant heat and the thermal protective performance with internal heat production and sweating. For this, the thermo-physiological effect and thermal protective performance of clothing were evaluated under radiant heat. The effects of radiant heat, perspired moisture and material properties on the physiological and protective performance were thoroughly discussed. This study contributes to the understanding of clothing thermal characteristics and human thermal behaviors under radiant heat and helps toward the improvement of occupational and general health and safety.

Methodology

Materials

Twelve commercially available thermal protective material systems with different physical properties were selected for the study, including seven single-layered station uniforms (SU) and five multi-layered turnout gears (TG; Table 1). As liquid sweating was investigated in our study, material hydrophilicity and hydrophobicity were identified by measuring the contact angle of the innermost side of the material systems. Pho-SU2, SU5, TG9 and TG10 are hydrophobic materials, while the other samples are hydrophilic materials. Pho-SU2 and SU5 were treated with a fluorocarbon finish providing the hydrophobic property. No further information about the hydrophobic treatment of TG9 and TG10 is available. The radiation wavelength range of the infrared lamps (SICCATHERM infrared lamps, OSRAM Licht AG, Germany) used to simulate the external radiant heat in our study is 590–2500 nm. Thus, material reflectivity (r), transmissivity (τ) and emissivity (ɛ) were also measured and averaged over this wavelength range. The main related physical properties of the material samples are presented in Table 1.
Table 1.
Physical properties of material samples (color online only)

Fabric systems Fiber content Physical properties*

Color Weight^a (g/m²) Thickness^b (mm) Contact angle^c (°) R_ct^d (10^–3·K·m²/W) R_et^e (m²·Pa/W) ɛ^f

Station uniform (single-layered fabric)

SU1 50% meta-aramid/50% fire retardant (FR) viscose 197 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 0.254

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

Turnout gear (multi-layered 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 2 130.8 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 0.834

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 2.3 0 71 16.8 0.901

Note: for all turnout gears, 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 a weighing balance, Mettler-Toledo, Switzerland), ^bISO 5084:1996 (by a thickness tester, 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,eISO 11092:2014 (by a hot plate tester, Hohenstein Institute, Germany) and ^fby an extended Fourier transform infrared spectrometer (VERTEX 80, Germany).^3,36,37 SU: station uniform; TG: turnout gear; OL: outer layer; ML: middle layer; IL: inner layer; PTFE: polytetrafluoroethylene; PU: polyurethane.

Experimental design

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. To accurately determine the heat transfer between the clothed human body and its surrounding environment, measurements were performed on a sweating torso manikin developed in our laboratory (Figure 1(a)).^38,39 The torso consists of a multi-layered main cylinder representing the surface area of an adult human torso and two heated aluminum guards. The main cylinder can maintain a constant surface temperature by controlling the heat input. 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 controlled valves for the accurate application of pre-set sweat rates. The test sample is wrapped around the main cylinder as clothing. More detailed information regarding the torso can be found in the literature.^40,41 Figure 1(b) gives the heat and moisture transfer pathways between the clothed torso and the surrounding environment.⁴³ The torso dissipated dry heat by conduction, radiation and convection. When sweating, the perspired moisture may (I) wick through the clothing, (II) evaporate or (III) drip from the clothed system. The evaporated sweat may then condense within the clothing layers. The heat from the environment transfers to the torso by conduction, radiation and convection.
Figure 1.
(a) Schematic diagram of the sweating torso manikin. (b) Schematic representation of heat and moisture transfer pathways when sweating. (c) Schematic diagram of three-phase experimental design. PTFE: polytetrafluoroethylene.

