Developing a hybrid cooling vest for combating heat stress in the construction industry

Abstract

Many frontline workers in the Hong Kong construction industry have to perform physically demanding work under hot working conditions, which could reduce work efficiency and time and increase the occurrence of heat cramps, heat exhaustion, and heat stroke. This study aimed to develop a hybrid, new cooling vest to combat heat stress in the construction industry. Following the functional clothing design process, a problem identification was conducted. Preliminary ideas were formed through the analysis of available types, research, a survey, literature review, and brainstorming. The design was refined through the use of desirable phase change material packs, fans with high wind velocity/long duration, and fabrics with thin, vapor-permeable, wind/water/abrasion-resistant properties, and UV protection, as well as clothing ergonomic design that considers fit, mobility, convenience, and safety. The desirable thermal functional performance in the new cooling vest was previewed through a computer-aided design platform S-smart system. The design criteria were established and a prototype was developed. The environmental chamber testing results showed that, in a hot environment, the mean skin temperature (35.8℃ vs. 36.59℃), heart rate (110 beats/min vs. 116 beats/min), and core temperature of the subjects with the new cooling vest were significantly lower than those with the control (without the new cooling vest ). A significantly longer exercise time was obtained with the new cooling vest compared with the control (22.08 min vs. 11.08 min). Significant improvements in levels of coolness, dryness, comfort, and physical recovery were observed with the new cooling vest. Results suggest that the new cooling vest can reduce the thermal stress of construction workers and improve their work performance and comfort.

Keywords

heat stress cooling vest functional clothing design process waterproof and breathable fabrics comfort

Many frontline workers in Hong Kong have to perform physically demanding work under hot working conditions, which can be as high as 35.4℃ and 95% relative humidity in summertime.¹ The hard work, combined with hot/humid weather, reduces work efficiency and time,² and could increase the occurrence of heat cramps and heat exhaustion, thereby resulting in high cardiovascular demand and the possibility of heat stroke.³ A search in local newspaper archives using the search engine WISENEWS shows at least 282 heat-related incidents (including 40 fatal cases) between 1998 and 2011 across all industries.⁴ Of the 282 heat-related incidents, construction contributed to 73 cases, 22 of which were fatal. Thus, measures to combat heat stress in the construction industry deserve considerable attention from all parties concerned. A properly designed personal cooling system (PCS) is one measure that can reduce the risk of heat-related injuries and illnesses.

Various studies have been conducted that centered on various PCSs, such as battery-driven ambient air fan-based cooling vests without external connections;^5,6 and passive cooling systems that employ phase change materials (PCMs) (e.g. ice, frozen gels) in vests/clothing.^7–11 Attempts have been made to introduce these products into construction workplaces, where workers endure a hot environment.¹² However, the effectiveness and applicability of these products in the construction industry have yet to be verified. Construction work is tough and demands additional requirements in a cooling vest. A cooling vest should have good cooling performance, be lightweight, durable, and easy to maintain. And the vest has to be suited to heavy tasks and sufficiently fit the body shapes of workers to avoid posing a hazard around moving parts while still providing flexibility. However, a cooling vest that is suitable for sports may not necessarily be appropriate for construction workplaces. Accordingly, developing a tailor-made cooling vest to protect construction workers from heat-related injuries while working in a hot environment is necessary. One method to realize this goal is to apply an objective and structured approach in the development of a new cooling vest.

Investigators in the textile and clothing fields have applied design process principles in their studies. Huck and Kim¹³ developed a coverall for wildland or grass firefighting by applying the design process of Dejonge.¹⁴ The preferability of the newly designed prototype coveralls to the current coveralls worn was based on the perceptions of fit and comfort and evaluated using objective and subjective measurements. LaBat and Sokolowski¹⁵ applied a three-stage design process (problem definition and research, creative exploration and development, and implementation) to an industry–university textile product design project: optimizing an athletic ankle brace. The results led to the redesign and improvement of the product, as well as the reduction of product returns. Other clothing items, such as occupational clothing for female pear farmers,¹⁶ thermal protective flight suits,¹⁷ and bicycle patrol uniforms,¹⁸ were designed to address the expectations of the wearer by employing the functional design process approach. Thus, the literature provides a systematic method to develop functional clothing.

The current study, which involves designing anti-heat stress clothing (a new uniform: a t-shirt made of moisture-wicking technical fabric and trousers made of Dry-Inside technical fabric) for construction workers in hot and humid weather, accurately followed the functional clothing design process of Dejonge.¹⁴ Moreover, the recent research further complemented this process by placing equal premium on fabric and design, rather than on merely the latter, in response to design situations.¹⁹ Fabrics with excellent heat/moisture-transporting properties and ergonomic clothing design (previewed and optimized using S-smart, a computer-aided design platform (CAD), before the formal design), which are associated with mobility, convenience, and safety, maximized the comfort of the wearer and improved the functionality of the clothing. Anti-heat stress clothing has been implemented in actual wearing conditions by the Construction Industry Council (CIC) in Hong Kong [Figure 1(a)]. Accordingly, this clothing was used as the basis of this research to design a hybrid, new cooling vest (NCV) for construction workers, who will wear the cooling vest outside the anti-heat stress clothing during considerably hot days in Hong Kong (criteria: daily maximum temperature ≥33.0℃).²⁰ The seven-step process of Lamb and Kallal²¹ was followed, and the design criteria establishment of Dejonge¹⁴ was integrated into the development of the cooling vest.

