Verification of a Comfort-Enhanced Liquid Cooling Vest

Abstract

Firefighter uniforms provide thermal protection and block radiant as well as high-temperature heat. However, they limit cooling and the dissipation of internal heat from the body. This study attempted to improve firefighter uniforms by developing and evaluating a comfortable and thermally balanced design. This study recruited six male college students for a within-subject comparison of vests with and without active liquid cooling. The participants used a questionnaire to report their comfort level in each body part while performing various motions. A biomechanical analysis was performed to objectively evaluate the comfort level of the cooling vest before and after the test. Subsequently, the participants’ blood pressure and ear as well as skin temperature were measured as they ran on a treadmill. The participants also responded to a questionnaire regarding their thermal perception. The results revealed that the cooling vest was comfortable and flexible. The data from the psychological questionnaire indicated that the participants were satisfied with the warmth, coolness, and other various aspects of the cooling vest. Moreover, the cooling vest positively affected the wearer’s microclimate and provided a comfortable thermal balance. The current findings demonstrate the feasibility of using human-factors-engineering-based objective verification methods for designing products.

1. Introduction

Firefighters are often exposed to dangerous environments with high temperatures and levels of radiation. Researchers have indicated that burns and thermal stress caused by the accumulation of heat in the microclimate within clothing, rather than thermal damage, are the main causes of injury [1]. The accumulation of heat in the thick and heavy uniforms with low air permeability worn by firefighters can increase the temperature in the microclimate between the clothing and the body [2]. When the temperature of the microclimate within clothing is 5 °C higher than the ambient temperature, increased perspiration can cause the body to rapidly lose water, create a salt–water imbalance [3], and exacerbate heat stress by increasing body temperature and heartbeat. This process may cause heat illness and can even threaten a firefighter’s life [4]. When core body temperature decreases by 2 °C, the low temperature may cause firefighters to lose consciousness, and can even result in death [5]. Therefore, research on the heat and humidity transfer mechanism of heat-protective clothing is crucial. Mitigating heat stress, preventing burns and physical injury, and increasing firefighters’ work efficiency can maximize the likelihood of successful rescues.

Several studies on the four-layer fabric used in firefighter uniforms have discovered that thermal protection can be enhanced by increasing fabric count and thickness; however, the increase in material would compromise the quality of the uniform and result in a loss of body heat [6]. Phase-change materials, air cooling, and liquid cooling are often used to reduce microclimate temperature [7]. Phase-change materials exhibit a fast thermal response, but the heat released in the solidification process and the heat accumulated in the multilayer firefighter uniform increase the risk of burns [8], which limits their use in firefighter uniforms. In terms of air cooling methods, externally connected pumps for air transmission can easily explode under high temperatures, and the forced convection accelerates water evaporation on the skin, thus endangering firefighters. Liquid cooling methods are widely used because of their safety and effectiveness. Liquid cooling is considered the most effective method of regulating heat within clothing [9]. Burton [10] designed liquid cooling clothing for pilots in the British Royal Air Force. Bennett [11] evaluated the effect of liquid cooling by placing various numbers of cooling strips in vests. The results revealed that the liquid cooling vests slowed the increase in skin temperature, maintained the clothing microclimate, and were comfortable.

Blocking external heat and reducing the temperature in the clothing microclimate are the primary solutions to addressing the thermal stress caused by firefighter uniforms. The commonly used methods of optimizing fabric performance and increasing layers of clothing to block heat disrupt heat dissipation and exacerbate heat stress. When the ambient temperature is higher than the skin temperature, body heat mainly dissipates through evaporation [12,13]. Evaporation is caused by differences in water vapor pressure, temperature, and relative humidity between the air and skin. During long periods of high-stress work, firefighters’ skin temperature increases, which reduces the difference between the clothing microclimate and ambient temperatures, thereby preventing the body from regulating temperature by removing excess heat through the evaporation of perspiration. A reduction in skin temperature could affect core temperature [14].