The experiment was designed to include three consecutive phases (Figure 1(c)), with a constant surface temperature of 35℃. In the first phase (P1), the torso was kept in the dry state for 1 hour. In phase two (P2), it began to sweat at a pre-set sweat rate for 2 hours. In the last phase (P3), a 96 cm × 188 cm radiant heat panel filled with the infrared lamps was applied from the front side of the manikin for another 2 hours while the sweat rate was kept at its previous value. As the heat flux in the routine thermal environment for firefighters remains lower than 1.67 kW/m²,⁹ and to guarantee the normal working of the torso, the radiation intensity on the torso surface was determined as 1 kW/m² to simulate the work environment. 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 torso, ensuring the set radiation intensity. Since the routine thermal environment was investigated, we considered that in this type of environmental condition, the human body is in a low to moderate level of sweating. Thus, three sweat rates of 100, 175 and 250 g/h, corresponding to sweat rates of 400–1000 g/h of a firefighter with low to moderate sweating level²² (assuming a body surface area of 1.8 m²),⁴⁴ were chosen. The test duration for each phase was determined according to the required time to reach the steady state of heat transfer. The three-phase experiment simulates the firefighting scenario: preparatory work without sweating followed by sweating and finally additional radiant heat exposure when entering the fire field.

Before the tests, thermal protective materials were preconditioned in the climatic chamber for at least 24 hours. Due to the complexity of the moisture effect, we considered in this study that the material contacted the torso surface homogeneously without an air gap and, thus, we were able to investigate liquid sweat transfer and phase-change in the clothing–human body system. The real-time weight change of the clothed torso was measured by a weighing scale (Mettler-Toledo KCC150, Mettler-Toledo GmbH, Greifensee, Switzerland; accuracy: 0.001 g) to obtain the amount of water stored in the fabrics. A thermographic camera (FLIR A40M, Flir Systems Inc., Wilsonville, Oregon, USA) was also used to record the thermal images of the clothed manikin, so as to observe the sweat transport and evaporation.

For most fabrics, 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 previous studies,⁴⁴ we assumed that the heat transfer of the clothed torso during the period of time reached a steady state. For each level of sweat rate, at least three replications of the fabrics were tested on the sweating torso, guaranteeing the CV of torso heating power among repetition tests to be less than 8.5%.

Calculations and dependent variables

In this study, instead of the THL theoretically calculated according to ASTM F1868,¹⁰ the THL, which was the heating power of the torso to maintain the surface temperature, was measured directly to characterize the heat dissipation performance at each phase of the experiment. The $THL$ in P1 $({THL}_{P 1})$ was obtained as dry heat loss. The $THL$ in P2 $({THL}_{P 2})$ was the combination of dry heat loss and heat loss caused by sweating. The $THL$ in P3 $({THL}_{P 3})$ also included the effect of radiant heat.

To understand the effective cooling caused by sweating, i.e. evaporative heat loss during P2 ( ${EHL}_{P 2}$ ), and effective heat gain caused by external radiant heat sources, i.e. radiant heat gain during P3 ( ${RHG}_{P 3}$ ), the lumped and effective values were calculated as per Equations (1) and (2). The heat pipe effect and wet conduction were not considered in detail, and thus the effective cooling caused by sweating ( ${EHL}_{P 2}$ ), which is also named the apparent evaporative heat loss,³ was assumed to be evaporative heat loss*
${EHL}_{P 2} = {THL}_{P 2} - {THL}_{P 1}$
(1)

${RHG}_{P 3} = {THL}_{P 2} - {THL}_{P 3}$
(2)

${THL}_{P 2}$ , ${THL}_{P 3}$ and ${EHL}_{P 2}$ were chosen as indicators of the thermo-physiological impact of protective clothing and ${RHG}_{P 3}$ was chosen as the thermal protection indicator characterizing the thermal behaviors of the clothed manikin.

Statistical analyses

Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) version 23.0 (IBM, Armonk, NY, USA). The combined effects of the sweat rate and material properties on THL without and with radiant heat, EHL and RHG were investigated with multiple linear regression. Linear regression was chosen due to its effective interpretability and evaluability. The force enter method was chosen for predictor selection. The multicollinearity assessment, casewise diagnostics, assumptions of linearity and homoscedasticity and normality of residuals were also investigated for all the multiple linear regression models. The individual effects of the sweat rate and radiant heat on the heat transfer indicators were analyzed by one-way analysis of variance (ANOVA) and followed by Tukey’s honest significant difference (HSD) or Tamhane’s T2 post-hoc tests.