Figure 1.

Newly designed cooling vest for the construction workers.

Materials and methods

According to Lamb and Kallal²¹ and Dejonge,¹⁴ the modified seven-step process includes the following components (Figure 2):

Problem identification.

Preliminary ideas.

Design refinement.

Design criteria establishment.

Prototype development.

Evaluation.

Implementation.

Figure 2.

Functional design process for the new cooling vest for construction workers. *CIC: Construction Industry Council.

In the development of anti-heat stress clothing, the textile technology used in fabric testing and CAD with regard to the inheritance and development of existing knowledge was continually employed and further expanded to test the battery-driven ambient air fan and PCMs for the NCVs.

Step 1: problem identification

The construction workers of Hong Kong have to undertake physically demanding activities in hot and humid conditions. Workers performing tasks in a hot and humid environment are at risk of heat-related injuries and illnesses. In a hot environment, regulating body temperature at a stable level is important for the human body. Hence, failure to do so can lead to several detrimental effects, such as dehydration, heat stroke, and elevated heart rate. A recent survey showed that 5% and 23% of construction workers had suffered from heat stroke and experienced signs and symptoms of heat stroke, respectively.⁴ Consequently, the increase in the number of heat-related incidents in the construction industry has led to public concern. After considering the additional requirements of the construction industry, the identified problem focused on developing a suitable cooling vest for construction workers to combat heat stress and reduce the risk of heat-related injuries and illnesses in the construction industry.

Step 2: preliminary ideas

In the second step of the process, preliminary ideas are formed from techniques such as sketching, brainstorming, research, surveys, and question-and-answer sessions to achieve the goals.²¹ These techniques were used in the present study.

Chan et al.²² conducted a meta-analysis and found that natural air-cooled garments and phase change material cooling garments were more suitable for most occupational workers because the workers move frequently. Furthermore, the cooling effect of a cooling vest with two fans and three ice packs was investigated in four Hong Kong industries, including construction, where the workers undertook wear trials and performed their usual work routines.²³ The cooling vest was perceived to have a long effective cooling time, and the workers felt it less clammy, sticky, damp, heavy, scratchy, rough, stiff, tight, and interferential with regard to job performance. These studies suggest that designing a hybrid cooling clothing based on vests with PCM packs and two fans is considerably suitable for construction workers. Therefore, preliminary ideas were sketched after the discussions of the research team, and PCM packs, fans, fabrics, and clothing design in the NCV were refined.

Step 3: design refinement

For all the aforementioned factors, this step should result in a few ideas that can then be tested for the fabrics, PCM packs, fans, and clothing design.

Fabrics

The aim of this research was to design a NCV for construction workers, who will wear this vest during extremely hot days in Hong Kong; the design demand of this vest is the same as the recent design for anti-heat stress clothing.¹⁹ Many factors in the development of anti-heat stress clothing, such as the thermal function provided by thin and vapor-permeable fabrics, UV protection, and abrasion resistance, were also determined as important concerns in the design of the NCV. Therefore, fabric properties, including weight, thickness, air permeability, thermal conductivity, water vapor permeability, ultraviolet protection factor (UPF), and abrasion resistance, were tested to determine the effects of the fabric on comfort and protection in a steady state. Moreover, the need to apply fabrics with a high water repellency was well understood because two fans with lithium batteries are used.

The researchers identified and tested 19 commercially available fabrics (i.e. 11 for shell fabrics and eight for lining fabrics) to select the ideal fabrics for use in the NCV. The fabrics in a commercially available cooling vest with a similar structure were also tested for comparison. Thus, a total of 21 fabrics were tested. Table 1 lists the fabric description, color, and mean physical characteristics. Prior to performing the various tests, all specimens were conditioned for at least 24 h in an air-conditioned laboratory at a temperature of 21 ± 1℃ and a relative humidity of 65 ± 2%. A total of 10 specimens for each fabric were tested in the laboratory, as required by ASTM D1776.²⁴

Table 1.

