Effect of vest structure, airflow velocity, and humidity on evaporative cooling capacity using a thermal manikin

Abstract

Climate change has exacerbated heat stress, particularly for outdoor workers. Evaporative cooling vests (ECVs) mitigate occupational heat stress. Their cooling capacity depends on the structural design, air temperature, air velocity, and humidity. This study investigated the cooling capacity and efficiency of four ECV designs under various environmental conditions. Four ECVs—polymer-based punched (ECVPP), polymer-based (ECVPB), cellulose-based (ECVCB), and TECHNICHE (ECVTECH)—were evaluated via a thermal manikin under controlled conditions (ambient temperatures: 35 °C and 40 °C; relative humidity: 20% and 40%; air velocities: 0.1, 0.4, and 1.0 m/s, respectively). The cooling capacity was measured by monitoring the energy consumption of the manikin over two-hour periods. The efficiency was calculated as the cooling capacity relative to the latent heat of water evaporation. Thermal imaging captured the surface temperature distribution. At 40 °C, ECVCB and 30 °C, the ECVPP consistently outperformed the other vests, achieving cooling capacities of 81.7 W and 78.5 W, respectively. These peak values were observed under higher air velocity conditions (1 m/s). Correlations (R = 0.9, p < 0.001) between the amount of evaporated water and the cooling capacity were observed. Thermal imaging confirmed that the progressive cooling capacity decreased due to evaporation—efficiency varied by vest design, highlighting the superiority of ECV_CB. This study highlights the performance of cellulose-based ECVs in hot, dry conditions, driven by enhanced evaporation rates. Higher air velocities improved the cooling capacity but reduced the efficiency, underscoring the need to balance airflow, humidity, and vest design for optimal performance. These findings provide insights for improving ECV functionality in occupational heat stress scenarios.

Introduction

Climate change has intensified global heat exposure, posing serious risks to human health, workplace safety, and economic productivity. Approximately 40% of the world’s population lives in hot or very hot regions where ambient temperatures frequently exceed 30 °C^1–3. Prolonged exposure to high temperatures, particularly during physically demanding work, can lead to adverse physiological and cognitive outcomes such as elevated core body temperature, dehydration, renal impairment, accelerated fatigue, reduced cognitive performance, and weakened immune response, ultimately increasing human error and reducing productivity^3–5.

The occupational and economic consequences of heat stress are especially pronounced in outdoor sectors such as agriculture and construction, where workers are exposed to high physical workloads, solar radiation, and protective clothing. Previous studies indicate that high ambient temperatures combined with heavy workloads and poor ventilation exacerbate physical and mental strain, reduce concentration, and increase injury and accident rates^6–15.

Due to the limitations of engineering controls in many workplaces, personal cooling systems have emerged as practical alternatives. These include circulating water-cooled garments, phase change material (PCM) vests, and evaporative cooling vests (ECVs)^16,17. While water-cooled garments provide effective heat removal, they require external power and add significant weight. PCM vests are lighter but have limited cooling duration and require recharging. ECVs, by contrast, are low-energy, lightweight solutions that utilize evaporative cooling and are particularly suitable for active work environments^17,18.

The effectiveness of ECVs depends on multiple factors, including garment design, fabric permeability, fit, ambient temperature, relative humidity, and airflow. High humidity and low airflow reduce evaporative efficiency by limiting water and sweat evaporation, thereby impairing thermoregulation^18,19. The heat transfer model proposed by Ankit Joshi et al. demonstrates that sensible and latent heat transfer within the skin–clothing system is strongly influenced by environmental conditions and garment permeability, with higher airflow and lower relative humidity substantially enhancing evaporative cooling²⁰.

Empirical studies support the effectiveness of ECVs in reducing heat strain. Dehghan et al. reported significant reductions in physiological and perceptual heat stress among construction workers exposed to extreme heat (49.0 °C, 5.5% RH) when using ECVs¹¹. Similarly, Ciuha et al. found that evaporative vests exhibit higher evaporative resistance compared to no vest or PCM vests^16–21. Karkalic also demonstrated that passive ECVs effectively reduce physiological heat strain during physical activity in hot conditions²².

