Modeling the efficiency and heat gain of a phase change material cooling vest: The effect of ambient temperature and outer isolation

Abstract

Cooling garments containing phase change materials are one of the practical techniques used for improving thermal comfort and worker’s performance in hot environments and under protective clothing. Such garments absorb the body's excessive heat and help the body reach thermal comfort situation by reducing the heat content of the body. The amount of heat absorbed by the phase change materials and the efficiency of the cooling garment can be affected by several parameters. This study uses a software modeling to investigate the effect of different factors such as ambient temperature, the use of protective clothing over the cooling garment, and the type of phase change material bags coating on the efficiency of the cooling garment. The results showed that temperature of the environment in which the cooling garment is used could largely affect the cooling efficiency of such garments. Increasing the ambient temperature from 30 to 40℃ can reduce the cooling efficiency of the garment up to 70%. Also, the use of cooling garment under protective clothing was simulated in this paper as a practical way to improve the thermal situation of the body. It was also shown that insulating the outer surfaces of phase change material bags against the surrounding environment could enhance the cooling power of the garment by 20 to 34% depending on the type of the coverings.

Keywords

Working Textiles properties Materials Industrial textiles

Introduction

The human body can suffer from thermal physiological strain resulting in reduced working endurance and performance and increased risk of heat illness when exposed to hot climates combined with physical work, such as firefighting, military drills, special work situations and sports [1]. This scenario is further confounded by the insulative effects of clothing ensembles (such as protective clothing) that are often required for a particular job [2,3]. Personal cooling garments (PCG) have been introduced as an effective and economical means of reducing heat stress and extending working times in thermally stressful environments. The use of cooling vests that reduce the temperature of the torso is recognized as the most practical for persons wearing protective clothing [4].

The working principle in personal cooling garments is based mainly on circulating cold air or liquids [5], phase change materials (PCM) like polymer gels, paraffin waxes or ice/water mixtures [6] or liquid evaporation [7]. However, most of the devices are bulky, weighty, depend on electric power and limit the wearer’s freedom of action [8]. So, in many situations, the only cooling technique that could be used in practice is a ‘phase change’ cooling vest. A PCM cooling vest consists of a torso garment containing pockets surrounding the chest cavity that holds the PCM packs. Body heat carried to the surface of the skin by the circulatory system is absorbed by the PCM packs. A garment loaded with PCM packs is completely unattached to any external devices, making it much more portable than liquid- or air-cooled garments [9]. During the phase change process of PCM, the temperature does not change; thus PCM cooling vest can stabilize body temperature.

The efficiency of the cooling garment is defined according to the ability of the garment to absorb the body's excessive heat. Numerous studies have been done to investigate the affecting parameters on the efficiency of the cooling garments [10 –13]. The results indicate that many factors such as the temperature gradient between skin temperature and the PCM melting/crystallizing temperature, PCM latent heat capacities, the mass amount of PCM and covering area, fabrics worn between the PCM layer and the skin and subcutaneous fat are the factors that affect the cooling efficiency of a PCM cooling garment [1,10,14]. However, it should be noted that the heat required for the phase change process in the PCM incorporated in the garment is absorbed from both the body of the person wearing the garment and the surrounding environment. Therefore, it can be inferred that the heat absorbed from the surrounding environment can have a significant impact on the cooling effect of the garment worn on the body. Finding the extent to which heat is absorbed from each of these two zones can seriously influence the cooling power of the garment. Naturally, if we create conditions in which most heat is absorbed from the body, the efficiency of the cooling garment in cooling the wearer can be improved. For example, in a study carried out by Smolander et al. [15], the amount of heat required for increasing the temperature and melting one kilogram of ice present in the cooling vest was calculated theoretically and it was found to be 514 kJ. Part of this amount of heat absorbed by the body, as measured by the thermal manikin, was reported to be 301 kJ. This was equal to 58% of the whole heat needed for converting the ice at –20℃ to water at 34℃ [15].

The amount of heat absorbed by body is dependent on the type, construction and arrangement of the layers on both sides of the cooling vest. The figure may be increased by reducing insulation layers between skin and ice-vest, and/or adding an insulative or reflective layer outside the vest [15]. So it is important to isolate the cooling garment from high ambient temperatures to increase its effectiveness; this is because some of the environmental heat is absorbed by the cooling system [16]. On the other hand, if the environment gets hotter, the temperature gradient between the garment and the surrounding environment is increased. This leads to the enhancement of the heat absorption capacity of the garment from the surrounding environment. Therefore, it can be concluded that in hot environments, the cooling capacity of the garment and its efficiency are decreased.