Fabric description, color, and mean physical characteristics

Fabric code	Description	Color	Fabric weight (gm/m²)	Fabric thickness (mm)	Air resistance (kPa S/m)	Thermal conductance (W/(m·K))	Water vapor permeability (g/m²/day)	UV protection factor	Abarison- 15000-weight loss (%)
Shell fabric (S)
S1	Polyester taffeta	Gray	0.73	0.06	0.04	0.023	846.19	50+	2.32%
S2	Polyester taffeta	Brown	0.65	0.06	0.02	0.022	846.38	50+	3.23%
S3	Nylon taffeta	Blue	0.60	0.08	0.01	0.026	828.81	50+	0.29%
S4	Polyester taffeta	Blue	0.72	0.06	0.03	0.022	858.46	50+	1.75%
S5	Polyester taffeta	Yellow	0.70	0.06	0.02	0.022	809.16	50+	3.07%
S6	Taslan	White	0.64	0.08	∞	0.024	656.47	5	0.68%
S7	Taslan	Blue	0.94	0.23	∞	0.033	425.81	50+	0.06%
S8	Taslan	Gray	1.37	0.32	∞	0.033	648.00	50+	2.06%
S9	Nylon taffeta	Green	1.30	0.32	1.18	0.038	1103.62	50+	0.82%
S10	Nylon taffeta	Gray	0.42	0.07	2.46	0.020	1052.31	50+	0.55%
S11	Nylon taffeta	Green	0.98	0.28	1.10	0.034	1014.30	50+	0.48%
CACV	Taffeta	Black	0.71	0.12	∞	0.035	938.50	50+	1.65%
Lining fabric (L)
L1	Polyester mesh	Gray	0.53	0.28	0.00	0.029	Miss	NA	NA
L2	Polyester mesh	Black	1.13	0.14	0.00	0.016	1046.39	NA	NA
L3	Polyester mesh	Green	1.65	0.52	0.07	0.038	1095.56	NA	NA
L4	Polyester mesh	Red	1.44	0.42	0.05	0.034	1035.55	NA	NA
L5	Polyester mesh	Blue	1.44	0.44	0.04	0.034	1119.12	NA	NA
L6	Polyester mesh	Gray	1.57	0.34	0.03	0.032	1041.39	NA	NA
L7	Polyester mesh	Orange	0.75	0.24	0.00	0.032	885.88	NA	NA
L8	Polyester mesh	Black	0.48	0.24	0.00	0.028	1253.48	NA	NA
CACV	Polyester mesh	Black	0.89	0.29	0.00	0.037	841.09	NA	NA

∞

: infinity; Miss: data were missed; NA: not available due to mesh fabric as lining fabric; CACV: commercially available cooling vest.

Weight and thickness. The fabric weight for all the fabrics was measured in accordance with ASTM D3776 Standard Method for Mass Per Unit Area of Woven Fabric. A Mettler balance, Shimadzu (Shimadzu Corporation, Kyoto, Japan), accurate to 0.001 g, was used. Fabric thickness was measured by a dial micrometer accurate to 0.0001 inch, according to ASTM D1777.

Air permeability. The air permeability of the textile fabrics was tested using KES-F8 API (Kato Tech Co., Ltd., Kyoto, Japan). The test of air permeability is used to measure the flow of air passing through a specific area of material. It is used to determine the ability of air to penetrate materials. During the test, the Automatic Air Permeability Tester supplies an air flow from high pressure to low pressure, and the resistance of the material to passing the air is determined by the tester. The flow of air passing through a unit area of material at a unit pressure difference across the tester over a unit period of time is recorded. During each test, the suction and discharge of air are carried out over five seconds, in order to measure the loss in air pressure caused by the air resistance ability of the material. The air resistance value ‘R’ is displayed directly on the panel. A specification regarding a cooling vest design indicated that the fabric properties should prevent air leakage and keep wind out,²⁵ which makes PCMs and wind from two fans form a cooling clothing microclimate. A fabric with a substantially high air resistance should be employed because a considerably high air resistance means a significantly low air permeability.

Thermal conductivity. Thermo Labo II (Kato Tech Co., Ltd. Kyoto, Japan) was used to measure the thermal conductivity of fabrics in this study.

The thermal conductivity was calculated using the following equation:

K = \frac{W \times D}{A \times Δ T} (W / (m \cdot K))

(1)

where K is the thermal conductivity, W is the power, D is the thickness of fabric in cm, A is the area of the hot plate (5 cm × 5 cm = 25 cm²), and ΔT is the temperature difference between the top and bottom of the hot plate (10℃). For details, see Chan et al.¹⁹

Water vapor permeability. Water vapor permeability tests the rate of water vapor that is diffused through the specimen. In accordance with ASTM E96,²⁶ 10 measurements of each fabric were performed by sealing fabrics over the open mouth of a dish that contained water and placing them in the standard testing atmosphere. After a period of time to establish equilibrium, successive weights of the dish were collected and the rate of the water vapor transfer through the specimen was calculated.

UPF. The UPF can be used to determine the rated UV protection factor of sun protection fabrics, which are designed to absorb or reflect the UV radiation of the sun as a means to protect the skin from damage. Three UPF protection categories are presented, as follows:²⁷

Range of 15–24: Good protection, effective ultraviolet radiation (UVR) transmission of 6.7%–4.2%

Range of 25–39, Very good protection, effective UVR transmission of 4.1% –2.6%

Range of 40–50, 50+: Excellent protection, effective UVR transmission of ≤2.5%

Abrasion. The abrasion weight loss percentage (%) was measured. The NCV, accompanied by anti-heat stress clothing, will mainly be worn from June to September in Hong Kong. Each ensemble will actually be worn for approximately 60 days due to alternate washing. In 15,000 times abrasion tests, a weight loss percentage for fabrics of below 8%, without the appearance of holes, is acceptable and offers sufficient protection for the actual wearing situation.¹⁹

Water repellency and wettability (spray test). This method measures the resistance of porous textile materials (or garments) to wetting by water and is shown as grades 0 to 5:²⁸

Grade 5: No sticking or wetting of the upper surface, thereby indicating the maximum water repellency.

Grade 0: Complete wetting of the entire upper and lower surfaces, indicating the lowest water repellency.