Liquid cooling clothing can protect those working in high-temperature environments and prevent injury and discomfort caused by heat. This study applied a human-factors-engineering–based methodology to design a cooling vest by quantitatively measuring factors related to the body and the surface conduction, convection, radiation, and heat evaporation of the vest [15] on the basis of physiology and thermodynamics [16]. The cooling properties of the vest were measured physiologically, and its comfort level was subjectively evaluated. The user-centered design places users’ experience at the core of the design process and requires the analysis of a product’s practicability; therefore, the users are central to testing the product. According to the International Organization for Standardization 13,407, the user-centered design process consists of four stages: (1) specify scenarios and contexts, (2) specify user needs, (3) propose solutions, and (4) verify the design (International Organization for Standardization, 1999). Design verification is a crucial stage. Adhering to the principles of human factors engineering by identifying users’ problems during product testing ensures the quality of the final product or service. Experiments based on the principles of heat exchange have transformed user needs into design elements for protype development [17]. Although the prototype may satisfy the initial design requirements, the design must be verified in terms of its ability to meet user expectations. Therefore, this study verified the design of the cooling vest to ensure it allowed for a normal range of motion, reduced physiological burden, and increased efficiency and comfort.

2. Analysis of Cooling Vests and Literature Review

2.1. Structural Analysis of Cooling Vests

Numerous cooling vests for firefighters have been developed with the aim of reducing skin temperature and humidity, thereby increasing comfort. White and Hodous [18] demonstrated that the design of sleeves and pantlegs strongly affects ventilation. Reischl et al. [19] revealed that open necklines effectively improve chest ventilation, that leg and crotch ventilation could be improved by opening the bottom of the pantlegs at the ankle, and that suspenders could replace belts to improve back, leg, and crotch ventilation. Stranskv et al. compared the difference in heat response between firefighter clothing with and without a microporous moisture barrier and revealed that under a high-temperature environment, firefighter uniforms with films increased body temperature, but not significantly. Graveling and Hanson [20] discovered that a change in the combination of fabrics would not affect physiological burden substantially but could affect the accumulation of body heat. Cooling vests such as the AirDown inflatable gilet use polyester and polytetrafluoroethylene as the fiber substrates, weighing only 70 g. The Gore Airvantage system uses carbon dioxide and the evaporation of a phase-change material to remove heat from the body. Another 1.5 kg system provides 1–2.5 h of cooling with ice cubes and a 500 mL/min water pump, but can only function in ambient temperatures below 60 °C. The portable cooling vest proposed in this study can assist firefighters by providing long-lasting effects and increasing working efficiency. Cooling vests should be light and provide enduring effects to protect those working in high-temperature environments from physical injury and discomfort. Such vests can also assist firefighters by increasing the time they can remain in high-temperature environments, thus increasing rescue efficiency.

2.2. Testing and Evaluation of Comfort

2.2.1. Analysis of Motions Required for Firefighting

Several studies have investigated the use of firefighter uniforms for long periods of medium-intensity work and short periods of high-intensity work [21]. They indicated that the uniforms are comfortable, safe, and highly efficient in work and training scenarios. However, some motions could not be easily performed [22]. Chen et al. utilized motion analysis technology to quantitatively evaluate firefighters’ motions during training. The results indicated that analyzing firefighters’ motion and developing standards to evaluate comfort could increase firefighters’ work efficiency, optimize their movements, and increase the effectiveness of training [23]. Motion analysis is used to examine the body’s movement during various tasks, with the aim of reducing unnecessary motion, reducing exhaustion, facilitating the completion of tasks, guaranteeing comfort and safety, and increasing efficiency. Motion optimization programs have been developed to standardize the movements required by certain tasks [24].

Firefighting involves a high degree of danger and difficulty. Heavy clothing can restrict the normal range of motion and increase temperature and humidity in the within-clothing microclimate. Therefore, analyzing firefighters’ motions is crucial.