As reported by Speckman et al., by the effect of increasing the temperature of the environment from 29℃ to 52℃ is to decrease the torso heat loss by –50% [16]. So the cooling efficiency is also dependent upon the temperature of the hot environment in which the cooling garment is used.

Also, in some working places, where people have to wear protective clothing, the cooling garment is worn under the protective clothing in order to provide body’s thermal comfort. The protective clothing, due to its insulating nature, can decrease the heat absorbed by the cooling garment from the surrounding environment, thereby improving the efficiency of the cooling garment containing PCMs in terms of cooling duration.

Xu et al. [17] used a series of manikin tests and showed that using an overgarment on the liquid cooling garment could increase its efficiency from approximately 45% to 70% in a dry environment. It should be, however, noted that the protective clothing have some effects on the body’s heat exchange with the environment. This, in some other respects, can undermine the efficiency of the cooling garment and therefore, the thermal comfort of the person wearing it. Preventing evaporative heat loss of the body and handling the heavy weights are some of the problems with wearing protective clothing. As this garment does not allow the body’s heat to be taken away, it leads to the increase in body’s heat, thereby resulting in temperature rise in the body of the person wearing it. Therefore, the efficiency of the cooling garment, when worn under the protective clothing, is affected by some positive and negative factors that were investigated in this study to determine the ultimate efficiency of the cooling garment in cooling the body.

In recent years, modeling has been employed as a new method with the property of high repeatability and speed in analyzing and evaluating the cooling power of the cooling garments [10 –20]. Modeling the use of the cooling garment on the body is a method by which we can study the heat exchange between the cooling garment and the body as well as the surrounding environment. The authors of the present paper have already conducted relevant and appropriate evaluation of the effects of the cooling garment on the body and heat exchange of a system having the three parts of body, cooling garment, and the environment using software simulation [21,22]. So, it has been attempted in this study to investigate the efficiency of the cooling garment as a function of its ability to absorb the body's heat. To this aim, the effects of some factors such as the ambient temperature and the coating used for PCM bags on their cooling power have been addressed. Also, the effect of using the cooling garment under the protective clothing, as a method widely used in the industry, on the cooling efficiency of the garment has been modeled and studied. In this way, the efficiency or inefficiency of the cooling garment in providing comfort for the individual in different conditions with or without the use of the protective garment was investigated.

Modeling

To simulate the effects of using the cooling vests on human’s body in this study, we used the ANSYS Workbench 14.5 software based on finite element method (FEM). For modeling phase change phenomenon, the enthalpy method was used in this model. To this aim, using the available resources, we defined the enthalpy relation dependent on the temperature for the PCM according to the enthalpy curve of the material as shown in Figure 1 [23]. Therefore, when the material reaches its phase change temperature, it begins to absorb body’s heat without increasing the temperature.

Figure 1.

Enthalpy-temperature curve of phase change material.

As can be seen, after the material reaches the phase change temperature, about 240 kJ/kg is absorbed by the material from the beginning of the phase change process to its end (320 − 80 kJ/kg), in order to complete changing the phase from solid to liquid. All this is used for changing the phase of material without any effect on its temperature. Therefore, according to the law of energy conservation, the heat absorbed by the material did not increase the temperature, but increased the enthalpy of the material.

This enthalpy diagram was used for defining the phase change phenomenon in the model, with some modification. The modification was related to changing the phase change temperature to the range of 18–20℃. This change could be due to the impurities present in paraffin obtained from the market, leading to the increase in phase change temperature.

Model hypotheses and boundary conditions

An attempt has been made in this model to investigate the effect of a cooling vest containing 1.5 kg of PCM (Hexadecane) on the torso of a person of average height. Properties of PCM defined in the model can be seen in Table 1.

Table 1.

Defined properties of phase change materials (PCM) in model.