PCM packs

A PCM is a material which has a high heat of fusion through melting and solidifying at a certain temperature to allow it to store and release an enormous amount of energy. Heat is absorbed when the material changes from solid to liquid, or released from liquid to solid; thus, PCMs are classified as “latent” heat storage materials.²⁹ Numerous PCMs absorb and release heat at a nearly constant temperature, and melt with a heat of fusion in any required range. Moreover, a few of these materials exhibit economic advantages, such as low cost and large-scale availability.²⁹ Thus, using PCM packs in the NCV is a logical approach.

In the current study, three commercially available PCMs were initially selected for comparison. Two out of the three PCMs were excluded due to unaffordable price and leaked packaging. ClimSel™ C28 (Climator Sweden AB, Sweden), a salt hydrate-based PCM that works by either charging or discharging energy at different temperatures, was eventually used. This PCM is affordable and showed the following desirable properties:

Moderate thermal conductivity.

Compatible with the packing material.

Easy maintenance and no known lifetime limit on PCM packs:

PCM packs can be stored in an air-conditioned room in construction sites at 10–26℃ to be solidified for use (a lower storage temperature is better in hot weather depending on need).

If PCM packs are correctly handled and the packaging remains uncompromised, then the product will continue to cycle as intended each time with no known lifetime limit.

Table 2 provides a list of the PCM size, main components, typical temperature stabilization span, and physical characteristics.

Table 2.

Functional criteria of components in the newly designed cooling vest

Item	Descriptions
PCM	The vest has eight pockets for PCM packs: four on the chest and four on the back [Figure 1(b)] PCM (C28) Size: width 115 mm × length 120 mm Main components: sodium sulfate, water, and additives Typical temperature stabilization span: 27–31℃ Physical data: Phase change temperature: solid 27℃ Phase change temperature: liquid 31℃ Latent heat of fusion: 45 Wh/kg–170 kJ/kg Specific gravity: 1.4 kg/litre Thermal conductivity: Solid 0.98 W/(m·K) Thermal conductivity: Liquid 0.72 W/(m·K)
Fan and battery		Fan Diameter (cm): 10 Rated power (W): 2.5 Weight (g): 95.5	Battery Material: lithium polymer Voltage (V): 7.4 Capacity (mAh): 4400 Input: DC8.4V1A Power: 20 WMax Size (cm): 8.6 × 7.1 × 2.2 Weight (g): 177
Shell fabric	Item Specification	Test method	Requirement
	1 Weight (g)	ASTM D3376	≥40
	2 Water vapor permeability	ASTM E96	>550 (g/m²/day)
	3 UV protection factor (UPF)	AS/NZS 4399:1996	>40
	4 Abarison-15000-Weight loss percentage (%)	ASTM D4966	≤5%
	5 Water repellency and wettability (spray test)	AATCC standard 22	Grade 5
Clothing ergonomic design	1. Narrow elastic bands in the collar and cuff of a sleeve: make the clothing more elegant and convenient for body activity. 2. Openings located on the collar and cuff of a sleeve: allow heat transfer out. 3. Two air-vents on the back transfer more hot air and enhance the evaporation rate and heat dissipation. 4. Wide and high resilience elastic bands in the waist: appear neat and tidy, keep the wind out, and enhance safety due to prevent that the clothing got caught on something in the workplace. 5. Opening zipper on the sides of the body: makes PCM packs closer to the body for cooling and the size adjustable. 6. Fans were firmly installed as if the cloth and fan are one: enhances the safety. 7. Washable retroreflective strips with different patterns on the front and back surfaces: enhances visibility for safety reasons.

Fans

This study initially selected two types of commercially available fans for comparison: fans that are driven by four AA alkaline batteries from a commercially available cooling vest with a similar structure, and fans that are driven by four rechargeable lithium batteries. Their wind velocities were continually tested using an anemometer (Beijing THY Science & Technology Co., Ltd, Beijing, China). The two types of fans have an adjustable wind velocity from high (grade 4) to low (grade 1). The wind velocity and duration of grade 4 in two types of fans were tested, as shown in Figure 6. Table 2 lists the physical characteristics.

NCV ergonomic design

This study aims to design a piece of hybrid cooling clothing based on a vest with two fans and PCM packs, in which conductive and convective heat loss mechanisms are involved. Clothing ergonomic factors are also important for improvement of the functionality of the NCV.

Design for conduction. Conduction is the heat transfer that occurs if a temperature gradient exists in a solid or stationary fluid medium. In this study, a temperature gradient exists between the skin and the PCM packs. When placing the PCM packs close to the skin of the body, heat flows in the direction of the PCM packs from the skin to decrease body temperature. Thus, the NCV was designed to appropriately fit the human body. Moreover, opening zippers were designed on the two sides of the body, thereby making the PCM packs significantly closer to the body for cooling and the size adjustable.

Design for convection. For the human body, heat transfers from the skin to the ambient air by convection through fabrics and garment openings.³⁰ Two fans were installed in the NCV, in which heat transfers out from the garment openings located on the collar and cuff of a sleeve. Moreover, two air vents on the back were added to transfer more hot air and enhance evaporation rate and heat dissipation.