Results

Heat dissipation and heat gain at different sweat rates

Figure 2 illustrates ${THL}_{P 2}$ , ${EHL}_{P 2}$ , ${THL}_{P 3}$ and ${RHG}_{P 3}$ at three sweat rates for all materials. Overall, ${THL}_{P 2}$ , ${EHL}_{P 2}$ and ${THL}_{P 3}$ of both SU and TG presented an increasing trend with sweat rates (p < 0.01). ${RHG}_{P 3}$ variations with sweat rate exhibited different patterns among the materials.
Figure 2.
Heat loss and heat gain at the three sweat rates. ${THL}_{P 2}$ of station uniform (SU) (a1) and turnout gear (TG) (a2). ${EHL}_{P 2}$ of SU (b1) and TG (b2). Max: maximum evaporative power when assuming all the sweat evaporated. ${THL}_{P 3}$ of SU (c1) and TG (c2). ${RHG}_{P 3}$ of SU (d1) and TG (d2). Note: SU1 at sweat rate 100 g/h and pho-SU2 at three levels of sweat rates did not reach steady state in P3 with acceptable test duration and, thus, the corresponding data are excluded in (c1) and (d1). Hydrophilic materials: solid line; hydrophobic materials: dotted line.

Relationship between material properties and heat dissipation/heat gain

Figure 2 demonstrates that the material properties have an effect on ${THL}_{P 2}$ , ${EHL}_{P 2}$ , ${THL}_{P 3}$ and ${RHG}_{P 3}$ . In most cases, hydrophilic and hydrophobic materials clearly behaved differently: (1) the hydrophobicity of SUs and TGs decreased ${THL}_{P 2}$ , ${EHL}_{P 2}$ and ${THL}_{P 3}$ noticeably by up to 330 W/m² compared with hydrophilic materials; (2) the sweat effect on hydrophobic materials was less pronounced than that on hydrophilic materials. For hydrophilic materials, the results of multiple linear regression between material properties and ${THL}_{P 2}$ , ${EHL}_{P 2}$ , ${THL}_{P 3}$ and ${RHG}_{P 3}$ are shown in Table 2.
Table 2.
Multiple regressions between material properties and heat dissipation/heat gain for hydrophilic station uniforms (SU) and turnout gear (TG)

Material R ² Regression coefficient Unstandardized coefficient (a1–a6, b) Standardized coefficient^a Material R ² Regression coefficient Unstandardized coefficient (a1–a6, b) Standardized coefficient

SU ${THL}_{P 2}$ TG ${THL}_{P 2}$

0.975 Constant 196.63 0.859 Constant 389.2

sweat rate 1.49 0.99 sweat rate 0.73 0.84

Weight* −0.16 −0.18

R_et −5.01 −0.34

${EHL}_{P 2}$ ${EHL}_{P 2}$

0.979 Constant 0.33 0.847 Constant 187.53

sweat rate 1.51 0.99 sweat rate** 0.73 0.88

R_et * −3.89 −0.28

${THL}_{P 3}$ ${THL}_{P 3}$

0.986 Constant −97.57 0.874 Constant −747.74

sweat rate 1.32 0.96 sweat rate 0.32 0.66

R_ct * 15.70 0.24 R_et −6.69 −0.83

Emissivity −145.5 −0.37 Emissivity 984.19 0.45

${RHG}_{P 3}$ ${RHG}_{P 3}$

0.976 Constant 357.12 0.815 Constant 1829.42

sweat rate 0.14 0.17 sweat rate** 0.41 0.71

Thickness* 127.16 0.4 *R_et*** 3.48 0.36

R_ct −21.79 −0.53 Emissivity −1873.16 −0.71

Emissivity** 178.7 0.74

Note: the form of the regression equation is as follows: y = a₁ * sweat rate + a₂ * weight + a₃ * thickness + a₄ * R_ct + a₅ * R_et + a₆ * emissivity + b, where y is ${THL}_{P 2}$ , ${EHL}_{P 2}$ , ${THL}_{P 3}$ and ${RHG}_{P 3}$ , respectively.

*
p < 0.05; **p < 0.001.

a
Standardized coefficients are the estimates resulting from a regression analysis that have been standardized so that the variances of dependent and independent variables are 1.