2.2.2. Physiological Analysis of Comfort

During tasks such as climbing ladders and maneuvering hoses [25], firefighters engage their muscles, and their basal metabolism, peripheral vasodilation, blood circulation, and skin temperature increase because of the temperature and humidity of the clothing microclimate, which imposes considerable thermal stress on the body. In general, firefighters can only work in environments with temperatures of < 65 °C for long periods of time. When the ambient temperature reaches 120 °C, firefighters’ activity is limited to approximately 45 s. Firefighters can only remain in an ambient temperature of 475 °C for 2 s [26]. Firefighter uniforms with low air permeability limit heat dissipation and cause heat to accumulate between the layers of clothing [27]. High-temperature environments and the heat produced by the human metabolism produce extreme thermal environments. Without proper cooling, such environments can disrupt the balance between the production and dissipation of heat from the body [28], which can result in burns that affect the internal organs in addition to general discomfort [29]. This condition can eventually cause disordered body temperature regulation, dizziness, heat stroke, seizure, shortness of breath, increased heartbeat, impaired reflexes and comprehension, and even death [30].

Heat stress is the main factor affecting firefighters’ work efficiency and has therefore been widely studied. Cooling vests contain devices that reduce the temperature of the clothing microclimate within firefighter uniforms, accelerate heat dissipation, alleviate heat stress, increase comfort, and reduce body temperature.

2.2.3. Subjective Evaluation of Cooling Vest Comfort

Studies have indicated that firefighter uniforms reduce tolerance to the temperature of the working environment. Cooling vests can accelerate convection, conduction, and the evaporation of perspiration to maintain a normal body temperature of 35–38 °C. Studies have also indicated that the temperature, relative humidity, and airflow in the within-clothing microclimate must be maintained at 32 ± 1 °C, 50% ± 10%, and 0.25 ± 0.15 m/s, respectively, for firefighters to feel comfortable. Comfort is difficult to define because it involves an individual’s physiological, psychological, and physical experience of an environment [31]. Comfort can be analyzed in terms of the psychological and physical aspects of an environment or item of clothing as well as the psychological and physiological responses of the wearer [32]. An ambient temperature of 28–30 °C is defined as a comfortable somatosensory temperature. Studies have indicated that body temperature increases after 30 min of strength exercise because of changes in skin and ambient temperature. Feelings of discomfort are related to the rate of perspiration on the skin, and thermal discomfort is the result of the thermoregulation mechanism. Consequently, cooling vests for firefighters have a considerable effect on temperature regulation. Because thermal comfort involves subjective factors, a relationship between subjective feeling and physiological responses must be identified to quantitatively evaluate thermal comfort [33].

According to the literature, a subjective discomfort rating could be used to measure the comfort, physical stability, and balance of an article of clothing. Vests without a cooling function can cause wearers to experience discomfort from high temperatures and affect their movement. Cooling vests reduce body temperature and thus allow users to feel comfortable.

3. Methods

Six college students were recruited for the experiment. The experiment was comprised of two stages. The first stage involved motion analysis, in which the participants evaluated the comfort of the vest during various motions by responding to a questionnaire. The second stage was the treadmill experiment, in which physiological factors were measured and the participants reported their perceived thermal sensations before and after exercise by completing a questionnaire. Statistical analysis was performed to verify the design of the vest. The participants were informed of the objective, procedure, method, and physical demands of the study through “subject notice”, and the experiment was approved by the Institutional Review Board and Ethics Committee of National Taiwan Normal University.

3.1. Participants

Sharkey et al. [34] noted that firefighters often walk more than 4.8 km carrying 20 kg of equipment in less than 45 min. Most firefighters are young men because they often have high muscle strength, are adaptable to changes in ambient temperature, and degree of nervousness. The participants in this study were male college students with similar ages, heights, weights, and body mass indices (height: 178.3 ± 7 cm, weight: 65 ± 8.5 kg, and average age: 22 years). Psychological and physiological variables were measured as the participants executed various motions while wearing different articles of clothing. To ensure the completeness and accuracy of the data and the quality of the experiment, the participants were required to sleep at 11:00 p.m. the night before the experiment and to not consume alcohol during the two days prior to maintain their physical health. The participants had no muscle or bone injuries or any physical illnesses.

Table 1. Anthropometric measurements of participants.