Design of the clothing ergonomic factors. Recent research identified the ergonomic factors of anti-heat stress clothing, which include mobility, convenience, and safety.¹⁹ These factors also apply to the present study. The fit description in the section on design for conduction is also included. Narrow elastic bands in the collar and cuff of a sleeve were applied to make the clothing more elegant and convenient for body activity. Wide and high resilience elastic bands in the waist can appear neat and tidy, keep the wind out, and enhance safety due to prevent that the clothing got caught on something in the workplace. Fans were firmly installed as if the cloth and fan were one, thereby enhancing the safety. Moreover, washable retroreflective strips with different patterns on the front and back surfaces were incorporated into the NCV to enhance visibility for safety reasons, which complies with the Specifications for Supply and Delivery of Worker Uniforms for the Construction Industry Council in Hong Kong. The CIC advised that washable retroreflective strips should be incorporated onto polo shirts, vests, and winter jackets for workers.³¹ Figure 1 (b), (c), and (d), Figure 7, Table 2, and The Results and discussion section present the detailed designs.

Preview of the NCV thermal functional performance using a virtual CAD system

The S-smart system as a CAD tool was developed to design and preview multi-layered clothing assemblies to achieve desirable thermal functional performance in a virtual space.^32,33 The coupled heat and moisture transfer processes in clothing and the external environment have been considered based on the mathematical models developed by Gagge, Stolwijk, and Nishi³⁴ and Hensel.³⁵ Subjective perception of thermal comfort is predicted by applying a fuzzy logic system, in which a set of inference rules was developed by analyzing the subjective records of a group of people in the experiment.³² The previewed results include core temperature, skin temperature, skin relative humidity, and subjective comfortable sensations before the real NCV was engineered to screen and determine the NCV fabrics and design with desirable thermal functional performance.

In the pre-processing of a user-friendly interface (Figure 3), the wear activities (1, what to do) were arranged based on the real metabolic rates described in a recent field study:³⁶

Figure 3.

The interfaces for defining the wear activities (1), environmental conditions (2), human subject (3), garments (4), and computational simulation (5) in the S-smart system.

Resting (metabolic rates: 42 W/m²)

Working (metabolic rates: 234, 248, 230, and 167 W/m²)

Resting again (metabolic rates: 55 W/m²)

Environment (2) and subjects (3, who) were also provided by the current study. Garment (4) was only the fit design, and the shell and lining fabric testing data, such as thickness, were inputted in the fabric design (Table 1 and Figure 3). The PCM was also selected in the fabric specification. The control and boundary conditions were defined. Thereafter, simulation (5) was started.

Steps 4 and 5: design criteria establishment and prototype development

The design criteria were established and the prototype was developed based on the studies on the PCM packs, fans, fabrics, and predicted results by computational simulation for the NCV thermal functional performance. The Results and discussion section describes these processes in detail.

Step 6: evaluation

Design evaluations were conducted inside an environmental chamber and in the construction worksites. Healthy participants volunteered to participate in the studies after being informed of the experimental procedures. These procedures were approved by the Human Subjects Ethics Sub-Committee of the university.

Evaluations inside an environmental chamber

A total of 12 healthy participants randomly carried out two stage exercises on a motorized treadmill with/without the NCV in an environmentally controlled chamber (37℃, relative humidity: 60%, WBGT: 32.4℃) after 30 min of pre-exercise rest on a chair. The chamber conditions simulated a practical wet-bulb globe temperature (WBGT) collected during wear trials in Hong Kong construction sites.²³

Stage 1. Without the NCV: 6 min warm up (subjects exercised on a motorized treadmill at 6 km/h walking speeds for 3 min with 2% slope and 3 min with 4% slope); 48 min work (6 km/h walking speeds for 3 min with 8% slope and 3 km/h walking speeds for 3 min with 2% slope, alternately performed until the desired core temperature (i.e. 38.5℃) and maximum heart rate were achieved. This simulated a previous study, in which 38.5℃ was reached;³⁶ 6 min active recovery (3 km/h and 2 km/h walking speeds for 3 min, respectively, with 1% slope).

Passive recovery. With/without the NCV: 30 min rest on a chair. Cooling/no cooling intervention was implemented.

Stage 2. Without the NCV: 6 min warm up, 48 min work, 6 min active recovery (repeat Stage 1 protocol).

This protocol simulated the real working situations of the Hong Kong construction workers. In a given workday, they work during the entire morning and afternoon, and 15 min and 30 min breaks are intermediately provided. The NCV was used only during the passive recovery period (the simulated break). We assume that cooling during passive recovery could provide a relatively low initial core temperature to enhance the working performance of the workers for the succeeding exercise period. Skin temperatures were measured using temperature probes (Nikkiso-YSI, Japan) attached to the chest (T_chest), upper arm (T_upper arm), upper leg (T_upper leg), and lower leg (T_lower leg) at a sampling frequency of 30 s. Mean skin temperature was calculated using the formula of Ramanathan:³⁷ 0.3 T_chest + 0.3 T_upper arm + 0.2 T_upper leg + 0.2 T_lower leg. Core temperature was collected through an ingestible core body temperature capsule (HQ, Inc., Palmetto, FL, US) and recorded at a sampling frequency of 30 s. In order to ensure functionality and accuracy, the capsule was calibrated before use. It was put into a water cup at temperatures ranging from 30 to 42℃ and the temperature values recorded continually. The recorded values were compared by a certified temperature probe with an accuracy of (0.15 + (0.002 × T) ℃ (Lutron®, Taiwan). Capsules falling outside a degree of accuracy of ±0.1℃ were not used in the test. The participants swallowed the calibrated capsule with warm water 4–6 h before the test to avoid the confounding effect of food and drinks.³⁸

Evaluations by wearing the NCV in construction sites

A total of 173 construction workers (171 males, 98.84%; two females, 1.16%) from two training centers in Hong Kong participated in wear trials during the summer. Their average age, height, and weight were 32.1 years (SD = 9.2), 171.7 cm (SD = 5.5) and 69.7 kg (SD = 13.9), respectively. The trade distribution comprised screw-plate workers (N = 70, 40.46%) and rebar workers (N = 103, 59.54%).