Water amount stored in the clothed torso

As the accumulated water may have an influence on thermal conduction and heat capacity, the amount of water accumulated in materials during the steady state of P2 was measured as the water accumulation capacity of materials. For hydrophilic SU (Figure 3(a)), the accumulated water showed a positive relationship with fabric thickness. For hydrophilic TG (Figure 3(b)), the effect of thickness is less significant, which may be because of the multi-layer structure. TG7 (thickness: 3.2 mm) with a four-layer structure showed a greater amount of accumulated water compared with the other two TG (thickness: 2.3, 3.8 mm) with a three-layer structure.
Figure 3.
Relationship between the thickness of hydrophilic materials and the water accumulated in materials in P2 at three sweat rates: (a) station uniform (SU); (b) turnout gear (TG) and (c) amount of water accumulated in hydrophobic SU and TG in P2.

For hydrophobic materials, TG showed a greater amount of accumulated water than SU (Figure 3(c); p < 0.05 sweat rates 100 and 250 g/h, p > 0.05 at sweat rate 175 g/h). This should be related to the multiple layer structure of TG, which accumulated more water between the layers, as observed after tests.

Discussion

THL comparison between environments without and with radiant heat

The comparison between ${THL}_{P 2}$ (without radiant heat) and ${THL}_{P 3}$ (with radiant heat) for all materials at three sweat rates (Figure 4(a)) shows that the radiant heat significantly reduced $THL$ (SU: 154–347 W/m², TG: 124–314 W/m², p < 0.001). This is clearly indicated, as no measurement crosses the reference line in Figure 4(a). In addition, the $THL$ reduction ( $THL (RHG) = {THL}_{P 2} - {THL}_{P 3}$ ) is positively related with the incident radiation intensity (Figure 4(b); p < 0.001). $THL$ also differed among materials. Table 3 presents the regression of ${THL}_{P 3}$ by using ${THL}_{P 2}$ , and further considering basic material properties (i.e., emissivity, hydrophilicity, thickness and weight). It shows that material emissivity is the most important material property that causes a difference between ${THL}_{P 2}$ and ${THL}_{P 3}$ . This led to the conclusion that a material with greater ${THL}_{P 2}$ than other materials may have a smaller ${THL}_{P 3}$ (e.g., at sweat rate 250 g/h, ${THL}_{TG 11, P 2}$ (408 W/m²) > ${THL}_{TG 9, P 2}$ (316 W/m²), ${THL}_{TG 11, P 3}$ (95 W/m²) < ${THL}_{TG 9, P 3}$ (192 W/m²), p < 0.001). This demonstrates that errors may be caused if the clothing physiological effect evaluated in an environment without radiant heat is applied in an environment with radiant heat.
Figure 4.
(a) Comparison between ${THL}_{P 2}$ and ${THL}_{P 3}$ for all materials (including all station uniforms [SU] and turnout gears [TG]) at all sweat rate levels. Ref: the reference line on which total heat loss (THL) in P2 equals to that in P3. SU1 at sweat rate 100 g/h and pho-SU2 at the three sweat rates did not reach steady state in P3, and ${THL}_{P 3}$ is zero during acceptable test duration. (b) The relationship between the incident radiant intensity and the radiant heat gain. The data are from European Union (EU) project THERMPROTEC (internal reports), ISO 11092:2014³ and our study. Note: EU-Heatman, EU-NEWTON and EU-TORE are the data from different thermal manikins of the EU project without mentioning the dry and wet state of underwear. ISO 11092:2014³dry and ISO 11092:2014³ wet are the data from Figures 5(a) and (b) of ISO 11092:2014³ with dry and wet underwear. The data from our study includes the results of all materials and all sweat rates.