Despite the availability of domestically produced ECVs with varying fabric structures and air permeabilities, systematic evaluations of their performance under different environmental conditions remain limited. The present study addresses this gap through a controlled laboratory assessment of four ECVs, including three prototype designs and one commercial vest (TECHNICHE), using a thermal manikin, examining the effects of air velocity and relative humidity on cooling capacity and evaporative efficiency. This standardized and quantitative analysis provides insights into optimizing personal cooling strategies and identifying climatic conditions that enhance or limit ECV performance. Accordingly, the study aims to evaluate the influence of ECV structure, air velocity, and relative humidity on cooling performance and efficiency.

Air velocity was set at 0.1 m/s (near-stagnant), 0.4 m/s (typical indoor airflow), and 1.0 m/s (near the upper limit for evaporative cooling).

Methods

Cooling vests

Four evaporative cooling vests (ECVs) were evaluated (Table 1): three prototypes developed by the authors under the HIFITCOOL brand and one commercially available cooling vest (TECHNICHE), which served as a reference. The HIFITCOOL models comprised two polymer-based cooling vests, including a punched (ECVPP) and a non-punched (ECVPB) design, as well as one cellulose-based vest (ECVCB). The selection of the HIFITCOOL cooling vests was based on their availability for this study.

Table 1.

Basic information on types of ECVs.

No.	Cooling vest	Abbreviation	Brand	Cooling Area(m2)	Waist circumference size(cm)	weight (kg)
						Dry state (Decharge)	Wet state (charge)
1	Evaporative cooling vest polymer-based	ECVPB	HIFITCOOL	0.40	96	0.66	1.36
2	Evaporative cooling vest polymer-based- punched	ECVPP	HIFITCOOL	0.44	96	0.40	1.10
3	Evaporative cooling vest cellulose-based	ECVCB	HIFITCOOL	0.42	96	0.53	1.54
4	Evaporative cooling vest	ECVTECH	TECHNICHE	0.42	100	0.26	0.54

ECVPP comprised three layers: an inner waterproof fabric, an outer permeable fabric, and a 5 mm superabsorbent polymer cooling layer (60–105 μm, ρ ≈ 0.6 g/cm³) enclosed in cotton fabric, with 7 mm ventilation holes to enhance vapor release (Fig. 1). ECVPB had an identical structure without ventilation holes. ECVCB consisted of a waterproof inner layer, a permeable outer layer, and a 5 mm cellulosic substrate containing superabsorbent granules of similar size and density, covered with fine cotton fabrics (Fig. 1). The commercial ECVTECH (TECHNICHE) included a waterproof nylon inner layer, a permeable polyester outer layer, and cooling core middle layer (68% polyester, 32% polymer) (Fig. 1).

Fig. 1.

Types of evaporative cooling used in this research.

All vests were fitted to a thermal manikin; looser garments (e.g., ECVTECH) were tightened at the waist, outside the cooling area, to standardize fit and minimize variability.

The thermal manikin

Cooling performance was assessed using a torso thermal manikin (Sina, Iran) compliant with ASTM F1291:2022 and ASTM F2371:2016 standards²³. The 19.3 kg copper manikin consisted of four independently controlled zones (chest, abdomen, upper back, lower back), with a total surface area of 0.67 m² and a clothing contact area of 0.44 m² (Fig. 2). Each zone was equipped with LM35DZ temperature sensors and RTD control sensors, with heating provided by 60 W resistive elements regulated by a PID system. Temperature, power, and cumulative energy consumption were recorded in real time.

Fig. 2.

Climatic Chamber and Torso thermal manikin.

Experiments were conducted in a climatic chamber at 35.0 ± 0.5 °C, 40 ± 5% RH, and 0.4 ± 0.1 m/s air velocity. The manikin surface temperature was maintained at 35.0 ± 0.5 °C until thermal equilibrium (near-zero power input for 120 min) was achieved. When ambient temperature exceeded skin temperature (e.g., 40 °C), heating was reduced automatically; application of an ECV increased power demand, enabling direct quantification of cooling capacity from net energy consumption.

