Effects of environmental temperature and humidity on evaporative heat loss through firefighter suit materials made with semi-permeable and microporous moisture barriers

Abstract

The goal of this research was to understand how firefighter protective suits perform in different operational environments. This study used a sweating guarded hotplate to examine the effect of environmental temperature (20–45°C) and relative humidity (25–85% RH) on evaporative heat loss through firefighter turnout materials. Four firefighter turnout composites containing three different bi-component (semi-permeable) and one microporous moisture barriers were selected. The results showed that the evaporative resistance of microporous moisture barrier systems was independent of environmental testing conditions. However, absorbed moisture strongly affected evaporative heat loss through semi-permeable moisture barriers coated with a layer of nonporous hydrophilic polymer. Moisture absorption in mild environment (20–25°C) tests, or when testing at high humidity (>85% RH), significantly increased water vapor transmission in semi-permeable turnout systems. It was also found that environmental conditions used in the total heat loss (THL) test (25°C and 65% RH) produced moisture condensation in bi-component barrier systems, making them appear more breathable than could be expected when worn in hotter environments. Regression models successfully qualified the relationships between moisture uptake levels in semi-permeable barrier systems and evaporative resistance and THL. These findings reveal the limitations in relying on THL, the heat strain index currently called for by the NFPA 1971 Standard for Structural Firefighter personal protective equipment, and supports the need to measure turnout evaporative resistance at 35°C (Ret), in addition to THL at 25°C.

Keywords

Moisture barrier evaporative resistance heat stress firefighter protective clothing

The thermal burden of firefighter turnout suits is a primary contributor to firefighter heat strain and to heat-related injuries that occur in structural firefighting.^1,2 The NFPA 1971 Standard for Structural Firefighter personal protective equipment (PPE) establishes a minimum heat strain requirement for turnout suits materials based on measured total heat loss (THL).³ Because the THL requirement is strictly valid only for predicting turnout-related heat strain in mild environmental conditions (25°C, 65% RH), there is interest in examining performance metrics across a broader range of the environmental temperature and humidity conditions. This has led to consideration of an evaporative resistance or Ret requirement. Ret is an index of water vapor resistance measured in hotter conditions (35°C, 40% RH) according to standard ASTM F1868, Part B.⁴ Ret is an isothermal measurement wherein the only heat loss possible is by evaporative heat transfer. THL and Ret test conditions can produce different breathability rankings of tested turnout materials, depending on the technology of the moisture barrier incorporated in the turnout composite construction.⁵

Heat loss through turnout materials is measured in the laboratory using a guarded sweating hot plate apparatus, sometimes called a sweating skin model. Sweating guarded hot plates are called sweating skin models because they measure heat loss through clothing fabrics using the same mechanisms responsible for heat loss from the human body through clothing, or by conductive, convective and radiant heat transfer, and by heat loss through sweat evaporation from the skin. The testing principle is simple: heat loss is measured as the power, in W/m², needed to maintain the guarded heated plate surface at 35°C (roughly skin temperature) as heat is lost from the plate surface through the test sample into an environmentally controlled test chamber. The guarded plate itself “sweats,” or delivers a controlled source of evaporative heat energy. The test chamber can be set to maintain air temperature and humidity and to control airflow across the surface of the fabric test sample. THL is the sum of dry heat transfer and wet heat transfer. “Dry” heat transfer results from the temperature differential between the simulated skin surface and a cooler ambient environment. The second (and usually larger) fraction of THL comes from heat loss through evaporative heat transfer occurring because of the differences in the moisture vapor pressure on the skin or sweating plate side of the test material (assumed to be moisture saturated, or 100% RH) and the less humid ambient test environment.

The NFPA 1971 Standard for Structural Firefighter protective clothing³ specifies non-isothermal testing conditions (25°C, 65% RH) or ASTM F1868, Part C,⁴ for measuring the THL of firefighter suit materials. This enables assessment of the insulation as well as the evaporative properties of turnout materials. Because of the 10°C temperature difference across the turnout test sample and ambient environment, it is possible for moisture condensation to occur in the test sample. This phenomenon results in a higher reading of heat loss. This is why the evaporative resistance measured in non-isothermal conditions is called “apparent” evaporative resistance. Because of moisture condensation in the test sample, some test materials can appear to become more moisture vapor permeable when tested in cooler environments.