	Subject 1	Subject 2	Subject 3	Subject 4	Subject 5	Subject 6	Mean	STD
Height (cm)	176	179	187	174	177	176	177.2167	7.129056
Weight (kg)	64	67	72	61	65	62	65.1667	8.539479
Age	22	22	22	22	22	22	22	0
Chest width (cm)	92	89	81	82	87	88	86.54381	5.453872
Shoulder (cm)	46	45	47	43	42	43	44.38542	2.697412
Waistline (cm)	72	81	82	69	75	79	76.3965413	8.385687
Hip (cm)	95	97	98	89	93	93	94.149235	4.919876

presents data on the participants.

3.2. Cooling Vest Design

Liquid cooling vests are worn between the firefighter uniforms and body. During rescue missions, firefighters perform a wide range of physical activities. The fabric used for uniforms must be flame retardant, flexible, and provide thermal protection to decrease physical burden. Veghte [35] reviewed the history of the materials and standards for firefighter uniforms, as of 1986, to develop materials that offer superior thermal protection and comfort. The current study used single-layer sleeveless vests.

Table 2. Cooling vest parameters.

Material Basic Data/Sample Number#1
Hardness (SHOER A)	58
Tensile strength (MPa)	6.9
Elongation at break (%)	260
Young’s modulus (100% elongation) (MPa)	2.14
Tearing strength (kN/m)	12
Compressive deformation (22 h/177 °C) (%)	38
Resilience (%)	55
Linear shrinkage (%)	2.6
Material Basic Data/Sample Number#2
Wide in width	45 inch
Code weight	180–190 g
Content	67% nylon/33% spandex

presents the results of the physical performance test and basic parameters of the vest’s fabric. Material A was a polyurethane heat exchange tube used to transfer the cooling liquid, and material B was a 67% nylon and 33% spandex chain-lock air-permeable fabric. The clothing was designed using user-centered human factors engineering to provide maximal efficiency, effectiveness, safety, comfort, and user satisfaction [36]. The size and fit of clothing affect its thermal protection properties, and looseness can affect the within-clothing microclimate [37]. Therefore, anthropometric measures were used to develop the heat exchange mechanism of the cooling devices. Chest width, waistline, length, front neck, and front collar measurements were used [38] (Figure 1). The front–back pattern allows for water to circulate through the tube and cool the body. Moldflow was used to analyze the various physical characteristics of the materials in the filling, pressure-holding, cooling, and warping processes and to edit the design. The tube covered the entire body to reduce heat near the heart and reduce core temperature through conduction and convection, thereby alleviating heat stress [39] while maintaining comfort and allowing for easy put-on and take-off. The control group wore vests without a cooling function.

3.3. Experimental Design

3.3.1. Ambient Temperature and Humidity

In accordance with the methods of Niu [40], the experiment involved a simulation of a high-temperature environment in which firefighters work. The temperature was 40 ± 2 °C and the relative humidity was 50 ± 2%.

3.3.2. Task Motions

The motions used by firefighters during rescue operations were identified for decomposition, classification, and evaluation. The motion analysis was performed to identify the main physical movements executed during certain tasks, and the hands and eyes were marked for emphasis. The data were formatted into charts to identify points for improvement. In accordance with the methods of Son and Tochihara [41], the body was divided into upper and lower parts. The upper part comprised the neck, shoulders, arms, elbows, and back, and the lower part comprised the thighs and knees. During testing, the participants emulated the typical motions of firefighters, and these motions were recorded for later observation. The participants performed 13 motions, and their level of comfort in various parts of the body was evaluated through a questionnaire after each motion. A t-test was used to analyze the effect of repeating a motion five times on the performance of the motions.

3.3.3. Walking Time and Speed

The four-layer design of firefighter uniforms prevents perspiration from evaporating through the fabric and thus prevents heat from dissipating. Fast walking can cause physical fatigue, which can affect the execution of motions, whereas walking slower than a normal pace can result in an abnormal gait. Studies have used various walking speeds to suit their respective objectives. In two studies, firefighters performed tasks for 30–60 min, then rested for 10–20 min to prevent physical injury caused by thermal stress [4]. However, cooling through heat dissipation depends on the evaporation of perspiration, and such a short rest time cannot effectively alleviate thermal stress. The participants in this study walked on the treadmill for 30 min during the formal experiment. In accordance with the methods of Smolander [42], the incline was 0° and the speed was 4 km/h. The questionnaire regarding body temperature was administered after the experiment to subjectively measure the participants’ comfort and perceptions of the cooling effect of the vest.