Type of PCM	Density (kg/m³)	Latent heat of fusion (J/g)	Melting point (℃)
Hexadecane	770	240	18–20

The heat generated in the person’s body is selected as the metabolic activity of a person walking at the speed of 2 km/h, equal to 110 W/m² [24]. As there is no gap between the body and the clothing, it has been assumed that heat transfer from the body to the vest is only through thermal conductivity. Also, it has been assumed that a cotton T-shirt is the first layer covering the surface of the skin with the cooling vest coming immediately after it. The initial temperature of the skin has been assumed to be 33℃. The initial temperature of the PCM, upon contact with the body, has been assumed to be 10℃. Therefore, given that the melting point of materials was 18 to 20℃, the PCMs were solid at the beginning of the process.

The rate of evaporative (E), radiative (R), and convective (C) heat loss of the body were calculated based on the ASHRAE standard and ISO 11079 [25,26]

E = \frac{ω (P_{sk, s} - P_{a})}{R_{e} + \frac{1}{f_{cl} h_{e}}}

(1)

R = h_{r} f_{cl} (t_{cl} - t_{r})

(2)

C = h_{c} f_{cl} (t_{cl} - t_{a})

(3)

where ω is the wettedness of the skin, p_sk,s and p_a are the saturated water vapor pressure at the skin and the ambient air pressure, R_e is the evaporative resistance of the clothing, f_cl is the cover factor of the clothing, t_cl is the temperature of clothing, and t_r and t_a are the radiative temperature and the ambient temperature, respectively.

Model geometry

Geometrically, the system consisted of four major parts: torso, cotton T-shirt, polyethylene packets containing PCM, and PCM. The torso considered for the model included back, breasts, and belly, collectively covering a surface of 0.65 m² based on ISO 8996 [24]. The vest covering the surface of the torso contained 14 PCM packets with the size of 12 × 10 × 1.16 cm. Six bags were in front of the torso and eight at the back (Figure 2). The surface covered by PCM packets was 0.168 m², which was 26% of the torso. The PCM was modeled inside of polyethylene pockets with a thickness of 0.2 mm.

Figure 2.

(a) Front view and (b) back view of the model geometry.

Results and discussion

To examine the effect of receiving heat from the surrounding environment on the efficiency of the cooling garment, modeling the use of the cooling garment was done in the following states and the effect of each of these was investigated.

The effect of the cooling garment at different ambient temperatures (30, 35, and 40℃)

The effect of wearing a protective clothing on the cooling garment

The effect of using an insulative covering layer on the exterior of PCM bags

The effect of the ambient temperature

To investigate the effect of the ambient temperature on the efficiency of the cooling garment, simulation was done for the three temperatures of 30, 35, and 40℃. The results obtained by these three models in this part of the study were studied in terms of the analysis of heat flux, time analysis of PCM melting process, the temperature of PCM, and the resulted mean skin temperature.

Heat flux to the PCM bags

Figures 3 and 4 show the heat flux curves absorbed by PCM from the outside environment and the body, respectively, for three different environmental conditions.

Figure 3.

Heat flux to PCM bags from environment in different environmental temperatures.

Figure 4.

Heat flux to PCM bags from the body in different environmental temperatures.

As can be seen in the curves, the heat absorption of PCMs was high at the beginning of putting them on the body, but gradually, with the increase in the temperature of PCM and the decrease in temperature gradient, heat absorption from the body and the surrounding environment was decreased.

This was due to the reduction of temperature difference between packets and the body as well the gradual change of phase from solid to liquid.

It was also observed that ambient temperature variations had a negligible effect on the heat absorbed from the body, but the heat absorbed from the environment did show a considerable increase with the increase in the ambient temperature. The proportion of heat absorbed from the environment, within the first hour of putting the bags on the body, to the total heat absorbed (from body and the environment), was calculated according to Table 2.

Table 2.

The amount of absorbed heat by phase change material (PCM) bags in environment temperature of 30, 35 and 40℃.

	30℃		35℃		40℃
	From body	From out	From body	From out	From body	From out
Absorbed heat (W)	46.1	44.6	48.6	54.3	49.1	64.5
Percent to total	51%	49%	47%	53%	43%	57%

It was found that at the temperature of 30℃, environment contribution in the absorption of the heat by PCMs was less than the heat absorbed from the body, but with the increase of the ambient temperature, this portion was increased, reaching to more than a half at the temperature of 40℃.