Ret is an index of evaporative heat resistance measured using a guarded sweating hot plate method (ASTM F1868, Part B) that is configured for isothermal environmental testing conditions (35°C, 40% RH).⁴ Because it is an isothermal test, Ret is a “truer” or “pure” measure of the evaporative resistance of clothing materials since moisture condensation effects in this method avoid these complications. On the other hand, Ret does not measure the thermal insulation, or the resistance to “dry” heat transfer of turnout materials.

Turnout suit constructions consist of a woven heat resistant outer shell fabric with inner layers including moisture barrier and thermal liner components. The moisture barrier functions to protect the firefighters from liquids – water, biological materials and chemicals – while allowing transmission of evaporated sweat from the body.⁶ Evaporative heat transfer is the primary mechanism of body heat loss in hot conditions, and the moisture barrier layer is the main determinant of the evaporative resistance of turnout systems.⁷ The composition and structure of moisture barriers are key to understanding how environmental conditions may affect the evaporative heat resistance of different moisture barrier technologies.

Moisture barriers are typically composed of thin semi-permeable membranes laminated to a fire-resistant woven or nonwoven base fabric. Current membrane technologies fall into three categories: microporous, solid hydrophilic and bi-component membranes. Among microporous membranes, expanded poly(tetrafluoroethylene) (e-PTFE) membranes are widely used in firefighter clothing constructions. These membranes have pore sizes ranging from 0.02 to 50 micrometers.⁸ They resist penetration of liquid water while allowing water vapor to diffuse through the interconnected channels.⁹ Nonporous films, such as polyurethanes (PUs) containing hydrophilic components, are also commonly used as breathable, waterproof membranes in firefighter suits. Vapor transmission through solid film barriers occurs by an absorption–diffusion–desorption mechanism.¹⁰,¹¹ The diffusion coefficient of water vapor through hydrophilic polymers depends on the moisture content of the membrane.^12–14 Bi-component membranes are the combination of a microporous membrane with solid hydrophilic film. Almost all commercially available moisture barriers in firefighter suits are bi-component e-PTFE moisture barriers, which feature a fine coating of solid hydrophilic film on an e-PTFE microporous membrane. The addition of a solid hydrophilic layer provides more durability to the moisture barrier and resistance to water penetration, but this surface coating increases the evaporative resistance, making the evaporative resistance strongly dependent on the moisture content inside the hydrophilic layer.¹⁵

Ding et al.^16,17 developed a heat and mass transfer model and studied the influence of the environmental conditions, materials properties and air gap on the thermal and evaporative resistance of a single-layer fabric system. One of the findings was that ambient conditions, including temperature, humidity and wind speed, have little effect on the evaporative resistance of the fabric itself without an air gap. He and Yu¹⁸ investigated the effect of ambient temperatures on the evaporative resistance of firefighter turnout composite using a sweating guarded hotplate and found that there was no obvious change in evaporative resistance under the chamber temperature from 20°C to 40°C. However, only one sample with a PTFE moisture barrier was selected. The above-mentioned studies did not address the effect of ambient conditions on mass transfer through different moisture barriers. Gibson¹⁴ studied the effect of temperature and humidity on vapor diffusion through polymer membranes or membrane/textile laminates with a Dynamic Moisture Permeation Cell. It was reported that the diffusion resistance of water molecules through the hydrophilic membrane strongly depended on the moisture contents.¹⁴ However, there was no temperature gradient applied across membranes during testing, and the influence of ambient conditions on moisture gain in the membranes was unknown. It is therefore necessary to investigate how ambient conditions affect moisture gain inside moisture barriers, and how the moisture absorption would affect the evaporative resistance of firefighter composites containing different moisture barriers types.

It is critically important to understand how firefighter protective clothing with different types of moisture barriers transfers heat in different environmental conditions. In this paper, firefighter turnout composites with four different moisture barriers, including three bi-component and one microporous moisture barrier, were selected. We explored how environmental conditions (temperature and humidity) affect the water uptake in each layer of the firefighter fabric systems along with how cooling through evaporative heat loss changed in different conditions. We also considered the relationship between evaporative resistance and the moisture content inside the moisture barriers.

Experimental method

This research conducted experiments designed to qualify environmentally related differences in heat transfer in firefighter turnout materials incorporating different breathable moisture barrier technologies.