In one out of the two experimental days (the environmental WBGT: 31.56℃), the construction workers performed their usual daily routine, working for approximately three hours in the morning and afternoon. A total of 15 min and 30 min breaks in the morning and afternoon, respectively, were provided intermediately. The construction workers randomly wore either the NCV on the outside of the anti-heat stress clothing or the anti-heat stress clothing only (control) during the two breaks. At the end of the two breaks, the participants rated their levels of coolness, dryness, comfort, and physical recovery based on a rating scale of “1” (poorest possible rating) to “7” (best possible rating).^19,39,40 For the NCV group, the participants answered whether they preferred the NCV and whether the NCV fitted their body size and facilitated the dissipation of heat with seven-point rating scales.

Statistical analysis

For the core temperature and exercise time, two-way ANOVA with repeated measures (condition × time) was conducted to detect differences between the NCV group and controls. The significance level was accepted at p < 0.05. For subjective evaluation in the construction site wear trials, the change between the NCV and control groups (%) was calculated.

Results and discussion

Objective measurement of the fabric properties and preview of the NCV thermal functional performance

As suggested in the section on design refinement of fabrics, the fabric properties used in the NCV are as follows:

Thin and vapor-permeable.

High air resistance.

High water repellency.

Very good UV protection and abrasion resistance.

With reference to Table 1, among the eight lining fabrics tested, L2 was the thinnest and had high water vapor permeability. Thus, L2 was selected as the lining fabric in the real cooling vest and simulation. Among the 11 shell fabrics tested, the thickness was divided into two groups: S1–6, S10 < 0.1 mm; S7–9, S11 > 0.2 mm. S1–5 showed poor air resistance with low values, and S6–11 showed good air resistance with high values or infinity. Water vapor permeability was divided into three groups: S6–8 < 800; 800 < S1–5 < 1000; S9–11 > 1000 g/m²/day. UPF was > 40 for all shell fabrics except S6, and abrasion resistance of all shell fabrics was excellent ( < 5%). With regard to the representatives of different levels of thickness and water vapor permeability, S2, S4, S7, S8, S9, and S10 were selected and entered into the simulation for the prediction of thermal functional performance by the S-smart system to determine the final selection of shell fabric.

Figure 4 shows the predicted core/skin temperatures and skin relative humidity when wearing the virtual cooling vest. In hot conditions, core/skin temperatures increased from resting values (37.2℃ and 33.0℃, respectively) to top working values (38.1℃ and 36.3℃, respectively). Skin relative humidity also increased from 55.25% at the start of the resting period to 80.3% by the top of the working period. The results indicate that the considerably high metabolic rates during the working period significantly elevated core/skin temperatures and skin relative humidity. Temperature values were lower in S10 compared with those of the commercially available cooling vest (CACV) and other fabrics, thereby suggesting that body heat dissipation is faster in fabric S10. The skin humidity resulting from S10 was 3% to 5% lower over 40–70 min and 5% to 10% lower during other times than those from CACV and other fabrics (except S4), thereby indicating that fabric S10 could maintain the dryness of the skin most of the time. S10 was light, thin, and had very good water vapor permeability and excellent UV protection and abrasion resistance. Furthermore, S10 had high air resistance and water repellency (Grade 5). Thus, S10 was eventually selected as the shell fabric based on the fabric testing results and computer simulation for thermal functional performance.

Figure 4.

The predicted temperature and humidity wearing the virtual cooling vest.

Figure 5 presents the predicted comfortable sensation when wearing the virtual cooling vest. The comfort value is acceptable when wearing the NCV, which could be associated with low skin relative humidity. Skin humidity reached 100% during difficult tasks in the recent research.¹⁹ In the present study, the two fans may accelerate the processes of sweat evaporation; however, skin humidity was merely 80.3% at the top of the working period. Therefore, the comfort value is acceptable because humidity is the driving force behind comfortable or uncomfortable perceptions. Previous studies reported that a significantly positive relationship existed between increasing humidity and the perception of discomfort in the subject.^41,42

Figure 5.

The predicted comfortable sensation wearing the virtual cooling vest.

Figure 6.

The wind velocity and duration tested in two kinds of fans (a) Fan with lithium batteries (b) Fan with alkaline batteries.

Figure 7.

The newly designed cooling vest prototype.

Fans and PCM packs

Figure 6 shows the average wind velocity and duration of the two types of fans. The fans with lithium batteries (Shen Zhen Ideal Energy Co., Ltd., Shen Zhen, China) were eventually used because they have a higher and more stable average wind velocity and longer duration compared with those of the fans with alkaline batteries (3.26 vs. 1.27 m/s and 7 vs. 4 h, respectively), which have a gradually declining wind velocity.