Thermal imaging cameras

Infrared images were captured using a Fluke VT04 Visual IR Thermometer (accuracy ± 2 °C). The device was calibrated prior to testing and positioned at a fixed distance perpendicular to the torso. A uniform emissivity of 0.95, appropriate for textile materials, was applied to all vests to standardize temperature measurements²⁴.

Climatic conditions

Tests were performed in a 3 × 4 × 1.7 m climatic chamber in accordance with ASTM F2371:2016. Two ambient temperatures were selected to represent typical Iranian hot climates: 35 ± 0.5 °C and 40 ± 0.5 °C. Relative humidity levels of 20 ± 5% (dry) and 40 ± 5% were applied. Air velocity was set at 0.1 m/s (near-stagnant), 0.4 m/s (typical indoor airflow), and 1.0 m/s (near the upper limit for evaporative cooling). Airflow was generated by two axial fans and measured at nine points using a hot-wire anemometer, yielding a coefficient of variation < 10%, consistent with ISO 7726 and ASHRAE 55 (Table 2).

Table 2.

Specifications of air velocity and mode trials.

Air Speed(m/s)	35 °C/RH 20%	35 °C/RH 40%	40 °C/RH 20%
0.1	X	–	X
0.4	X	X	X
1	X		X

Methodology

The manikin was first validated under isothermal conditions (35 °C) for 120 min. Vests were soaked in ~ 20 °C water for 5 min, drained, and hung for 5 min before testing. Manikin surface temperature was controlled at 35.0–40.0 °C (± 0.1 °C) using a closed-loop LabVIEW system. Cooling capacity was calculated from manikin energy consumption and vest water evaporation rate. Each condition was tested at least twice for 2 h, with data recorded every second.

At 40 °C, environmental heat gain was added to the measured energy to isolate vest cooling. Evaporative efficiency was calculated as:

1

Where is the actual heat removed (J) and is the theoretical maximum cooling, based on evaporated water mass and latent heat of vaporization (~ 2430 J/g at 35–40 °C)²⁵.

Environmental conditions were monitored throughout, and thermal imaging was used to assess surface temperature distribution.

Statistical analysis

Data were analyzed using IBM SPSS Statistics 27.0. Normality was assessed with the Shapiro–Wilk test. Paired t-tests or Wilcoxon signed-rank tests were used to compare fan conditions for a given vest, while one-way ANOVA or Kruskal–Wallis tests were applied to compare vests under fixed conditions. Tukey’s post hoc test was used when appropriate. Statistical significance was set at p < 0.05.

Results

This study assessed the cooling capacity of four evaporative cooling vests (ECVs) under hot–dry conditions (35 °C and 40 °C, 20% RH) with three airflow levels (fan off, 0.4 m/s, 1.0 m/s), as well as under moderate humidity (35 °C, 40% RH, 0.4 m/s). Statistical analyses were conducted using parametric tests (ANOVA) when normality assumptions were satisfied and nonparametric tests (Kruskal–Wallis) otherwise. A p-value < 0.05 was considered statistically significant for both parametric and non-parametric tests.

Effect of the cooling vest type on the cooling capacity

At 35 °C and 20% RH, cooling capacity differed significantly among vest types and airflow conditions (Table 3). Without airflow, ECVCB and ECVPP exhibited higher average cooling capacities than ECVTECH (parametric P < 0.001). At 0.4 m/s, ECVCB and ECVPB showed higher performance, with ECVCB achieving the highest average cooling capacity. At 1.0 m/s, ECVPB reached the highest peak and average cooling capacities, significantly exceeding ECVTECH and ECVPP (nonparametric P < 0.05). Overall, ECVCB and ECVPB consistently demonstrated higher cooling capacities, whereas ECVTECH showed the lowest cooling capacity across all airflow levels.

Table 3.

Comparison of cooling power of evaporative cooling vests (ECVs) in different air temperature, velocity, and humidity conditions.