Test materials

The turnout composites studied utilize different membrane compositions to achieve a range of evaporative resistance (Table 1). Three of the samples are examples of e-PTFE bi-component membranes (Samples A–C). Bi-component membranes incorporate a fine hydrophilic layer coated onto a microporous e-PTFE film, as shown in Figure 1(a). The nonporous hydrophilic coating increases resistance to high-pressure water penetration but impacts the evaporative resistance of the membrane. It makes the evaporative resistance of the membrane strongly dependent on the moisture content of this hydrophilic layer.¹⁵ Two of the semi-permeable bi-component membranes (Samples B and C) are commercially available products compliant with the NFPA 1971 Standard for Structural Firefighter PPE.³ Sample A is a specially designed bi-component e-PTFE membrane coated with a proprietary nonporous layer. Sample MM is an example of a truly moisture vapor permeable e-PTFE microporous membrane (no hydrophilic coating), as shown in Figure 1(b). Membranes A and MM are special moisture barrier constructs specially obtained for this study.

Table 1.
Moisture barriers

Moisture barrier Membrane type and composition Weight (g/m²) Thickness (mm) Images

A Bi-component e-PTFE membrane, bonded to woven aramid fabric 216 0.42

B Bi-component e-PTFE membrane, bonded to spun laced-aramid base fabric 167 0.76

C Bi-component e-PTFE membrane, bonded to woven aramid fabric 170 0.40

MM Microporous e-PTFE membrane, bonded to woven aramid fabric 144 0.38

e-PTFE: expanded poly(tetrafluoroethylene).

Figure 1.
Schematics of the (a) bi-component moisture barrier (A–C) and (b) microporous moisture barrier (MM).

We tested each of the four different moisture barrier technologies as inner-layer components of layered turnout fabric systems that were otherwise identical, as shown in Figure 2. They used the same outer shell fabric (meta-aramid blend, plain weave, weight 256 g/m² and thickness 0.61 mm) and thermal liner (needle punched aramid nonwoven quilted to 100% meta-aramid woven fabric, weight 287 g/m² and thickness 1.99 mm). During use, the base fabric of the moisture barrier faced outward, and the membrane surface faced the human skin. Since the only difference between firefighter composites is the moisture barrier, we used the moisture barrier name to represent the firefighter composite. Unless specified, the notations of A–C and MM refer to firefighter composites.

Figure 2.
Firefighter turnout fabric system.

Measuring heat loss of turnout systems in different ambient temperatures and humidities

We measured heat loss through turnout samples in different environmental temperatures and humidities using a sweating plate housed in an environmental chamber (Figure 3). The hot plate apparatus incorporates specialized instrumentation, including an embedded heat flux sensor, liquid-cooled hot plate and thermal control system that enables measurements that are not possible using a standard hot plate apparatus, specifically when ambient air temperature exceeds the surface temperature of the guarded sweating plate.

Figure 3.
Schematic of the sweating guarded hot plate.

The hot plate system consisted of a 20.3 cm $\times$ 20.3 cm heated plate surrounded by a 5.1-cm guard ring (Figure 3). Twenty-five volumetrically controlled sweat ports supplied moisture to the plate. An integrated liquid-cooling loop and an embedded heat flux sensor enabled heat flux measurements in ambient conditions above the plate set temperature. An embedded heat flux sensor measured heat loss in the hottest testing condition (45°C). Power supplied to hotplate indicated heat loss for other ambient temperatures (20°C, 25°C and 35°C). We maintained the surface of the hot plate at 35°C to simulate human skin temperature during the tests. The device used (DHS–8.2, Thermetrics, USA) fulfills all requirements in ASTM F1868-17, but it does not meet the requirement of ISO 11092-2014. The ISO standard requires the use of porous metal for the construction of hotplate surface, and the device test surface in this study was made of composite materials.¹⁹

We measured heat loss at four different chamber conditions (20°C, 88.1% RH; 25°C, 65% RH; 35°C, 36.6% RH; 45°C, 21.5% RH). We set the relative humidity levels to maintain a constant ambient vapor pressure gradient of 3.57 kPa between the wet hotplate surface and the chamber environment to eliminate the influence of a vapor pressure gradient on evaporative resistance measurements. When tested at 45°C, the incorporated cooling system was activated by circulating coolant under the hotplate surface to prevent it from overheating. With the additional cooling, the power required to keep the plate at a constant temperature could no longer be used as it included the power required to overcome the cooling. Therefore, the heat loss measured by the embedded heat flux sensor was utilized at the condition of 45°C.