3.3.4. Physiological Measurement

For the physical test, the physiological acceptability and heat tolerance of the participants wearing the cooling vests were evaluated to identify the changes in and distribution of body temperature [43]. Comfort was evaluated using physiological data obtained from the experiment. A t-test was performed to determine whether the means differed among the groups. The difference in physiological values before and after the experiment was determined for the two groups. Standard error was used to determine the accuracy of the data [44]. The physiological measurements were as follows: (1) Ear temperature: An ear thermometer was used to measure temperature. Normal body temperature is 35.9–37.6 °C. (2) Heart rate: An index for the amount of exercise. The heart rate is t heartbeat frequency, that is, the number of heart beats per minute. The normal heart rate is 60–100 times per minute. (3) Blood pressure: Lateral pressure of blood in a unit area of a blood vessel. General blood pressure refers to blood pressure in an arterial blood vessel. Ideal blood pressure is below 120/80 mmHg, normal blood pressure is below 139/89 mmHg, moderately high blood pressure is between 140/90 and 160/95 mmHg, and high blood pressure is above 180/90 mmHg. (4) Mean skin temperature (Tsk): Average temperature of various body parts. According to Burton and Murlin [10], the weighted equation for Tsk is 0.5 × 3I (D) + 0.14 × 4I (L) + 0.36 × 6I (R). Seven points, including the middle of the sternum, the arm, and the middle of the calf, were measured to generate the weighted skin temperature (

Table 3. Skin temperature measurement positions.

No. of Measuring Point	LT-851	Body Part	Weight
1 (A)	CV1	Forehead	0
2 (B)	CV2	Cheek (left)	0
3 (D)	CV3	Chest width (top left, middle of sternoclavicle)	0.5
4 (L)	CV4	Forearm (left)	0.14
5 (M)	CV5	Hand web (left)	0
6 (R)	CV6	Shank (left)	0.36
7	CV7	Index finger (left)	0

).

3.4. Procedure

3.4.1. Motion Analysis

All participants wore firefighter uniforms. The control group did not wear the cooling vests, whereas the experimental group did.

3.4.2. Treadmill Experiment

The treadmill experiment had a total duration of 53 min. Five minutes were allocated to explain the experiment and allow the participants to fill in their basic health data. Another 5 min were used to prepare the measurement equipment. A 3 min rest period was allotted after the participants put on their uniforms and vests. The exercise test lasted 30 min, and the participants completed the questionnaire in the final 10 min of rest.

Table 4. Treadmill experiment.

Experiment Time	5 min	5 min	3 min	30 min	10 min
Experiment explanation	Explanation before experiment	Measurement equipment preparation	Rest before experiment	Treadmill exercise mode (slope = 0°, speed = 4 km/h)	Rest
Physiological value measurement			Ear temperature, blood pressure, and heart beats		Ear temperature, blood pressure, and heart beats
Perceived thermal sensation questionnaire					Weight, humidity, and sultriness

presents the procedure. The physiological measurements in this experiment were blood pressure, ear temperature, heart rate, and skin temperature. Blood pressure, ear temperature, and heart rate were measured before and after the experiment, and skin temperature was measured while the participants walked on the treadmills.

3.5. Statistical Analysis

A Likert-type scale was used for the questionnaire. The scores for items in the same section were added together, and standalone items were not factored in. The scale comprises a series of statements scored from one to seven points. The participants indicated their level of agreement with each statement, and the results were used to determine their subjective perceptions. The data were analyzed using SPSS (version 17.0). Descriptive statistics and a paired-samples t-test were used for analysis to identify the mean scores for each group and determine whether they were the same. The use of two independent variables allowed for the identification of differences in the physiological values between the groups as well as before and after the experiment, differences for which p > 0.05 was nonsignificant [45].