Mode trials	Parameters	ECVPP	ECVPB	ECVCB		ECVTECH	Significant Comparisons (P-value < 0.05(
NOFAN 35.20	P max (W)	56.5	52.9	60.13		38.6	ECVPP vs. ECVPB	P0.05 <
	P avg (W)	43.6	43.2	47.8		22.4	ECVPP vs. ECVCB	P0.05 <
	P min (W)	35.2	35.3	40.4		13.9	ECVPP vs. ECVTECH	P < 0.001
							ECVPB vs. ECVCB	P0.05 <
							ECVPB vs. ECVTECH	P0.05 <
							ECVCB vs. ECVTECH	P < 0.001
FAN 0.4.35.20	P max (W)	78.4	104.1	92.3		49.0	ECVPP vs. ECVPB	P0.05 <
	P avg (W)	39.2	76.4	77.6		25.6	ECVPP vs. ECVCB	P < 0.05
	P min (W)	7.7	59.4	67.8		15	ECVPP vs. ECVTECH	P0.05 <
							ECVPB vs. ECVCB	P0.05 <
							ECVPB vs. ECVTECH	P < 0.001
							ECVCB vs. ECVTECH	P < 0.001
FAN 1.35.20	P max (W)	78.6	105.6	95.4		66.9	ECVPP vs. ECVPB	P < 0.05
	P avg (W)	35.9	78.5	75.5		43.9	ECVPP vs. ECVCB	P < 0.05
	P min (W)	5.2	60.5	61.2		28	ECVPP vs. ECVTECH	P0.05 <
							ECVPB vs. ECVCB	P0.05 <
							ECVPB vs. ECVTECH	P < 0.001
							ECVCB vs. ECVTECH	P < 0.001
NOFAN 40.20	P max (W)	80.36	76.34	65.68		51.38	ECVPP vs. ECVPB	P0.05 <
	P avg (W)	57.8	57.0	58.1		29.4	ECVPP vs. ECVCB	P0.05 <
	P min (W)	42.0	49.3	54.7		19.1	ECVPP vs. ECVTECH	P < 0.05
							ECVPB vs. ECVCB	P0.05 <
							ECVPB vs. ECVTECH	P < 0.001
							ECVCB vs. ECVTECH	P < 0.001
FAN 0.4.40.20	P max (W)	101.5	104.3	96.5		69.7	ECVPP vs. ECVPB	P0.05 <
	P avg (W)	54.5	73.1	78.4		39.2	ECVPP vs. ECVCB	P0.05 <
	P min (W)	12.0	57.0	67		20.0	ECVPP vs. ECVTECH	P0.05 <
							ECVPB vs. ECVCB	P0.05 <
							ECVPB vs. ECVTECH	P < 0.05
							ECVCB vs. ECVTECH	P < 0.001
FAN 1.40.20	P max (W)	97/1	94/8	111/1		75/8	ECVPP vs. ECVPB	P0.05 <
	P avg (W)	39.8	68.3	81.7		37.1	ECVPP vs. ECVCB	P0.05 <
	P min (W)	4/7	55	60/7	23		ECVPP vs. ECVTECH	P0.05 <
							ECVPB vs. ECVCB	P0.05 <
							ECVPB vs. ECVTECH	P < 0.05
							ECVCB vs. ECVTECH	P < 0.001
FAN 0.4.35.40	P max (W)	44.2	46.2	49.3		36.3	ECVPP vs. ECVPB	P0.05 <
	P avg (W)	41.1	41.2	46.5		29.0	ECVPP vs. ECVCB	P < 0.05
	P min (W)	40.3	37.5	43		21	ECVPP vs. ECVTECH	P < 0.05
							ECVPB vs. ECVCB	P < 0.05
							ECVPB vs. ECVTECH	P < 0.05
							ECVCB vs. ECVTECH	P < 0.001

Similar trends were observed at 40 °C and 20% RH. ECVCB and ECVPB showed higher cooling capacities than ECVTECH under all airflow conditions (parametric P < 0.05). ECVCB achieved the highest average cooling capacity at 1.0 m/s (P_avg = 81.7 W), while ECVTECH consistently exhibited the lowest values (Table 3).

Under 35 °C, 40% RH, and 0.4 m/s airflow, ECVCB again showed higher cooling capacity than the other vests (parametric P < 0.05). ECVCB recorded the highest P_max (49.3 W) and P_avg (46.5 W), whereas ECVTECH showed the lowest P_max (36.3 W), P_avg (29.0 W), and P_min (21.0 W).