We also conducted tests at five different levels of ambient relative humidity (25% RH, 40% RH, 55% RH, 70% RH and 85% RH) to study the effects of environmental humidity on heat loss. Tests were conducted at isothermal temperature conditions (35°C) to determine the influence of ambient humidity levels on evaporative resistance. The test duration was 1 hour. Steady-state heat transfer through test samples typically occurred within 30 minutes of initiating the test. Average heat loss was measured after achieving steady-state conditions.

Test samples were preconditioned at 21°C and 65% RH for at least 12 hours prior to hot plate testing. The duration of all wet tests was controlled to be 1 hour. After the wet test, each layer of the firefighter composites was weighed immediately, starting with the outer shell, then the moisture barrier and the liner. The weight of each layer of the turnout composite was measured on a precision scale (Mettler PM460). Before measurement, all samples were sealed inside a plastic bag to minimize moisture loss.

Calculations

THL from the hot plate to the environment through the composites measured in the environment of 25°C, 65% RH was calculated with the following equation⁴
$THL = \frac{Δ T}{Rct - R_{cbp} + 0.04} + \frac{Δ P}{{}^{A}{Ret - R_{ebp}} + 0.0035}$
(1)
where $R_{cbp}$ (°C·m²/W) and $R_{ebp}$ (kPa·m²/W) are the thermal resistance and evaporative resistance of the boundary air layer measured with the bare plate, respectively. The values of 0.04 and 0.0035 are standard bare plate values for dry and wet tests at a wind speed of 2 m/s, respectively.²⁰ Here, $Rct - R_{cbp}$ equals Rcf (intrinsic thermal resistance) and ${}^{A}{Ret - R_{ebp}}$ is ^ARef (intrinsic apparent evaporative resistance). The first part of Equation (1) is dry heat loss due to conduction, radiation and convection. The second part is wet heat loss mainly caused by evaporation. For the tests conducted at non-standard conditions, we named them predicted THL to distinguish from the standard THL test.

Total thermal resistance was calculated by
$R_{c t} = \frac{Δ T}{H_{d}}$
(2)
where ΔT is the temperature gradient between the hotplate surface and ambient conditions, °C, and H_d is the electrical power supplied to maintain the hotplate at 35°C during the dry test, W/m².

Total apparent evaporative resistance (^ARet) was calculated by⁴
${}^{A}{Ret} = \frac{Δ P}{H_{tot} - \frac{Δ T}{Rct}}$
(3)
where ΔP (kPa) is the vapor pressure gradient between the water saturated hotplate surface and the chamber; $Rct$ (°C·m²/W) is the total thermal resistance of the samples; $H_{tot}$ (W/m²) is the electrical power supplied to maintain the hotplate at 35°C during the wet test. When testing at 35°C, $Δ T$ was zero, and the ^ARet result calculated equals the “true” evaporative resistance (Ret). We used the Antoine vapor pressure equation²¹ to calculate the water vapor pressure difference between the hotplate surface and ambient conditions (ΔP).

The weight gain W (g) after the wet test for each sample was calculated with
$W = W_{2} - W_{1}$
(4)
where W₁ (g) is the weight of the preconditioned sample before the wet test and W₂ (g) is the weight of the sample after the wet test. To reduce the evaporation during measurement, we placed the digital scale near the hotplate device so that samples after the wet test could be weighed immediately. The weight measurements occurred at room temperature with minimal airflow, which can help to reduce evaporation. However, evaporation during testing is unavoidable.

Statistical analysis

The statistical analysis was performed using JMP Pro 15 software (SAS, Cary, NC, USA). The evaporative resistances measured at different environmental conditions for the firefighter composites were compared using a one-way analysis of variance (ANOVA), followed by Tukey's honestly significant difference (HSD). A p-value of less than 0.05 was chosen to indicate a significant difference.