Table 3.
Regressions between ${THL}_{P 3}$ and ${THL}_{P 2}$ , further considering material properties (i.e., material emissivity (ɛ), hydrophilicity, hydrophobicity, thickness and weight)

Materials Regression model Coefficient of determination (R²) Significance

SU ${THL}_{P 3} = 0.653 * {THL}_{P 2} (p < 0.001)$ −77.638 0.728 p < 0.001

${THL}_{P 3} = 0.839 * {THL}_{P 2} (p < 0.001) - 221.763 $ ɛ(p<0.001)−63.911 0.979 p < 0.001

TG ${THL}_{P 3} = 0.368 {THL}_{P 2} (p < 0.01)$ −17.332 0.161 p < 0.01

${THL}_{P 3} = 0.513 * {THL}_{P 2} (p < 0.001) - 226.651 $ ɛ(p<0.001)+122.435 0.840 p < 0.001

SU and TG ${THL}_{P 3} = 0.703 {THL}_{P 2} (p < 0.001)$ −109.899 0.724 p < 0.001

${THL}_{P 3} = 0.630 * {THL}_{P 2} (p < 0.001) - 136.791 *$ ɛ(p<0.001)+0.688 0.902 p < 0.001

SU: station uniform; TG: turnout gear.

Sweat behaviors in radiant heat

In our study, sweat behaviors on heat transfer in radiant heat can be divided into three types (Table 4). One, for hydrophilic SU and TG, with sweat rate increase $Δ {THL}_{P 2}$ > $Δ {THL}_{P 3}$ > 0 (p < 0.05), that is, $Δ RHG$ > 0. This demonstrates the dual effect of perspired moisture on heat transfer for hydrophilic materials: more sweat can strengthen both the effective cooling and the radiant heat absorption of the clothed human body. For hydrophilic SU, $Δ RHG$ due to perspired moisture (15.0 W/m²) was about 11% of $Δ {THL}_{P 3}$ (132.3 W/m²). For hydrophilic TG, $Δ RHG$ was 41.1 W/m², while $Δ {THL}_{P 3}$ was 31.9 W/m². This shows that, in our cases of hydrophilic SU and TG, perspired moisture can bring more physiological cooling benefit than radiant heat absorption. Two, for hydrophobic TG9, with sweat rate increase $Δ {THL}_{P 3}$ > $Δ {THL}_{P 2}$ (p < 0.05), that is, $Δ RHG$ < 0. Three, for hydrophobic SU5 and TG10, the sweat rate only had very slight positive effect (SU5) or no significant effect (TG10) on ${THL}_{P 3}$ and there is no significant difference between $Δ {THL}_{P 2}$ and $Δ {THL}_{P 3}$ (p > 0.05), that is, $Δ RHG$ = 0. $Δ {THL}_{P 2}$ > 0, due to sweating causing the increase of evaporation, wet conduction and possible evaporation–condensation process. For hydrophilic materials, the possible reason for $Δ RHG$ > 0 is that sweat increased the thermal conductivity and radiation absorptivity of materials greatly, thus increasing the conductive and radiant heat transfer from the environment to the torso. On the other hand, sweat did not change the thermal conductivity and radiation absorptivity greatly for hydrophobic materials and added an additional water layer, which may act as minor thermal insulation for radiant heat transfer, leading to $Δ RHG$ < 0. The different behavior between TG9 and SU5/TG10 is possibly due to less water accumulation for SU5/TG10 (Figure 3(c)), causing the thermal insulation effect of the water to be insignificant.
Table 4.
Sweat effect on heat dissipation and heat gain: $Δ {THL}_{P 2}$ , $Δ {THL}_{P 3}$ , $Δ RHG$ for each 100 g/h sweat rate increment

Type of moisture effect Materials $Δ {THL}_{P 2}$ (W/m²) $Δ {THL}_{P 3}$ (W/m²) $Δ RHG$ (W/m²)

I Hydrophilic SU 147.3** 132.3* 15.0**

Hydrophilic TG 72.9* 31.9 41.1**

II Hydrophobic TG9 18.6 28.4* –9.8*

III Hydrophobic SU5 13.0 14.6** –1.5

Hydrophobic TG10 4.1 7.8 –3.7

*
p < 0.05; **p < 0.005.

SU: station uniform; TG: turnout gear; THL: total heat loss; RHG: radiant heat gain.