Temporal trends in cooling capacity

Temporal variations in cooling capacity are illustrated in Fig. 3. Under no-airflow conditions at both 35 °C and 40 °C, most vests showed a gradual decline in cooling capacity over time, with ECVTECH consistently showing lower cooling capacity. At 0.4 m/s, ECVPB and ECVCB maintained higher and more stable cooling capacities, whereas ECVPP showed a sharp decline after approximately 120 min. At 1.0 m/s, ECVCB and ECVPB again showed higher cooling capacities, while ECVPP exhibited a rapid decline within the first hour. Across all conditions, ECVTECH remained the vest with the lowest cooling capacity.

Fig. 3.

Cooling power of four evaporative cooling vests under three air velocities (no fan, 0.4 m/s, 1 m/s). (A, B) No fan; (C, D, G) 0.4 m/s; (E, F) 1 m/s.

Effect of the airflow velocity on the cooling capacity

Comparison at similar airflow velocities

At 35 °C and 20% RH, maximum cooling capacities were observed for ECVCB at 0.4 m/s (77.6 W) and ECVPB at 1.0 m/s (78.5 W), both significantly higher than fan-off conditions (parametric P < 0.001) (Table 4). ECVTECH showed the lowest capacities overall but exhibited a significant increase at 1.0 m/s (43.9 W, parametric P < 0.001). In contrast, ECVPP showed lower cooling capacity at higher airflow.

Table 4.

Cooling power and efficiency of evaporative cooling vests (ECVs) at different air temperatures, air velocities, and relative humidities.

mode Trials		ECVPP		ECVPB		ECVCB		ECVTECH
		Cooling Power(W) & water evaporated (gr)	Efficiency (%)	Cooling Power(W) and water evaporated (gr)	Efficiency (%)	Cooling Power(W) and water evaporated (gr)	Efficiency (%)	Cooling Power(W) and water evaporated (gr)	Efficiency (%)
tm:35 °C,20%RH	No fan Vs 0.4 m/s	43.6 (242) 39.2 (299)	52.0 38	43.2 (198) ** 76.4 (453)	64 49	47.8 (265) ** 77.6 (530)	64 43	22.4 (139) 25.6(225)	47 35
	No fan Vs 1 m/s	43.6 (242) 35.9 (302)	52.0 35	43.2 (198) ** 78.5 (474)	64 48	47.8 (265) ** 75.5 (574)	64 38	22.4 (139) ** 43.9 (233)	47 55
	1 m/s vs. 0.4 m/s	35.9 (302) 39.2(299)	35 38	78.5 (474) 76.4 (453)	48 49	75.5 (574) 77.6 (530)	38 43	43.9 (233) * 22.6(225)	55 35
tm:40 °C,20%RH	No fan Vs 0.4 m/s	57.8 (278) 54.5 (310)	61 51	57.0 (240) * 73.1 (489)	69 44.0	58.1 (316) ** 78.4 (579)	54 40	29.4 (165) * 39.2 (269)	52 43.0
	No fan Vs 1 m/s	57.8 (278) 39.8 (323)	61 36	57.0 (240) * 68.3 (501)	69 40	58.1 (316) * 81.7 (606)	54 40	29.4 (165) * 37.1 (323)	52 34
	1 m/s Vs 0.4 m/s	39.8 (323) * 54.5 (310)	36 51	68.3 (501) * 73.1 (489)	40 44.0	81.7 (606) 78.4 (579)	40 40	37.1 (323) 39.2 (269)	34 43.0
tm:35 °C,40%RH	0.4 m/s	41.13(278)	43.5	41.2 (333.6)	36	46.5 (328)	42	29.0 (314)	27

** Significant Comparisons) P value < 0.001 (.

* P value < 0.05 indicates significant differences (P value < 0.05) .