Results and discussion

Table 2 shows the measured THL and evaporative resistance of the turnout study materials. THL, intrinsic thermal resistance (Rcf) and apparent evaporative resistance (^ARet) were measured in non-isothermal conditions (25°C, 65% RH) following ASTM F1868, Part C. Evaporative resistance (Ret) was measured in isothermal conditions (35°C, 40% RH) following procedures described in ASTM F1868, Part B.⁴ These data indicated only about 12 W/m² difference in total heat transmitted by the bi-component membrane systems A and C. However, the system that incorporated a microporous moisture barrier (composite MM) transmitted 28 W/m² more heat than composite C in these non-isothermal test conditions. All the turnout systems exceeded the 205 W/m² minimum THL performance level required by the NFPA 1971 standard.³ In comparison, significant differences were indicated in the ability to transmit evaporative heat in isothermal conditions, as shown by measured Ret values. The system containing the microporous membrane (MM) was more than three times less resistant to isothermal evaporative heat transmission than the system with the monolithic hydrophilic coating layer (composite A).

Table 2.
Total heat loss (THL) and evaporative resistance of firefighter turnout materials

Composite Weight (g/m²) Thickness (mm) Rcf (°C·m²/W) ^ARet (Pa·m²/W) THL (W/m²) Ret (Pa·m²/W)

A 747 3.11 0.1346 (2.65% CV) 23.1 (0.35% CV) 221 56.7 (1.44% CV)

B 711 3.58 0.1583 (1.33% CV) 22.7 (1.57% CV) 218 40.4 (1.72% CV)

C 695 3.15 0.1441 (2.68% CV) 21.3 (1.23% CV) 233 31 (2.43% CV)

MM 666 3.07 0.1378 (1.66% CV) 18.8 (2.18% CV) 261 18.7 (2.22% CV)

Note: Rcf (intrinsic thermal resistance), ^ARet (total apparent evaporative resistance), THL (total heat loss) and Ret (total evaporative resistance) were tested according to ASTM F1868.⁴

Significant differences in Ret values were observed in all three of the systems that incorporated bi-component membranes (A > B > C) (p < 0.05). Therefore, these data confirmed a material-dependent relationship between heat loss through turnout systems and environmental testing conditions.

Influence of air temperature on heat loss and evaporative resistance

Figure 4 shows heat loss through the turnout systems and environmental temperature in guarded hot plate tests conducted at 20°C, 25°C, 35°C and 45°C. The vapor pressure gradient between the sweating plate and chamber environment was held constant at 3.57 kPa for these tests.

Figure 4.
Heat loss through turnout composites measured by the hotplate at different ambient air temperatures.

These data show that a higher air temperature had a more pronounced effect on lowering heat loss through turnout systems that contain semi-permeable bi-component moisture barriers (composites A–C) than on the system incorporating a more vapor permeable microporous moisture barrier (composite MM). We could observe that the heat loss was similar through the turnout systems in cool environments (20–25°C). However, significant differences in composite heat loss appeared when ambient temperatures exceeded 30°C. The observed differences were substantial when the air temperature reached 35°C or higher.

Figure 5 shows that the apparent evaporative resistance of the composites increased with temperature, following the same trend as THL. This finding confirmed the effects of moisture barrier composition on the relationship between system heat loss and environmental temperature.

Figure 5.
Evaporative resistance of turnout composites at different ambient temperatures according to Equation (3) (the CV of replications ranged from 0.35% to 7.97%).

Figure 5 shows that, while the apparent evaporative resistance of the turnout systems was similar when measured in cool environments (20–25°C), it increased sharply with ambient air temperature for composites containing bi-component membranes (A–C). In contrast, apparent evaporative resistance (^ARet) remained constant for the composite made with the microporous membrane (MM) (p < 0.05).

Figure 6 shows the calculated fraction of the THL through the composites broken out by evaporative heat transfer (wet heat loss) and by dry heat loss.

Figure 6.
Calculated dry and wet heat loss according to Equation (1) under different ambient temperatures at a constant vapor pressure gradient of 3.57 kPa

At cooler ambient temperatures (20–25°C), the largest fraction of heat loss came through evaporative heat transfer through all the systems tested. The primary cause of the lower heat loss in composites containing bi-component moisture barriers was the observed increase in the apparent evaporative resistance of these systems when ambient air temperatures exceeded about 30°C. Significantly, reduced evaporative heat loss was not present in the composite that incorporates the microporous moisture barrier.