The results demonstrate that continuous sweating has a negative effect on the thermal protective performance of hydrophilic materials, but a positive effect for hydrophobic materials. The exact role of perspired moisture on the thermal protective performance depends on the material surface properties and water accumulation capacity. To our knowledge, this is the first time the effect of perspired moisture on radiant heat gain for hydrophilic and hydrophobic materials has been quantified, exploring the role of continuous sweating on heat transfer of the clothed human body (Figure 5).
Figure 5.
Effect of perspired moisture on heat transfer. (a) Hydrophilic materials: perspired moisture increases both evaporative heat loss and radiant heat gain. (b) Hydrophobic materials: perspired moisture increases evaporative heat loss and decreases radiant heat gain. The start and end of the arrow “evaporative heat loss” is the human body and the environment, while the start and end of the arrow “radiant heat gain” is the environment and the human body.

Characterizing the thermal comfort/heat stress of thermal protective clothing with liquid sweating and/or radiant heat – the limitations of standard R_ct and R_et

Firstly, fabric hydrophilicity and hydrophobicity play a crucial role in characterizing physiological burden in both environments without and with radiant heat ( ${THL}_{P 2}$ , ${THL}_{P 3}$ ). Human trials⁴⁵ have shown that hydrophilic clothing exhibited more favorable thermal physiological performance than hydrophobic clothing. This is in agreement with most cases of our study: ${THL}_{P 2}$ and ${THL}_{P 3}$ of hydrophilic materials were higher than that of hydrophobic materials regardless of the relative magnitude of fabric $R_{ct}$ and $R_{et}$ (Figures 2(a1, (a2), (c1) and (c2)). This could be due to the large amount of dripping water for hydrophobic materials that could not provide evaporation cooling, wet conduction and heat pipe effect. The exceptions of ${THL}_{TG 9, P 2}$ and ${THL}_{TG 10, P 2}$ at sweat rate 100 g/h demonstrated that at lower sweat rates, the negative effect of material hydrophobicity on physiological cooling was not obvious. The exception of ${THL}_{SU 5, P 3}$ at sweat rate 100 g/h and ${THL}_{TG 9, P 3}$ at three sweat rates was due to the lower emissivity of SU5 and TG9.

For hydrophobic pho-SU2 and SU5, even though $R_{ct}$ and $R_{et}$ of SU5 are both higher than that of pho-SU2, SU5 showed a greater ${THL}_{P 2}$ and ${THL}_{P 3}$ than pho-SU2 at three sweat rates. In addition to the lower emissivity of SU5, another possible reason may be inferred from the infrared images (Figure 6). It is clear that SU5 has a larger evaporative area than pho-SU2 and we hypothesized that because SU5 has a better capacity to hold water between the material and the torso surface, more water can evaporate with SU5 and also more evaporative cooling occurs. To confirm this point, a moisture management test⁴⁶ was conducted to compare the interaction behavior of pho-SU2 and SU5 with perspired moisture. The results (time for wetting top surface: pho-SU2: 10.8 ± 4.3 s, SU5: 8.2 ± 2.4 s) seemingly showed that SU5 did have a better moisture absorption capacity than pho-SU2.
Figure 6.
Infrared images of pho-SU2 (a) and SU5 (b). From left to right: Sweat rates 100, 175, 250 g/h in the steady state of P2, respectively, and sweat rate 175 g/h immediately after taking off the materials when the tests end.

In addition, it is worth noting that in our study, the sweat glands of the sweating torso manikin are a rough resolution as compared to human skin. Fifty-four sweat nozzles are distributed on the surface of the torso device (surface area: 0.4335 m²), while the real human torso has about 100 sweat glands per cm². Thus, for hydrophobic materials the drip-off water in the torso test will be more than that in the human case because there will be less local moisture saturation for the human body. Therefore, for hydrophobic materials, evaporation cooling may be more notable in the human body than the torso, in both environments without and with radiant heat. Caution needs to be taken when extending the results here on hydrophobic materials to the actual clothed human body.

Secondly, our study showed the significant effect of fabric emissivity, $R_{ct}$ , $R_{et}$ and thickness on the heat transfer indicators, while the previous study⁴ demonstrated a negligible influence of material properties, except material reflectivity. The first possible reason is that the previous measurements were conducted using an anatomically formed thermal manikin, which led to the formation of an air gap between the body and the clothing, making material effects less pronounced. In addition, previous experimental studies only considered pre-wetted underwear rather than continuous liquid sweating; and in the theoretical model³⁵ the evaporative heat transfer was neglected. However, for thicker hydrophilic SUs in our study, the clothed torso accumulated a greater amount of water (Figure 3(a)) and gained more radiant heat (Table 2). This indicates the importance of combine analyses of liquid sweat transport and its thermal effects in the clothed system.