Temperature of the PCM

Immediately after putting the PCMs on the body, the temperature of these materials was increased in surfaces near the skin surface, and reached the phase change temperature sooner than other parts of the bags. After each point in the bags reached the phase change temperature, the temperature rise was stopped until they absorbed the amount of latent heat needed and they could be converted to liquid. After the material passed the phase change temperature, the temperature of the liquid was again increased quickly and this process was continued until its temperature became the same as that of body. Figure 5 shows mean temperature variations of PCMs in three states of the ambient temperature.

Figure 5.

Variation of average temperature in PCM bags in different environmental temperatures.

As can be seen in the graph showing the average temperature of PCM bags, the tendency for the overall temperature increase of the bags was stopped for a while after reaching the melting temperature range (18 to 20℃) and during this time, absorbing heat led to melting the main parts of material in the bags. The melting operation in the remaining parts of PCMs caused the reduction in the rate of temperature rise of mean temperature in the next steps.

Melting phenomenon in different parts of PCM bags does not happen at the same time. Different parts of the bags reach the phase change temperature at different times. The melting progress in PCM bags can be represented as in Figure 6. This graph shows the temperature distribution of PCM bags after 1 hour of their touch with the body in an environment with temperature of 35℃. As can be seen, the first part that reached the phase change temperature, with the highest temperature, referred to the inner layers of the bags as they were in contact with the body and due to the high temperature difference between these surfaces. Following them, the outer layers of the bags, which had convective and radiant heat gain from the surrounding environment, were subject to melting. Finally, the middle layers of the bags, which had the lowest temperature relative to other points, experienced melting as the last stage.

Figure 6.

Temperature distribution in PCM bags after 1 hour.

When the last point in the PCM bags exited the melting zone, all materials turned to the liquid state. The needed time for complete melting of the PCMs at different ambient temperatures can be seen in Figure 7. Naturally, with the increase in the ambient temperature, the heat transfer from the environment to the bags was increased and, therefore, the bags were melted completely within a shorter period of time.

Figure 7.

Needed time for complete melting of PCM in different environmental temperatures.

The time required for complete melting of the PCM cannot be necessarily taken as the cooling period of the cooling garment. In fact, the optimal cooling period is less than this as when the middle layers of PCM bags exit the phase change zone, the temperature of PCM inner layers which are closest to the skin is the same as that of body. Therefore, due to the low thermal conductivity of PCMs, at this time there was no effective thermal transfer between the body and PCM bags that would result in cooling the body. Subsequently, it is suggested in this study to determine the cooling period of the garment based on another criterion such as the ability of cooling garment in controlling the skin temperature, as following.

Skin temperature

In many studies, skin temperature is taken as a sign showing the cooling capacity of the cooling garment [27 –29]. In fact, the cooling garment can be effective in cooling the body by absorbing the excessive heat of the body and preventing the temperature rise in the body and skin surface. The mean skin temperature obtained by the model, when it is in contact with the cooling garment, and under the condition without the cooling garment (as the control condition) at the ambient temperature of 35℃, can be seen in Figure 8.

Figure 8.

Mean skin temperature with and without the cool-vest.

As can be seen in the Figure, it can be inferred that if the body is in a hot environment, mean skin temperature is gradually increased. The skin temperature in control condition can reach 38.2℃ without the use of cooling vest which is an uncomfortable state for the body. The increase in skin temperature will be higher if the worker is doing a higher level of activity. This is due to a much more level of heat produced through the metabolism.

In the second curve it is observed that the use of PCM packets on the skin could greatly reduce mean skin temperature by absorbing the body's excessive heat. Mean skin temperature fell very fast in the first minutes of the contact with the PCM packets due to the low temperature of the PCM (10℃) and then gradually increased. This resulted in a reduction of 2.8℃ in mean skin temperature after 1 hour (37.2–34.4℃). This reduction in the mean skin temperature could result in increasing endurance threshold and individual’s performance in hot working conditions.

Also, it should be noted that skin temperature varies for the person wearing the cooling garment depending on the temperature of the environment. Figure 9 shows these variations at three temperatures of 30, 35, and 40℃ of environment.

Figure 9.

Mean skin temperature with the use of cooling vest in different environmental temperatures.

As can be seen, the increase of ambient temperature undermined the efficiency of the cooling garment and so the mean skin temperature in hotter environments more quickly reached 36℃, thereby causing discomfort for the body according to physiological sources [30,31].