Therefore, we can attribute the effect of ambient temperature on heat loss in some turnout systems to reduced apparent evaporative heat transfer associated with moisture condensation in the composite. Figure 7 shows the moisture absorption of the different systems measured at different ambient temperatures.

Figure 7.
Water uptake of individual layers after wet tests conducted at different ambient temperatures.

These data showed that all the turnout systems absorbed significant amounts of moisture through condensation when the ambient air temperature was cool (20–25°C), or when a positive temperature gradient existed between the sweating plate surface temperature (35°C) and the air in the test chamber. Most of the absorbed moisture resided in the moisture barrier and thermal liner layers of the turnout composites. Higher ambient air temperatures, or temperatures greater than 30°C, significantly reduced moisture absorption in all of the semi-permeable bi-component and vapor permeable microporous membrane systems. The weight gain at 20°C and 25°C observed in Figure 7 was likely due to moisture condensation in the composite. Condensation occurs when the water vapor pressure reaches saturation point in the fabric due to lower local temperatures.²² The amount of moisture uptake measured decreased with temperature, as was also observed by Rossi et al.²³ Interestingly, at 45°C with the same vapor pressure in the environment, all composites showed a weight loss. This is because water absorption is an exothermic reaction, and water uptake decreases as temperature increases.²⁴ The results suggest that Ret is advantageous over THL in predicting heat loss in hot conditions. THL is measured at 25°C, 65% RH, and would cause a large amount of condensation during testing, making the bi-component moisture barriers more breathable. Compared with the THL test, Ret is measured at higher temperature (35°C, 40% RH), and could reduce moisture absorption inside the moisture barrier and produce an evaporative resistance value for bi-component moisture barrier systems (A–C) closer to that in a hotter environment. For example, if we want to predict the wet heat loss of sample A at 45°C, 21.5% RH using THL (25°C), the predicted wet heat loss would be 165 W/m², indicating a large difference of 132 W/m² from the measured value (33 W/m²) at 45°C. In contrast, if using Ret, the difference between the predicted value (62 W/m²) and measured evaporative heat loss (33 W/m²) is only 29 W/m², as shown in Figure 6. This reveals the advantage of using Ret in predicting heat loss at high temperatures.

We can attribute observed differences between the bi-component systems (A–C) and microporous system (MM) to the effects that absorbed water had on swelling the hydrophilic coating layer present in bi-component moisture barriers. Moisture induced swelling and produced more free volume between polymer chains, thereby facilitating the transmission of water vapor. This led to lower evaporative resistance, as indicated by Figure 5. In addition, absorbed water acted as a plasticizer, potentially lowering the activation energy of diffusion through the membrane.¹³ In comparison, the evaporative resistance of the firefighter composite containing the microporous moisture barrier (MM) did not depend on moisture absorption and was relatively constant. Apart from the influence of moisture content, the temperature could increase the water diffusion rate through bi-component membranes. However, compared with the effect of moisture content on the evaporative resistance of bi-component moisture barriers, the temperature effect was much less important, especially for the small temperature range we studied (20–45°C). A similar conclusion was reported by Gibson.¹⁴

It is arguable that higher evaporative heat loss at 20°C or 25°C (Figure 6) might be due to the increase in wet heat conduction caused by the moisture condensation in the system, instead of the change in evaporative resistance observed in Figure 5. For example, because of condensation at 20°C the moisture content in the composite A was 13%. The moisture condensation increased the thermal conductivity of the system, causing a corresponding decrease in the insulation value. The real evaporative heat loss of composite A at 20°C might have been 33 W/m² instead of 194 W/m² as shown in Figure 6. The difference between 194 and 33 W/m², which is 161 W/m², could be due to extra thermal conduction caused by water condensation in the environment of 20°C, instead of the dependence of evaporative resistance on moisture gain for bi-component moisture barriers. Therefore, dry heat loss, defined as the heat loss pathways by conduction, radiation and convection, should be the sum of 161 W/m² heat transfer by wet conduction and 84 W/m² by dry insulation. As a result, the total insulation value, including air layer resistance (0.04°C·m²/W), was 0.061°C·m²/W. This means that the intrinsic insulation of the fabric with 13% moisture content was reduced to 0.021°C·m²/W. This represented a significant 85% insulation loss compared to the insulation of the dry fabric (0.135°C·m²/W), which was not realistic. Akcagun et al.²⁵ studied the insulation properties of woven fabric in wet state with an Alambeta instrument, and it was found that a 13% moisture gain in a wool/polyester woven fabric only resulted in about 8% decrease in thermal resistance. Romeli et al.²⁶ reported a 9% decrease in thermal resistance for a cotton woven fabric containing 12% moisture, measured with a ThermoTex. Thus, it is safe to conclude that the larger evaporative resistance or less wet heat loss at higher temperatures for firefighter composites containing bi-component moisture barriers (A–C) was mainly due to the dependence of evaporative resistance on the amount of water uptake.