Thirdly, the two-way role of clothing/material $R_{ct}$ in heat transfer may require further study. Generally, when a material has a greater $R_{ct}$ and $R_{et}$ and smaller $THL$ than other materials, it is assumed that the material has worse thermal comfort.^7,10 In contrast, some researchers have pointed out that with higher $R_{ct}$ , more thermal protection can be provided to wearers.^4,35 For the hydrophilic SU in our study, with 1 × 10^–3 K·m²/W $R_{ct}$ increase, the human body dissipates 15.70 W/m² more heat and heat gain decreases by 21.79 W/m² when other input parameters (i.e., sweat rate, fabric emissivity and thickness) are constant (Table 2). Moreover, in P3, for SU1 of sweat rate 100 g/h and pho-SU2 at three sweat rates, the surface temperature of the clothed torso remained above 35℃, not reaching steady state during the experiment. However, for all TG, the clothed torso reached steady state, that is, the surface temperature decreased to 35℃ and the heating power reached a certain value above zero for at least half an hour at the end of P3. That is, for SU1 and pho-SU2 with lower $R_{ct}$ and $R_{et}$ than all TGs, they cannot achieve heat balance in radiant heat while all TGs can. These results indicate the limitation of standard $R_{ct}$ and $R_{et}$ to characterize the clothing physiological effect and serve as experimental evidence of the two-way role of $R_{ct}$ in heat transfer.

The results in the present study are beneficial for understanding the heat transfer of the contact area between clothing and the human body, for example, the upper chest, upper back, upper arm and posterior pelvis in the standing posture and also the lower arm and shin in exercising postures.^47,48,49 For body regions that have a lower contact area, further studies considering different sizes and distributions of the air gap are required.⁵⁰

Conclusions

In the present study, to understand thermal the physiological impact and thermal protection of clothing simultaneously and investigate the effect of perspired moisture and material properties on these two clothing thermal characteristics, the external thermal hazard and internal heat production were combined, and perspired moisture and a range of common thermal protective clothing were applied. The results show how the effect of radiant heat on clothing thermal characteristics interacts with the sweat rate and material properties. The dual role of perspired moisture in heat transfer for hydrophilic clothing was quantified: perspired moisture contributes to the evaporation cooling and also reduces protective performance. On the other hand, perspired moisture can increase evaporative cooling and decrease the radiant heat gain for hydrophobic materials. Standard fabric R_ct and R_et may be not sufficient for characterizing the heat stress in radiant heat. Material hydrophilicity and hydrophobicity, emissivity and thickness are also critical when assessing the thermal physiology and thermal protection with profuse sweating.

The results demonstrate that the fabric surface properties, the two-way effect of thermal resistance and the dual roles of perspired moisture are crucial factors that influence the thermal comfort and protection of thermal protective clothing. Further studies are required to investigate these factors within a larger range of materials and quantify the effect of material properties (e.g., thermal resistance, emissivity and thickness) and moisture in a variety of hot environments, including extreme conditions with a high intensity of radiant heat and flash fires. Such studies will finally contribute to the understanding of clothing thermal characteristics and human thermal behaviors in realistic conditions and guide the design and selection of materials for high-performance thermal protective clothing, improving occupational and public health and safety.

Fabric systems	Fiber content
Station uniform (single-layered fabric)
SU1	50% meta-aramid/50% fire retardant (FR) viscose	197	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	0.254
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
Turnout gear (multi-layered 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	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	0.834
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	2.3	0	71	16.8	0.901