If we take the reaching to mean skin temperature of 36℃ as a criterion showing the cooling period of the garment, the period of optimal efficiency of the garment for different ambient temperatures can be obtained as shown in Table 3. Therefore, it can be said that increasing the ambient temperature by five degrees can reduce the cooling period of the garment or its efficiency up to 50%.

Table 3.

The cooling period of cooling garment.

Environmental temperature (℃)	30	35	40
The time of cooling period (min)	210	108	62

The effect of using the cooling garment under a protective clothing

Very often, protective clothing has high thermal resistance and evaporative resistance. So when an individual wears it, there is a considerable reduction in his body's thermal exchange with the environment. Also, in some protective garments, as those used by firefighters, the outer surface of the garment is covered in such a way that it would have high reflectivity and low emissivity. Therefore, the radiant heat gain from the environment is minimized if the protective garment is worn.

Based on the above, in this section of modeling, the use of the cooling garment under the protective garment was modeled so that improvement of body's thermal conditions when using these two garments could be studied.

The protective clothing modeled in this part of the study was that used by firefighters with the thermal resistance of 2.78 Clo (0.431 m²℃/W). Its torso was simulated on the model, with the thickness of 4 mm. It was at a distance from the body such that an air layer with an average thickness of 6.35 mm was between the inner surface of the protective garment and the outer surface of the cooling garment. Only on the shoulders there was contact between the layers of the garments due to their weight. The coefficient of radiation heat transfer on the surface of the protective clothing was equal to 0.11, which was almost one-fourth of the emissivity of the usual garments.

The results of modeling at the ambient temperature of 35, as can be seen in Figure 10, showed that the use of the cooling garment under the protective garment led to some reduction in the heat gain of the PCM bags from the surrounding environment, compared to the time when the cooling garment was used without the protective garment. The reduction in the heat absorption of PCM was due to the high thermal resistance of the protective garment which showed resistance against heat exchange between the bags and the surrounding environment. It should also be noted that there was no significant difference in the intensity of absorbing heat from the body in either cases.

Figure 10.

Outer heat flux of the PCM bags with different coverings.

Therefore, as the environment did not play a significant role in heat gain of PCM bags in the situation of wearing protective clothing, the average temperature of the bags was increased with a lower speed as shown in Figure 11. Accordingly, the complete melting of PCMs occurred after 4 hours when, in comparison to the time the protective clothing was not used, it took 90 more minutes.

Figure 11.

Average temperature of PCM bags with the use of different coverings.

Figure 12 also shows variations in mean skin temperature with and without using the protective clothing on the cooling garment. As can be seen, the use of the protective clothing, due to reducing the body's evaporative heat loss, increased the mean skin temperature more quickly, reaching to 36℃, only after 58 minutes and causing discomfort for the wearer. It can be, therefore, concluded that the use of the protective clothing on the cooling garment, in spite of reducing heat absorption of the bags from the surrounding environment, reduced the efficiency of the cooling garment in providing thermal comfort due to the increase in mean skin temperature.

Figure 12.

Mean skin temperature with the cool-vest and different coverings.

As the thermal exchange of the bags with the surrounding environment was minimized in case the protective clothing was worn over the cooling garment and skin temperature was quickly increased under this condition, it could be concluded that in order to reach the highest efficiency when both garments were used, it is necessary to maximize the covering surface of the PCM bags on the skin. For example, by keeping the mass of PCM on the skin fixed and reducing the thickness of the bags to a half, it is possible to double their coverage surface on the body, thereby preventing skin temperature increase more effectively. As the heat absorption of PCM bags from the surrounding environment is minimized if the protective clothing is used, doubling the coverage surface does not reduce the efficiency of the cooling garment and it is even increased with the better control of the skin temperature.