Influence of relative humidity on heat loss and evaporative resistance

Figure 8(a) shows heat loss at relative humidity levels ranging from 25% to 85%, measured in sweating plate tests conducted in isothermal conditions (35°C). Because of the reduced vapor pressure gradient, an expected reduction in heat loss occurred at higher ambient humidity levels. Figure 9(a) shows that the evaporative resistance (Ret) of the systems decreased with ambient humidity. Although differences persisted across a range of humidity levels, moisture barrier-related differences in evaporative resistance decreased at high humidity levels (>80% RH). Figure 8(b) shows that moisture absorption in the turnout systems increased with ambient humidity. Smaller evaporative resistance, combined with a lower moisture vapor pressure gradient, explained the reduction in the differences in THL through the turnout composites observed at higher humidity levels (Figure 9(b)). Differences in heat loss among turnout systems were minimal at the highest humidity testing condition (85% RH).

Figure 8.
(a) Heat loss measured by the hotplate and (b) the water uptake of individual layers after wet tests conducted with varied relative humidity (25%, 40%, 55%, 70%, 85% RH) at a constant temperature of 35°C.

Figure 9.
(a) Evaporative resistance and (b) calculated wet heat loss under different levels of relative humidity (25%, 40%, 55%, 70% and 85%) at a constant ambient temperature of 35°C (the CV of replications for evaporative resistance test ranged from 0.31% to 2.71%).

Relationships between moisture absorption and evaporative resistance in turnout systems

The experiments conducted to study the effects of ambient temperature and humidity on the heat loss and evaporative resistance of turnout composites have clearly demonstrated the effects of moisture pickup by turnout samples during sweating plate tests on their measured heat loss. Figure 10 shows the strongly significant inverse correlations observed between moisture absorbed by the moisture barrier in turnout systems and their evaporative resistance.

Figure 10.
Relationships between evaporative resistance and water uptake in the moisture barriers.

Figure 10 shows that the evaporative resistance of the microporous barrier system (MM) was unchanged by absorbed moisture. In comparison, evaporative resistance dropped off exponentially with absorbed moisture for semi-permeable barrier systems (A–C), reaching a minimum level when the system was saturated with moisture. These data indicated that relatively small amounts of absorbed moisture were required to produce saturation and to minimize the evaporative resistance of semi-permeable barrier systems. Commercial systems B and C achieved the minimum evaporative resistance level with 3–5% moisture uptake. System A saturated at about 10% water uptake. It is necessary to study the chemical composition and morphology of the moisture barriers in order to explain the observed differences among semi-permeable bi-component moisture barriers in Figure 10, and this is to be conducted in our future research. The exponential regression models developed for relating the evaporative resistance of semi-permeable turnout systems to absorbed moisture are a useful outcome of this analysis (Figure 10). They could be used to predict the evaporative cooling and THL of firefighter turnout systems based on the moisture barrier type and moisture uptake.

The predicted heat loss at different chamber temperatures (20°C, 25°C, 35°C and 45°C) under the same vapor pressure gradient was calculated according to Equation (1), as shown in Figure 11. The green bar shows the predicted THL in different environments calculated with the apparent evaporative resistance value measured at 25°C, 65% RH, as in the standard THL measurement. The blue bar displays the predicted THL calculated with the actual evaporative resistance values shown in Figure 5, which was measured at the chamber condition to be predicted. It illustrates how measurements of evaporative resistance in a mild environment (25°C) led to the significant overestimates of potential heat loss through turnout systems containing bi-component moisture barriers at elevated testing temperatures (35°C and 45°C). This means that relative comparisons of the heat strain performance of different turnout materials could have dramatically different outcomes, depending on which of the indexes is used for the ranking.