At 40 °C and 20% RH, ECVCB consistently achieved the highest average cooling capacity across all airflow levels (58.1–81.7 W), while ECVTECH remained the vest with the lowest values (29.4–37.1 W). Airflow significantly enhanced cooling capacity in ECVCB, ECVPB, and ECVTECH (parametric P < 0.05), with ECVCB showing the largest increase at 1.0 m/s. However, increasing airflow from 0.4 to 1.0 m/s resulted in lower cooling capacity for ECVPB and ECVPP (nonparametric P < 0.05), consistent with reductions in evaporated water and changes in evaporative efficiency (Table 4).

Overall, airflow strongly influenced cooling output and efficiency, but responses varied by vest design, reflecting differences in material properties, fabric structure, and moisture retention. The time-dependent nature of cooling highlights the transient behavior of evaporative processes and their implications for effective service life.

Comparison across different airflow speeds

Figure 4 illustrates cooling capacity trends across airflow conditions. For ECVPB, airflow initially enhanced cooling but led to sharper declines after ~ 60 min at both temperatures. ECVTECH showed a gradual decline under all airflow levels, with faster reductions at 40 °C. ECVCB demonstrated relatively stable cooling, particularly under no-airflow conditions at 40 °C, while airflow accelerated performance decline. ECVPP showed stable cooling without airflow but experienced marked reductions under forced convection, especially at 1.0 m/s.

Fig. 4.

Cooling capacity of four evaporative cooling vests under three velocity conditions (no fan, 0.4 m/s, and 1 m/s) at temperatures of 35 °C and 40 °C.

Effects of evaporated water on the cooling capacity

Relationship between the cooling capacity and the amount of evaporated Water

A strong positive linear relationship was found between cooling capacity and evaporated water mass (Eq. 2):

2

The Pearson correlation coefficient was 0.90 (p < 0.001), with R² = 0.81, indicating that 81% of the variation in cooling capacity was explained by water evaporation.

Efficiency of cooling vests based on the latent heat of evaporated water

Cooling efficiency generally decreased with increasing airflow. At 35 °C and 20% RH, all vests showed lower efficiency under airflow, with the largest decrease in ECVCB (64.0% to 38.0%). ECVTECH was the only vest to show an increase in efficiency at 1.0 m/s (to 55.0%). At 40 °C and 20% RH, efficiencies decreased for all vests at higher airflow, with ECVPP showing the sharpest reduction. At 35 °C and 40% RH, ECVPP achieved the highest efficiency (43.5%), while ECVTECH remained the vest with the lowest efficiency (27.0%) (Table 4).

Thermal images of the cooling vests

Thermal images (Fig. 5) showed progressive drying of all vests over time, indicated by a reduction in low-temperature (blue) regions and an increase in surface temperature. Drying and temperature rise were more pronounced at higher airflow (1.0 m/s) than at low airflow (0.1 m/s), with notable differences among vest types, reflecting variations in evaporation dynamics and cooling persistence.

Fig. 5.

Comparative thermal image and surface temperature analysis of four cooling vest types under two air velocity conditions (0.1 and 1.0 m/s) and three time intervals (0, 1, and 2 h) under hot environmental conditions (35 °C and 20% relative humidity).

Discussion

This study investigated the cooling capacity and efficiency of evaporative cooling vests (ECVs) under varying temperature, airflow, and humidity conditions, highlighting the role of vest structure and environmental parameters.

Impact of vest structure on cooling capacity

At both 35 °C and 40 °C under dry conditions (20% RH), ECVCB and ECVPB consistently showed higher average cooling capacities than the other vests. The performance of ECVCB can be attributed to its cellulosic substrate combined with coarse cotton layers and superabsorbent granules, which enhance air permeability, water absorption, and evaporation (Table 1). Given that approximately 80% of cooling variation is governed by evaporated water (Eq. 2), this structure enables more effective evaporative cooling.

ECVTECH showed lower cooling capacities under all conditions, largely due to its lower water-holding capacity and rapid water depletion (Table 1), leading to faster drying and reduced cooling over time (Fig. 3). Although ECVPB and ECVPP share similar polymer-based materials, their structural differences resulted in different cooling capacities and temporal cooling patterns. The perforated design of ECVPP promoted rapid, localized evaporation under high airflow, causing uneven moisture distribution and accelerated drying, as confirmed by thermal imaging (Fig. 5). In contrast, ECVPB retained moisture more uniformly, providing more sustained cooling.