4. Results and Discussion

The effect of the cooling vests on the physiological values indicating subjective comfort level was analyzed. The results revealed no considerable change in the dependent variables before and after the experiment. Therefore, the cooling vests alleviated thermal stress caused by inadequate heat dissipation.

4.1. Analysis of Firefighters’ Motion

The participants used the Likert-type scale to score their comfort while performing the 13 motions.

Table 5. Analysis of motion during tasks (Likert-type scale; mean ± standard deviation; and unit: %).

Movement Analysis Item	a. General Vest	b. Cooling Vest	p	Unplanned Comparison Result
Slowly swing the right hand up and down by parallel with the body	0.67 ± 0.68	0.68 ± 0.72	0.000 *	b > a
Fast swing the right hand up and down by parallel with the body	0.53 ± 0.71	0.54 ± 0.78	0.000 *	b > a
Make a fist on the right hand and move up and down by vertical to the body	0.64 ± 0.69	0.63 ± 0.78	0.000 *	b > a
Cross the arms over the chest to do the motion of opening arms	0.57 ± 0.59	0.58 ± 0.67	0.000 *	b > a
Raise hands up and keep parallel and then move from the top of head down to touch knees	0.56 ± 0.45	0.53 ± 0.91	0.000 *	b > a
Put feet together and then move outwards to the same width as shoulders	0.61 ± 0.59	0.62 ± 0.87	0.000 *	b > a
Jump in place	0.65 ± 0.89	0.67 ± 0.93	0.000 *	b > a
Lift the leg to vertical 90°	0.56 ± 0.23	0.57 ± 0.82	0.000 *	b > a
Lift the right leg outwards to the right waist	0.52 ± 0.67	0.55 ± 0.79	0.000 *	b > a
Squat in place, raise both hands up with parallel, and then move from the top of head to touch knees	0.45 ± 0.81	0.46 ± 0.78	0.000 *	b > a
By simulating the rescue situation of firefighters, lift an injured person, weighted 73 kg, and move backward for 3 m	0.57 ± 0.59	0.58 ± 0.78	0.000 *	b > a
Raise both hands closed, move downward to knees, and swing to the right behind the neck	0.58 ± 0.87	0.60 ± 0.76	0.000 *	b > a
Squat and move forward for 3 m	0.47 ± 0.83	0.48 ± 0.62	0.000 *	b > a

* stands for significant difference, p < 0.05.

presents the results. For motion one, the participants slowly swung their right hand up and down, parallel to their body. The score for the normal vests was four–seven points, and that for the cooling vests was five–seven points, but this difference was nonsignificant (p > 0.05). For motion two, the participants rapidly swung their right hand up and down, parallel to their body. The score for the normal vests was four–seven points, and that for the cooling vests was five–seven points, but the difference was nonsignificant (p > 0.05). For motion three, the participants made a fist with their right hand and moved it up and down, parallel to their body. The score for both the normal and cooling vests was five–seven points. For motion four, the participants crossed their arms over their chest and then opened their arms. The score for both vests was four–seven points. For motion five, the participants raised their hands and kept them parallel with each other, then moved them from the top of their head to their knees. The scores for the normal and cooling vests were four–seven and five–seven points, respectively, but this difference was nonsignificant (p > 0.05). For motion six, the participants placed their feet together then spread them shoulder-width apart. The scores for both vests were five–seven points. For motion seven, the participants jumped in place. The scores for the normal and cooling vests were five–seven and four–seven points, respectively, but this difference was nonsignificant (p > 0.05). For motion eight, the participants lifted their leg to a 90° angle. The scores for both vests were three–seven points. For motion nine, the participants lifted their right leg to the right. The scores for the normal and cooling vests were four–seven and three–seven points, respectively, but this difference was nonsignificant (p > 0.05). For motion 10, the participants squatted in place, raised both hands parallel to each other, and moved them from the top of their head to their knees. The scores for the normal and cooling vests were four–seven and three–seven points, respectively, but this difference was nonsignificant (p > 0.05). For motion 11, the participants simulated a rescue mission by lifting a person weighing 73 kg and moving backward for 3 min. The scores for the normal and cooling vests were five–seven and four–seven points, respectively, but this difference was nonsignificant (p > 0.05). For motion 12, the participants raised both hands together, moved them to their knees, and swung them to the right behind the neck. The scores for the normal and cooling vests were five–seven and four–seven points, respectively, but this difference was nonsignificant (p > 0.05). For motion 13, the participants squatted and moved forward for 3 min. The scores for the normal and cooling vests were five–seven and four–seven points, respectively, but this difference was nonsignificant (p > 0.05).

