Abstract
Aim
This study aimed to experimentally verify the safety and effectiveness of evaporative plus convective cooling used in heat stroke treatment using a doll simulating a patient with heat stroke.
Methods
Evaporative plus convective cooling was simulated by blowing air through a fan (speed: approximately 1.0 or 2.5 m/s) and using normal (20°C) or slightly warm (40°C) water on a doll whose surface body temperature was set at 40°C. We measured the change in surface body temperature using a surface heater attached to the back of the doll's chest cover and observed aerosol generation (size: ≥5 μm) using a particulate visualization system. Three particle counters were placed to measure the generated particles that were not captured by the particulate visualization system.
Results
The cooling effect of the 2.5 m/s wind speed was greater than that of the 1.0 m/s wind speed. No particles >0.5 μm were observed, and no aerosol particles were generated.
Conclusions
Our results thus suggested that wind force has a significant effect, and there was no risk of aerosol‐related viral infection in evaporative plus convective cooling. However, this does not rule out the risk of droplet infection.
Keywords: aerosols, COVID‐19, heat exhaustion, heat stroke, wind
Heat stroke treatment simulation was conducted using dolls resembling heat stroke patients, and a greater cooling effect was observed with a typical electric fan than with a handheld fan. Because no aerosol was generated from the body surface by evaporation, there was no risk of aerosol‐related viral infection.
INTRODUCTION
Heat stroke has become a public health concern worldwide. 1 , 2 Evaporative plus convective cooling (EPCC) is a frequently used method that decreases a patient's body temperature by moistening the body surface and using a fan to evaporate and remove the heat. However, the relationship between the temperature of the liquid that moistens the body surface and the wind power of the fan and its cooling effect has not been reported. 3
It is difficult to distinguish heat stroke and coronavirus disease 2019 (COVID‐19) based on appearance alone. 4 The virus is likely to be present on the body surface of patients with COVID‐19. 5 If aerosols could be generated when patients with COVID‐19 were treated for heat stroke with EPCC and water evaporated from the body surface, the aerosols could catch the virus on the patients' body surface and become pathogenic particles. Fan‐blown air can spread the pathogenic particles over a wider area than the 2–3 m range, which represents a risk range of droplet infection and increased COVID‐19 infection. Considering these risks, the implementation rate of EPCC within active cooling for heat stroke treatment in Japan has decreased from 77.9% to 61.6%. 4 , 6
This study aimed to experimentally verify the safety and effectiveness of EPCC used for treating heat stroke, using a doll to simulate a patient with heat stroke treated with EPCC, in relation to the actual cooling effect with respect to the water temperature that moistens the body surface, the wind force of the fan, and the aerosol generated from the body surface.
MATERIALS AND METHODS
Definition
We defined infectious aerosols as particles >0.5 μm, consistent with that reported in a previous study. 7
Study design
In this simulation study, we first determined whether aerosols were generated as the water evaporated at temperatures <100°C. Next, we applied the EPCC method to a doll with a surface body temperature of 40°C to simulate a patient with heat stroke. Four patterns of EPCC methods were performed with normal (20°C) or slightly warm (40°C) water on the body surface and at a wind speed of 1.0 m/s or 2.5 m/s. The doll was placed in a quiet environment in a clean room. We observed the generation of aerosol particles from the doll surface using the particulate visualization system “Type‐S” (Shin Nippon Air Technologies Company Limited, Tokyo, Japan), which visualizes the scattering of droplets generally >0.5 μm.
The study protocol was reviewed, and the need for approval was waived by the Teikyo University Ethical Review Board for Medical and Health Research, as this study did not include humans.
Equipment
We used the particulate visualization system to visualize the dispersal of droplets generally >5 μm while maintaining noiseless conditions in a clean room.
We collaborated with the Shin Nippon Air Technologies Company Limited (Tokyo, Japan) (SNK) to use the particle visualization system and conducted the experiment with the following setup. We adopted a light‐emitting diode light source dedicated to particle visualization, “Parallel Eye D” (SNK product), as lightning, high‐sensitivity camera dedicated to particle visualization, “EyeScope7” (SNK product) as a camera, basic image processing package software “Particle Eye” (SNK product) for image processing, portable particle visualization system “Type‐S” (SNK product) and particle counter “Model 9303” (TSI Incorporated, Minnesota, USA, product) for particle counting, thermal imaging camera “A6751” (FLIR Systems Incorporated, Oregon, USA, product) and data logger “GL800 (T thermocouple)” (GRAPHTEC Incorporated, Kanagawa, Japan product) as temperature measurement and anemomaster “Model 6006‐D0” (KANOMAX JAPAN Incorporated, Osaka, Japan product) for wind speed measurement.
The particle counter “Model 9303” can measure the number and concentration of particles in the three particle size ranges (0.3–0.49 μm, 0.5–4.9 μm, and >5.0 μm). The portable particle visualization system “Type‐S” can measure the number of particles sized >0.5μm and with a large measurement area (20 cm × 4 cm).
Setting
Were aerosols generated as the water evaporated at temperatures <100°C?
We filled two stainless steel pads with water at 40°C and 80°C each and blew air on the water surface with a fan to observe whether aerosols were generated from the surface as the water evaporated at temperatures that did not reach 100°C. Aerosol generation from the water surface during the 5 s after the pad lid was removed was compiled as an accumulated image.