Corcoran⁴³ reported that workers have some concerns which may restrict the use of cooling garments in the market: that they spend more time on waiting for the vest to activate and it does not function. The construction workers can wear the NCV for one week on a single charge when it is worn during the two morning and afternoon breaks because the fans with lithium batteries can work for seven hours under a stable wind velocity. This type of storage battery can be easily charged by the workers during their weekends, thereby increasing the availability of the NCV.

Construction sites in Hong Kong are furnished with air-conditioned rooms of approximately 20–24℃ for meetings and toolsheds. The PCM packs can be stored in the air-conditioned rooms to ensure they are solidified ready for use when the NCV is not worn. Thus, the NCV is workable in the construction sites in Hong Kong.

Establishment of design criteria and prototype development (Step 4 and Step 5)

Considering various factors for the PCM packs, fans, fabrics, and design features, computational simulation of the thermal functional performance of the NCV in the previous section (Fans and PCM packs) has demonstrated the desired effects on body temperature, skin relative humidity, and comfortable sensation. The design criteria for the NCV were established (see Table 2). Moreover, the Materials and methods section suggests that the PCM packs, fans, and shell fabrics selected have to address the functional criteria listed in Table 2. Thereafter, the NCV prototype was developed and is illustrated in Figures 7 and 1(b), (c), and (d).

Evaluation (Step 6)

Evaluations inside an environmental chamber

The environmental chamber testing results showed that, during passive recovery, the mean skin temperature and heart rate following use of the NCV (35.8℃ and 110 beats/min, respectively) were significantly lower than those for the controls (36.59℃ and 116 beats/min, respectively; p < 0.001 for all measurements). The difference in core temperature between the NCV group and controls was insignificant during the 30 min pre-exercise rest and Stage 1 exercise. However, a significantly lower core temperature was obtained during the passive recovery and Stage 2 exercise for the NCV group compared with the control group [p < 0.001 for both; Figure 8(a)]. The difference in exercise time between the NCV and control groups was insignificant during the Stage 1 exercise. However, a significantly longer exercise time was obtained during Stage 2 exercise for the NCV group compared with the controls (22.08 min vs. 11.08 min, p < 0.001; Figure 8(b)).

Figure 8.

The core temperature (a) and exercise time (b) wearing the newly designed cooling vest. NCV: newly designed cooling vest; Con: control (wearing the anti-heat stress clothing only); NS: not significant; ***p < 0.001.

In the present study, the cooling intervention was not implemented during the 30 min pre-exercise rest and Stage 1 exercise, and thermoregulatory responses were insignificant between the NCV and the control groups. The core temperature was merely 37.3℃, which is a nearly normal value, during the 30 min pre-exercise rest and before Stage 1 exercise [Figure 8(a)]. Thus, employing a cooling intervention in the Stage 1 exercise is unnecessary. However, the core temperature elevated to over 38℃ at the end of the Stage 1 exercise. Thus, the cooling intervention was immediately implemented and lasted during the passive recovery. Cooling wearing the NCV during passive recovery in a heated environment significantly attenuated the increase in core/mean skin temperatures and heart rate during this period. The properties and design of the NCV contributed to the results. The NCV possesses a PCM and two fans. In the present study, the temperature of the PCM used was 28℃. According to the prediction with S-smart, in hot conditions, skin temperature increased from resting values of 33.0℃ to top working values of 36.3℃, which means a temperature gradient of about 8℃ between the skin and the PCM packs during the working period. When placing the PCM packs considerably closer to the body skin, heat flowed from the skin in the direction of the PCM packs and heat was absorbed when the material changed from solid to liquid; thus body temperature was decreased. A fitted design and opening zipper on the sides of the NCV made the PCM packs considerably closer to the body for cooling. The latter accelerated the processes of sweat evaporation and heat dissipation. The effects of conductive and convective heat transfer wearing the NCV in the heat are suggested to reduce thermal strain in the subjects.

Furthermore, cooling wearing the NCV during passive recovery also delayed the increase in core temperature and contributed to the prolonged work time during the Stage 2 exercise. The results were in agreement with those of Webborn et al.,⁴⁴ thereby showing that the mean core temperature and perceived exertion were lower during cooling wearing an ice vest before exercise (precooling) compared with the no cooling control. The authors suggested that the reduced perceived exertion may translate into an improved functional capacity. For the present study, cooling wearing the NCV during the passive recovery can be seen as a precooling for the Stage 2 exercise.

Evaluations by wearing the NCV in construction sites

Table 3 presents the results of the subjective ratings in the wear trials of the construction sites.

Table 3.

Subjective evaluations in the wear trials of construction sites

	Mean value NCV (Mor)	Mean value Control (Mor)	Change between NCV and Control (%) (Mor)	Mean value NCV (Aft)	Mean value Control (Aft)	Change between NCV and Control (%) (Aft)
I feel cool level during the break	4.84	3.45	28.66	4.66	3.36	27.91
I feel dry level during the break	4.26	3.27	23.37	4.25	3.25	23.41
I feel comfortable level during the break	4.87	3.50	28.13	4.60	3.49	24.12
I feel physically recovered level after the break	4.63	3.76	18.76	4.61	3.63	21.35
Preference for the NCV	5.04			4.69
Whether the NCV fits the body sizes	4.89			4.84
The dissipation of heat in the NCV	5.13			4.95

Note: A “7” was the best possible rating; a “1” was the poorest possible rating. NCV: newly designed cooling vest; Mor: morning; Aft: afternoon.