4.2. Subjective Evaluation of Thermal Sensation

After the treadmill experiment, the participants ranked their thermal sensations using the Likert-type scale (

Table 6. Perceived thermal sensation (Likert-type scale; mean ± standard deviation; and unit: %).

Degree of Perceived Thermal Sensation Questionnaire	a. General Vest	b. Cooling Vest	p	Unplanned Comparison Result
Effects of weight on motion	0.50 ± 0.58	0.54 ± 0.81	0.000 *	b > a
Humid feeling of vests	0.56 ± 0.59	0.53 ± 0.36	0.000 *	a > b
Effects of sultriness of vests in the experiment	0.52 ± 0.87	0.58 ± 0.62	0.000 *	b > a
Satisfaction with the looseness of vests in the experiment	0.55 ± 0.72	0.58 ± 0.61	0.000 *	a > b
Coolness of vests in the experiment	0.50 ± 0.81	0.31 ± 0.56	0.000 *	b > a

* stands for significant difference, p < 0.05.

). In terms of the effect of the added weight on motion, the score for the normal vests was five–seven points, and that for the cooling vests was four–seven points, but this difference was nonsignificant (p > 0.05). In terms of humidity, the score for the normal vests was four–seven points, and that for the cooling vests was 5–7 points, but this difference was nonsignificant (p > 0.05). In terms of the effect of the vests’ sultry draftness on motion, the score for the normal vests was five–seven points, and that for the cooling vests was four–seven points, but this difference was nonsignificant (p > 0.05). In terms of satisfaction with the looseness of the vests, the score for the normal vests was four–seven points, and that for the cooling vests was five–seven points, but this difference was nonsignificant (p > 0.05). In terms of coolness, the score for the normal vests was four–seven points, and that for the cooling vests was two–five, which was a significant difference (p < 0.05). The control group indicated discomfort due to the high temperature.

4.3. Physiological Analysis of Comfort

The participants’ physiological values were scored using the Likert-type scale (

Table 7. Physiological measurements (Likert-type scale; mean ± standard deviation; and unit: %).

Physiological Value Measurement	a. General Vest	b. Cooling Vest	p	Unplanned Comparison Result
Ear temperature	0.54 ± 0.71	0.54 ± 0.65	0.000 *	a > b
Heart rate (HR)	0.54 ± 0.73	0.53 ± 0.87	0.000 *	a > b
Blood pressure—diastolic blood pressure	0.51 ± 0.69	0.59 ± 0.38	0.000 *	a > b
Blood pressure—systolic blood pressure	0.50 ± 0.69	0.58 ± 0.23	0.000 *	a > b
Mean skin temperature (TSK)	0.52 ± 0.36	0.30 ± 0.21	0.000 *	b > a

* stands for significant difference, p < 0.05.

). For ear temperature, the score for the normal vests was five–seven points, and that for the cooling vests was four–seven points, but this difference was nonsignificant (p > 0.05). For heart rate, the score for the normal vests was four–seven points, and that for the cooling vests was five–seven points, but this difference was nonsignificant (p > 0.05). For diastolic blood pressure, the score for the normal vests was five–seven points, and that for the cooling vests was four–seven points, but this difference was nonsignificant (p > 0.05). For systolic blood pressure, the score for the normal vests was five–seven points, and that for the cooling vests was four–seven points, but this difference was nonsignificant (p > 0.05). For average skin temperature, the score for the normal vests was four–seven points, and that for the cooling vests was two–five points, which is a significant difference (p < 0.05). This finding indicates that the cooling effect of the vest was substantial.