It should be noted that ECVPP is designed to enhance sweat evaporation during direct skin contact; this mechanism is not represented in thermal manikin tests, partly explaining its lower apparent cooling at higher airflow. Overall, the cellulose-based ECVCB exhibited the highest observed cooling capacities, aligning with previous findings emphasizing the importance of fiber composition and fabric structure in heat and moisture transfer^20,26.

Effect of increased temperature on cooling capacity

Cooling capacity generally increased at 40 °C compared to 35 °C (Table 3), due to lower relative humidity and reduced water vapor partial pressure, which enhance evaporation. However, at higher airflow rates (0.4 and 1.0 m/s), ECVPP and ECVTECH showed lower cooling capacities, attributed to excessive evaporation and rapid drying of larger surface areas, ultimately lowering cooling efficiency (Fig. 5).

Effect of increased airflow rate on cooling capacity

Increasing airflow significantly increased cooling capacity for ECVCB and ECVPB, particularly at 0.4 and 1.0 m/s, confirming the role of airflow in accelerating evaporation^16,18,27. Conversely, ECVTECH showed limited change, and ECVPP showed reduced cooling at higher airflow due to lower water storage and premature drying. These results are consistent with previous studies reporting a strong dependence of evaporative cooling capacity on airflow speed¹⁹. Overall, airflow is a key determinant of cooling performance, though its effects depend on vest design and water retention capacity.

Effect of increased relative humidity on cooling capacity

Raising relative humidity from 20% to 40% at 35 °C and 0.4 m/s resulted in an average 28% reduction in cooling capacity (Table 3), due to suppressed evaporation rates. This confirms that evaporative cooling is most effective in dry environments and is strongly limited by high humidity, in agreement with earlier studies^18,28 and thermodynamic models of heat and mass transfer²⁰.

Efficiency of evaporative cooling tests based on evaporated water

Cooling efficiency varied with vest structure, airflow, temperature, and humidity. Although increased airflow generally increased cooling power, it often reduced efficiency due to excessive water consumption and partial loss of latent heat to the environment. For most vests, higher airflow led to lower efficiency, as observed in ECVPB and ECVPP. An exception was ECVTECH, which showed higher efficiency at 1.0 m/s, likely due to its design optimizing heat exchange. Dry conditions favored higher efficiency, whereas increased humidity reduced efficiency by diminishing the vapor pressure gradient. These findings highlight the importance of optimizing vest design to balance cooling power and water-use efficiency.

Effect of water evaporation on thermal images

Thermal images (Fig. 5) corroborated quantitative results, showing progressive drying and surface temperature increases with time, particularly at higher airflow. ECVCB and ECVPB maintained larger cool (blue) regions, reflecting better moisture retention and sustained cooling, whereas ECVTECH and ECVPP showed expanded warm zones due to faster drying.

Metabolic implications of increased garment weight in evaporative cooling vests

While ECVs reduce heat stress, water absorption increases garment weight, potentially elevating metabolic and thermal strain during physical activity. Such effects cannot be assessed with a stationary thermal manikin and warrant future human-based studies to balance cooling benefits against metabolic costs²⁹.

Toward a more realistic assessment: the role of underlayer garments in cooling vest performance

A limitation of this study is the absence of an underlayer garment between the vest and the manikin. In real use, ECVs are worn over clothing, which can alter heat and moisture transfer. Although the protocol followed ASTM F2371:2016, future studies should incorporate a standardized underlayer (e.g., a cotton T-shirt) to provide a more realistic assessment of cooling performance under practical conditions.

Conclusion

This study evaluated the cooling capacity and efficiency of evaporative cooling vests under varying ambient temperature, relative humidity, and airflow conditions, demonstrating that cooling vest performance is strongly governed by substrate material, water-holding capacity, airflow interaction, and structural design. Among the tested cooling vests, cellulose-based evaporative cooling vest, with high permeability and superior water retention, showed the highest performance in hot and dry environments (35–40 °C, 20% RH) due to enhanced evaporative cooling.