Material	R ²	Regression coefficient	Unstandardized coefficient (a1–a6, b)	Standardized coefficient^a	Material	R ²	Unstandardized coefficient (a1–a6, b)	Standardized coefficient
SU	${THL}_{P 2}$	TG	${THL}_{P 2}$
0.975	Constant	196.63		0.859	Constant	389.2
sweat rate**	1.49	0.99	sweat rate**	0.73	0.84
			Weight*	−0.16	−0.18
			R_et**	−5.01	−0.34
${EHL}_{P 2}$	${EHL}_{P 2}$
0.979	Constant	0.33		0.847	Constant	187.53
sweat rate**	1.51	0.99	sweat rate**	0.73	0.88
			R_et *	−3.89	−0.28
${THL}_{P 3}$	${THL}_{P 3}$
0.986	Constant	−97.57		0.874	Constant	−747.74
sweat rate**	1.32	0.96	sweat rate**	0.32	0.66
R_ct *	15.70	0.24	R_et**	−6.69	−0.83
Emissivity**	−145.5	−0.37	Emissivity**	984.19	0.45
${RHG}_{P 3}$	${RHG}_{P 3}$
0.976	Constant	357.12		0.815	Constant	1829.42
sweat rate**	0.14	0.17	sweat rate**	0.41	0.71
Thickness*	127.16	0.4	R_et**	3.48	0.36
R_ct**	−21.79	−0.53	Emissivity**	−1873.16	−0.71
Emissivity**	178.7	0.74

Materials	Regression model	Coefficient of determination (R²)	Significance
SU	${THL}_{P 3} = 0.653 * {THL}_{P 2} (p < 0.001)$ −77.638	0.728	p < 0.001
${THL}_{P 3} = 0.839 * {THL}_{P 2} (p < 0.001) - 221.763 *$ ɛ(p<0.001)−63.911	0.979	p < 0.001
TG	${THL}_{P 3} = 0.368 * {THL}_{P 2} (p < 0.01)$ −17.332	0.161	p < 0.01
${THL}_{P 3} = 0.513 * {THL}_{P 2} (p < 0.001) - 226.651 *$ ɛ(p<0.001)+122.435	0.840	p < 0.001
SU and TG	${THL}_{P 3} = 0.703 * {THL}_{P 2} (p < 0.001)$ −109.899	0.724	p < 0.001
${THL}_{P 3} = 0.630 * {THL}_{P 2} (p < 0.001) - 136.791 *$ ɛ(p<0.001)+0.688	0.902	p < 0.001

Type of moisture effect	Materials	$Δ {THL}_{P 2}$ (W/m²)	$Δ {THL}_{P 3}$ (W/m²)	$Δ RHG$ (W/m²)
I	Hydrophilic SU	147.3**	132.3*	15.0**
Hydrophilic TG	72.9*	31.9	41.1**
II	Hydrophobic TG9	18.6	28.4*	–9.8*
III	Hydrophobic SU5	13.0	14.6**	–1.5
Hydrophobic TG10	4.1	7.8	–3.7

Footnotes

Acknowledgements

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

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 Fundamental Research Funds for the Central Universities (Grant No. 2232018G-08), the National Nature Science Foundation (Grant No. 51576038), the Shanghai Municipal Natural Science Foundation (Grant No. 17ZR1400500) and the China Scholarship Council.

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Fabric systems	Fiber content	Physical properties*
Fabric systems	Fiber content	Color	Weight^a (g/m²)	Thickness^b (mm)	Contact angle^c (°)	R_ct^d (10^–3·K·m²/W)	R_et^e (m²·Pa/W)	ɛ^f
Station uniform (single-layered fabric)
SU1	50% meta-aramid/50% fire retardant (FR) viscose		197	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	0.254
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
Turnout gear (multi-layered 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	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	0.834
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	2.3	0	71	16.8	0.901

Effect of perspired moisture and material properties on evaporative cooling and thermal protection of the clothed human body exposed to radiant heat

Abstract

Keywords

Methodology

Materials

Experimental design

Calculations and dependent variables

Statistical analyses

Results

Heat dissipation and heat gain at different sweat rates

Relationship between material properties and heat dissipation/heat gain

Water amount stored in the clothed torso

Discussion

THL comparison between environments without and with radiant heat

Sweat behaviors in radiant heat

Characterizing the thermal comfort/heat stress of thermal protective clothing with liquid sweating and/or radiant heat – the limitations of standard Rct and Ret

Conclusions

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

References

Characterizing the thermal comfort/heat stress of thermal protective clothing with liquid sweating and/or radiant heat – the limitations of standard R_ct and R_et