The effect of using the insulating layer on the outer surface of PCM bags

In order to reduce the heat absorption of PCM bags from the surrounding environment, it is possible to coat the outer surface of the bags with an insulating layer. In this part of the study, the effect of coating the outer surface of the PCM bags by a nonwoven polyester layer with the thickness of 2 mm on the efficiency of the cooling garment was investigated. As expected, the intensity of heat flux absorbed by PCM bags was lower in the case of using an insulative coating at exterior part of PCM bags in comparison to the state that no covering was used (Figure 11). So, the time needed for the complete melting of PCMs, in comparison to the case without the coating, was increased up to 41%. More durability of PCMs at the lower temperature could decrease skin temperature further, thereby improving the efficiency of the cooling garment (as shown in Figure 12)

If a reflective layer is used on the outer surface of the bags, due to the considerable reduction in radiant heat absorption of the bags from the surrounding environment, the speed of temperature increase will be further slowed, thereby increasing the time required for complete melting of the materials up to 71%. The average skin temperature is also decreased, in comparison to the case in which a usual cloth is used for covering the bags, thereby improving the efficiency of the bags considerably. If we take the temperature of 36℃ as one for the end of the cooling period, covering the bags with a usual cloth and a reflective coating results in the efficiency increase of 20 and 34%, respectively, compared to the case without any covering.

Model verification with the experimental tests

Results of a series of subjective tests were used to verify the accuracy of the model. The experiments were conducted in a climatic chamber conditioned at a temperature of 30℃ and the relative humidity of 37%. Four college-age subjects participated in the wear trial tests.

The subjects wore working clothes consisting of their own undershorts, a cotton T-shirt, trousers, and socks. This clothing ensemble was estimated to be 0.88 clo by measurement with thermal manikin. PCM cooling vest with properties similar to those of the model was designed and worn on the working clothes. The control condition tests were also employed without the use of cooling vests.

Every subject was asked to go up and down 25 steps, each for 10 minutes, during the 60-minute experimental test. This activity was estimated to be 1.4 met (81.2 W/m²), the same as the metabolic heat assumption of the model. Skin temperature at chest and back was measured and stored each 4 minutes.

Figure 13 shows the variation of skin temperature with and without the use of cooling vest. The results of the model were also added to estimate the degree of correspondence between the two curves.

Figure 13.

Skin temperature with and without cooling vest (experimental and model results).

The independent T-test was used to analyze the equality of mean values between experimental and modeling results. Data were analyzed using SPSS 20.0 for Windows and Statistical significance was set at p < 0.05. Table 4 shows the statistical data used for analyzing the mean skin temperatures.

Table 4.

Statistical data.

Test condition	Group of data name	N	Mean	Std. deviation	Sig. (2-tailed) In t-test for equality of means
With cooling vest	Model results	8	30.5400	1.16997	0.121
	Experimental results	16	31.1188	.60467	0.121
Control condition	Model results	8	33.3087	.25453	0.426
	Experimental results	16	33.4188	.37277	0.426

As can be seen, the values obtained for skin temperature for the control condition in the experimental tests and modeling showed a good agreement. As the significance level in t-test for the equality of means was 0.462, the experimental and model results in control condition did not show a significant difference. This good agreement revealed that the heat exchanges between the body and the environment have been correctly simulated in the model.

In the case of using the cooling vest, the curves related to body’s temperature in both modeling and experimental conditions showed the reduction of skin temperature in comparison to the control condition. But the results of modeling in the first minutes of wearing the vest were somewhat overestimated, so the reduction of skin temperature during this time was more than its real value. This can be attributed to the inability of the model to simulate skin cutaneous vasculature. But, this effect was decreased in the model by the decrease in temperature difference between skin and the cooling vest. Therefore, after 1 hour, the results of modeling and human tests were converged, showing a good agreement. The significance level in t-test for the equality of means was 0.121, so the experimental and model results, with the use of cooling vest, did not show any significant difference with each other.

Conclusion

The effect of using a phase change cooling garment on the body's thermal situation was simulated in this paper with the help of FEM and ANSYS software. The effect of ambient temperature on the efficiency of the cooling garment was evaluated with the criterion of mean skin temperature. It was shown that the efficiency of the cool-vest in controlling the skin temperature decreased up to 70% with increasing the environment temperature from 30 to 40℃. But also, it was shown that the heat gain of PCM bags can be reduced by the use of insulating layers on the outer surface of PCM bags. This can increase the efficiency of cool-vest up to 20 and 34% in case of using an ordinary insulating layer or a reflective coating, respectively. In addition, the use of cooling garment under protective clothing was simulated in this paper. Wearing the protective clothing on the cooling garment can increase the time for complete melting of the PCM but reduces the efficiency of the cooling garment in controlling the skin temperature at the same time. So, it was suggested to use cooling garments with higher covering area underneath the protective clothing.

Footnotes

Declaration of Conflicting Interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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