The results further indicated that increasing ambient temperature generally enhances cooling performance by promoting evaporation, while airflow accelerates heat removal but may reduce long-term efficiency in cooling vests with insufficient water retention. These findings highlight the importance of optimizing water storage capacity; maintaining approximately 1 kg of stored water in the cooling layer per charge is recommended to ensure sustained performance.

Conversely, increased relative humidity significantly reduced cooling capacity and efficiency by suppressing evaporation, confirming that evaporative cooling vests are most effective in hot, dry climates. From a design perspective, cellulosic substrates showed higher cooling capacities compared with polymer-based materials, emphasizing the critical role of fabric selection and core-layer structure in maximizing evaporative efficiency. Proper airflow management, adapted to environmental conditions, is essential to balance cooling power and water conservation.

Overall, this study provides practical guidance for the design and application of evaporative cooling vests for users exposed to extreme heat, including industrial workers and athletes. It also establishes a foundation for future research on advanced materials, optimized structural designs, and improved evaluation methods, such as the use of sweating thermal manikins, to better represent human thermoregulation and real-world performance.

Limitations

A primary limitation of this study is the use of a dry thermal manikin, which cannot simulate sweating or evaporative heat loss. As a result, key aspects of human thermoregulation—including skin wetness, vapor diffusion through the garment, and moisture transport—were not represented. Given the central role of evaporation in cooling performance, this limitation may lead to an underestimation of the actual cooling potential of evaporative cooling vests, particularly those designed to enhance sweat evaporation.

Because a sweating thermal manikin was not available, the analysis was restricted to conductive heat transfer between the manikin surface and the garment under different environmental conditions. Although this approach is consistent with standardized laboratory evaluations, a complete assessment of garment thermal performance should also incorporate evaporative and convective heat transfer mechanisms.

In addition, even advanced thermal manikins cannot fully reproduce complex human physiological and behavioral responses, such as spatially and temporally variable sweating, transient skin wetness, body movement, and microclimate changes at the skin–garment interface. Accordingly, future studies should employ sweating thermal manikins or controlled human trials under realistic environmental and activity conditions to more comprehensively evaluate the combined effects of garment structure, fit, motion, and evaporative cooling on performance in practical use scenarios.

Suggestions for future research

The results of this study highlight the need for continued development of evaporative cooling materials and vest designs that better exploit airflow and temperature effects. Accordingly, future research should consider the following directions:

1. For environments with extreme heat (> 40 °C) and variable airflow, hybrid cooling systems combining evaporative materials with phase change materials (PCMs) should be investigated to enhance cooling efficiency and extend cooling duration.

2. Future studies should evaluate evaporative cooling vests using sweating thermal manikins to account for natural perspiration, enabling more accurate assessment of evaporative heat loss under different environmental conditions.

3. Performance assessments under real-world conditions and extended wearing periods, including human trials, are recommended to better understand durability, long-term effectiveness, and practical usability. Further research should also explore novel lightweight fabrics and highly permeable structures with improved water absorption and evaporation characteristics to optimize cooling performance.

Abbreviations

ECVs

Evaporative cooling vests

ECVPP

Polymer-based punched evaporative cooling vest

ECVPB

Polymer-based evaporative cooling vest

ECVCB

Cellulose-based evaporative cooling vest

ECVTECH

TECHNICHE evaporative cooling vest

Author contributions

H.D developed and planned the experimental design, and N.S and A.D conducted the experiments and analyzed the data. H.D and N.S were responsible for writing specific subsections and for editing all parts of this work. All the authors read and approved the final manuscript.

Funding

This research was supported by the Research Committee of Isfahan University of Medical Sciences and Health Services (No: 140126).

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The corresponding author (H.D.) is the designer of the HIFITCOOL cooling vests evaluated in this study. This potential conflict of interest has been disclosed and managed. The study design, data collection, analysis, and interpretation were conducted independently. All other authors declare that they do not have any competing interest.

Ethics approval and consent to participate

Ethics approval This research was approved by the Medical Ethics Committee of Isfahan University of Medical Sciences (IR.MUI. RESEARCH.REC.1402.074).