Figure 11.
Calculated total heat loss based on ^ARef measured at 25°C and ^ARef measured at prevailing conditions. (Color online only.) THL: total heat loss.

Limitations

In this paper, we studied the effect of ambient temperature and relative humidity on the evaporative resistance of firefighter turnout composites on fabric level. Three bi-component e-PTFE (A–C) and one microporous e-PTFE (MM) moisture barriers were selected, with only two samples (B, C) being NFPA 1971 certified. It is important to include more commercially available moisture barriers in future research. Firefighter composites are multi-layer system. Air gaps exist between the human body and clothing, as well between fabric layers.^27,28 The presence of air gaps influences negatively the wear comfort in terms of heat and water vapor transfer from the human body to the environment.¹⁸ We will investigate how air gap sizes along with environmental conditions could affect heat transfer through firefighter turnout composites. Apart from the fabric-level test, garment-level characterization is also necessary. The heat exchange between the human body and the environment through clothing is complicated, as it can be affected by moisture content, garment design, human posture, movement, wind, clothing fit, evaporation efficiency, etc.^29–36 In addition, for THL and Ret tests, the hotplate surface was kept as water saturated, which cannot reflect human thermoregulation actions. When in mild conditions or doing light activities, human skin may be dry due to less heat stress. In order to fully understand the influence of different turnout clothing on the human body in various environments, physiological manikin and human wear trials are required, which are to be conducted in the future. Exploring the relationships between the evaporative resistance of turnout composites with bi-component moisture barriers, A–C (observed in Table 2 and Figures 5, 9(a) and 10), and their chemical composition and membrane structure was beyond the scope of this study. Research to better understand these relationships would be an excellent topic for future study. Characterization including Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC) can be used.

Conclusions

Which is the better index for rating the heat strain of turnout materials, THL or Ret? We have shown that the answer to this question depends on the assumptions made about the temperature and humidity of the intended operating environment. Each heat loss index strictly correlates to a single set of conditions: THL predicts apparent evaporative heat and insulation in mild working conditions (25°C, 65% RH). Ret predicts best for hotter conditions (35°C, 40% RH). Each index provides complementary information about the potential heat strain contribution of turnout materials. Our study has demonstrated the significant effects that ambient temperature has on the evaporative resistance of firefighter turnout materials that contain semi-permeable moisture barriers.

These findings reveal limitations in using THL, the current testing method used to characterize the capabilities for firefighter clothing to release heat specified by the NFPA 1971 standard, as the only index for evaluating material-related heat strain potential. Because structural firefighters work in a range of environmental conditions, including mild and hot environments, the need to measure turnout evaporative resistance at 35°C (Ret) in addition to THL at 25°C is supported. The environmental condition used in the THL test (25°C and 65% RH) can result in condensation and makes firefighter composites with bi-component moisture barriers appear to be more breathable than they actually are without condensation. In some real-world firefighting scenarios, working conditions can be very hot. This study has definitively shown that hot environments produce less water inside moisture barriers, causing turnout systems with bi-component moisture barriers to have much higher evaporative resistance.

Moisture barrier	Membrane type and composition	Weight (g/m²)	Thickness (mm)
A	Bi-component e-PTFE membrane, bonded to woven aramid fabric	216	0.42
B	Bi-component e-PTFE membrane, bonded to spun laced-aramid base fabric	167	0.76
C	Bi-component e-PTFE membrane, bonded to woven aramid fabric	170	0.40
MM	Microporous e-PTFE membrane, bonded to woven aramid fabric	144	0.38

Composite	Weight (g/m²)	Thickness (mm)	Rcf (°C·m²/W)	^ARet (Pa·m²/W)	THL (W/m²)	Ret (Pa·m²/W)
A	747	3.11	0.1346 (2.65% CV)	23.1 (0.35% CV)	221	56.7 (1.44% CV)
B	711	3.58	0.1583 (1.33% CV)	22.7 (1.57% CV)	218	40.4 (1.72% CV)
C	695	3.15	0.1441 (2.68% CV)	21.3 (1.23% CV)	233	31 (2.43% CV)
MM	666	3.07	0.1378 (1.66% CV)	18.8 (2.18% CV)	261	18.7 (2.22% CV)

Footnotes

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 Federal Emergency Management Agency, through the Assistance to Firefighters Grants (AFG) program.

ORCID iDs

Huipu Gao

Emiel A DenHartog

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