During the break, significant improvements in the levels of coolness, dryness, comfort, and physical recovery felt were observed for the morning and afternoon. The NCV group changes from the control group (%) were 28.66, 23.37, 28.13, and 18.76 in the morning, and 27.91, 23.41, 24.12, and 21.35 in the afternoon. The scores that the participants provided with regard to their preference for the NCV, and whether the NCV fits their body size and facilitates the dissipation of heat were 5.04, 4.89, and 5.13 in the morning and 4.69, 4.84, and 4.95 in the afternoon. The responses were positive for wearing the NCV.

Previous literature reported that the subjective perceptions of warmth and wetness underneath the clothing correlated with objective measurements of temperature and relative humidity, and discomfort was strongly associated with the amount of moisture on the skin.^41,42 The reduced body temperature and moisture from the double heat dissipation effects of both conduction and convection with the aid of fit design with opening zipper/air vents wearing NCV in Section 3.4.1 contributed to the improved coolness, dryness, comfort, and physical recovery levels in the current study. Therefore, the participants prefer the NCV in the hot environment and high activity levels.

Other deliverables sharing common background and methodology, but mainly focusing on the laboratory human study and the on-site worker survey, indicated that thermoregulatory, physiological, and perceptual strains were significantly lower in the NCV group than those in controls during the recovery session (p ≤ 0.022), which were accompanied by a large effect of cooling (Cohen's d = 0.84–2.11). The rise in physiological strain index was reduced by 0.11 ± 0.12 unit min⁻¹ (p = 0.010) following the use of the NCV. The details on the laboratory human study and the on-site worker survey were described elsewhere.^45–47

Step 7: implementation

The NCV will serve as work clothes in construction sites in Hong Kong. The team will mainly focus on realizing the goal of successfully applying the current research into practical use, particularly in addressing industrial issues. The university may license the NCV technology to the CIC in Hong Kong based on the experiences from the anti-heat stress clothing. Consequently, the CIC will further promote the technology to contractors for site adoption to enhance the health and well-being of construction workers. The NCV may also receive extensive attention from academics, industries, and the public through a series of promotions, exhibition activities, and awards. Thereafter, an official launch of the NCV may be conducted during Construction Safety Week in Hong Kong. The CIC may initially produce a few NCV for workers, and the contractors may further produce additional clothing for use in construction sites.

Conclusions

The objective of this research was to design a new cooling vest for construction workers during extremely hot days in Hong Kong. Problem identification was undertaken after considering the additional requirements of the construction industry, as well as following the functional clothing design process of Lamb and Kallal²¹ and integrating the design criteria established by Dejonge.¹⁴ Preliminary ideas were formed based on the analysis of available types, research, surveys, literature reviews, and brainstorming: designing a hybrid cooling vest with PCM packs and two fans. Design refinement was obtained by selecting the available PCM packs with the ideal price and desirable properties; employing fans with high wind velocity and long duration; screening the used fabrics with thin and vapor-permeable properties, excellent air resistance, water repellence, UV protection, and abrasion resistance from 19 commercially available fabrics; and identifying clothing ergonomic design with fit, mobility, convenience, and safety. The desired effects on the NCV thermal functional performance were previewed through the S-smart system software platform. The design criteria for NCV were established and the prototype was subsequently developed. The NCV prototype evaluations were conducted using treadmill testing by human subjects inside an environmental chamber and wear trials in the construction sites under actual wearing conditions. Significantly lower core/skin temperatures, lower heart rate, and longer exercise time, and significant improvements in levels of coolness, dryness, comfort, and physical recovery felt were observed in wearing the NCV. Thus, the participants prefer the NCV.

The effects of conductive and convective heat transfer wearing the NCV in the heat are concluded to reduce thermal strain in the subjects, which has a profound influence on the work performance and subjective perception of discomfort. In the future, the NCV could be implemented by licensing the technology to the industry and conducting a series of promotions, exhibition activities, and awards.

The novelty of this study is the development of a new vest system which combines a PCM and fan ventilation. The cooling effectiveness of this new vest system can be validated by the results of the laboratory human study and the on-site worker survey, which are described here and elsewhere.^45–47

Footnotes

Acknowledgements

This paper forms part of the research project titled “Developing a Personal Cooling System (PCS) for Combating Heat Stress in the Construction Industry,” from which other deliverables will be produced with different objectives/scopes but sharing common background and methodology. The authors also wish to acknowledge the contributions of other team members including Dr Michael Yam, Dr Daniel Chan, Dr Edmond Lam, Dr Del Wong, Dr Jackie Young, Dr Song, WF, Dr Yi, W and Miss Zhao, YJ.

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 is funded by the Research Grants Council of the Hong Kong Special Administrative Region, China (RGC Project No. PolyU510513) and the Natural Science Research Project for Universities and Colleges in Jiangsu Province (No. 15KJB620004).

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