5. Conclusions and Suggestions

This study developed cooling vests for firefighters and evaluated the effectiveness of the design on the basis of human factors engineering to create a usable product. Although comfort was evaluated in terms of the wearers’ perceptions, it can indicate objective physiological differences in the temperature and humidity of the within-clothing microclimate. The comfort of the vest was evaluated using advanced measurement technology. The objective data from the experiment indicated that the air permeability of the vests’ fabric affected the comfort in terms of temperature and humidity. This study verified the user-centered nature of the vest design and thereby expanded upon the research on the design of heat exchange systems for cooling clothing. The results can be applied to designing firefighter cooling vests, and they evidence the practicability of objective human factors engineering for designing products.

In the next stage, the vest will be made using lightweight materials and a cooling medium to facilitate air cooling, increase comfort, and make the vests lighter and more flexible. Continuing to design clothing for unique environments is a viable direction for future research.

5.1. Effect of Firefighter Cooling Vest

This study yielded p values of > 0.05 for motions 1–13, indicating they were not significant, and the reliability of each motion was below 95%. The results for the two groups were similar, indicating that the cooling vest is comfortable.

In terms of subjective discomfort, the data from the psychological questionnaire indicated similar results for sultriness, coolness, and overall satisfaction with the cooling vest. No notable difference in satisfaction with looseness was observed between the groups. However, the control group indicated that the loose parts of the vest were more comfortable and that the chest, underarms, shoulders, and back were tight. The experimental group indicated that some parts of the vest were tight because of the cooling structure, and that the cooling vests affected their motion. They also described feeling the cooling effect of the liquid when they put on the vest. These results revealed that cooling vests for firefighters could effectively reduce the temperature of the within-clothing microclimate and regulate the body’s thermal balance.

The cooling vests reduced rectal temperature, heart rate, and oxygen consumption, and also increased movement time in people simulating firefighter movements. Therefore, the liquid cooling mechanism effectively reduced internal temperature and alleviated thermal stress. The physiological and psychological measurements of the participants wearing the cooling vests did not differ considerably before and after the experiment. The average skin temperature and standard deviation were 33.96 ± 0.31 and 34.18 ± 0.24 °C for the experimental and control groups, respectively. The results of the statistical analysis revealed that the cooling vest produced a notable cooling effect.

5.2. Application of Verification Method to Design

The verification method used in this study is often applied in human factors engineering to examine performance in addition to physiological and psychological burden through motion analysis; for example, the effect of firefighters’ poor posture caused by the weight of cooling vests. Physiological measurements are used to evaluate the comfort of cooling vests. However, few studies have applied scientific verification methods to designing products; new design concepts are mostly verified using subjective questionnaires and oral evaluations. The current study used a human-factors-engineering–based and user-centered verification method. In user-centered design, users contribute their ideas and experiences to guide the design process. Although user-centered design is a cost-effective method of solving product development problems in enterprises, it is not common in practical product development for general enterprises. A study in cooperation with a professional research team adopted user-centered design to identify the requirements and expectations for firefighters’ clothing, produce a prototype of a cooling vest, and verify the design. The results of this study can serve as a reference for user-centered product development.

5.3. Suggestions for Future Research

Firefighter uniforms are crucial for safety, and the fabric used in the uniforms has been widely studied. Subsequent studies should:

(1)

Incorporate wearable artificial intelligence technology into firefighter uniforms to monitor firefighters’ vital signs in real time and provide feedback during rescues. This would allow rescue centers to effectively assign the appropriate personnel for each situation and guarantee firefighters’ safety;

(2)

Develop new, intelligent fabrics for firefighter uniforms to address metabolic heat production and maximize dissipation. Such fabrics can transform thermal energy into power for internal cooling or intelligent devices, accelerate heat dissipation, and be an efficient use of resources;

(3)

Utilize monitoring technology for the inner air layer in firefighter uniforms, combine the air layer thickness between layers, develop a model of heat and humidity transfer in firefighter uniforms, and analyze heat and humidity transfer in firefighter uniforms in high-temperature environments.

This study has several limitations. Protective-clothing-related products still require certification from organizations of international standards, and the development process of such products is challenging. Therefore, verification methods must be more thorough.