Simulation experiment using EPCC on a doll with a surface body temperature of 40°C
To simulate the EPCC for a patient with heat stroke, air was blown using a fan on a doll whose surface temperature was controlled by moistening it with warm water. The generation of aerosol particulates observed using the particulate visualization system is shown in Figure 1.
FIGURE 1.
Setup of the particle visualization system with evaporative plus convective cooling. To ensure that there are no particles upstream of the airflow from the doll's chest, a particle counter was installed at the exit of the wind tunnel. To measure the particles evaporated at the chest, a particle counter was installed on each side of the downstream side of the airflow, and the portable particle visualization system “Type‐S,” which has a larger measurement area (20 cm × 4 cm), was installed in the center. The particle counters measure the concentration of particles across three size ranges (0.3–0.49 μm, 0.5–4.9 μm, and ≥5.0 μm), while the portable particle visualization system measures the number of particles sized ≥0.5 μm.
The cooling air was generated with a fan filter unit (FFU), so that particles other than those generated by evaporation would not be measured. Because the wind originates at a point slightly away from the patient, we used wind speeds of 1.0 m/s and 2.5 m/s at a distance of 60 cm away from the FFU for the experimental condition. The wind speed of 1.0 m/s is equivalent to a handheld fan and that of 2.5 m/s to a typical electric fan. The following four patterns of EPCC were used:
20°C water on the body surface and with the wind speed of 1.0 m/s;
20°C water on the body surface and with the wind speed of 2.5 m/s;
40°C water on the body surface and with the wind speed of 1.0 m/s;
40°C water on the body surface and with the wind speed of 2.5 m/s.
In addition, the room cleanliness was operated at a low level (to ensure no interference with the cooling airflow) to avoid measuring dust from people as much as possible. Since this study examined the risk of infection associated with aerosol generation, the method of applying moisture to generate aerosols was not investigated in this study.
Temperature management
Were aerosols generated as the water evaporated at temperatures <100°C?
We set the water temperature at 40°C and 80°C.
Simulation experiment using EPCC on a doll with a surface body temperature of 40°C
The upper body of the doll was photographed with a thermal imaging camera (FLIR Systems Incorporated, Oregon, USA, product). A point on the chest was pinpointed and measured using a data logger (GRAPHTEC Incorporated, Kanagawa, Japan product). In the experiment, the temperature data from the data logger were used as the doll's surface temperature. A rubber heater (surface heater) was attached to the back of the doll's chest cover, and the surface temperature was controlled by adjusting the input voltage (Figure 2).
FIGURE 2.
Controlling the surface temperature of the doll. A rubber heater (surface heater) was attached to the back of the doll's chest cover, and the surface temperature was controlled by adjusting the input voltage. (A) Back side of the doll's chest cover. (B) Rubber heater.
Assuming the body temperature of a patient with heat stroke to be 40°C, the surface temperature of the doll was set to 40°C at the beginning of EPCC. Low‐dust paper was used to moisten the body surface with slightly warm water at 40°C at the beginning of EPCC. The temperature of the doll's surface was measured with a thermometer every 10 s, while EPCC was being performed. The average temperature of the laboratory was 24.5°C, and the average humidity was 51.5%, which was almost constant.
The surface temperature of the doll was measured every 10 s. Because of the possibility of outliers, the maximum and minimum values were excluded from five measurement results, and the average of the three remaining results was calculated.
Particle counting
Were aerosols generated as the water evaporated at temperatures <100°C?
We visually assessed the accumulated still images.
Simulation experiment using EPCC on a doll with a surface body temperature of 40°C
A portable particle visualization system was placed in the center. One minute of accumulated still images was created from the start of cooling. This setup made it possible to visualize the particles generated in 1 min. Then, two particle counters were placed on each side of the downstream side of the airflow. On both sides of the doll, the number of particles generated was counted so that they were not missed.
To confirm that there were no particles upstream of the airflow from the doll's chest, a particle counter was installed at the exit of the wind tunnel. This made it possible to count the particles that were generated when the fan‐blown air exited the tunnel. These particles were not generated by the EPCC method and hence must be clearly distinguished.
Data analysis
Were aerosols generated as the water evaporated at temperatures <100°C?
We did not perform any statistical analysis of the data.
A simulation experiment using EPCC on a doll with a surface body temperature of 40°C
To determine whether cooling of the doll's surface was achieved for each pattern of wind speed and liquid temperature settings, we examined the doll's surface temperature and recorded the number of particles generated. We did not perform any statistical analysis of the data.
RESULTS
Were aerosols generated as the water evaporated at temperatures <100°C?
When the stainless pad was filled with water at 80°C, we could observe the generation of many particles, whereas only a small number of particles were generated at 40°C (Figure 3).
FIGURE 3.
Accumulated still images taken for 5 s after the pad lid was removed using the high‐sensitivity camera in experiments observing particle formation from the water surface at (A) 40°C and (B) 80°C. Accumulated still images indicate accumulated images in which video images are stored.
Simulation experiment using EPCC on a doll with a surface body temperature of 40°C
We assessed the accumulated still images from the high‐sensitivity camera for 1 min after the start of the measurement (Figure 4A–D).
FIGURE 4.
Accumulated still images taken for 1 min using the high‐sensitivity camera after measurement. Accumulated still images indicate accumulated images in which video images are stored. In this study, video images were stored for an arbitrary period of approximately 1 s after the start of cooling. No particle generation was observed in the accumulated still images. (A) 20°C water on the body surface and with the wind speed of 1.0 m/s; (B) 20°C water on the body surface and with the wind speed of 2.5 m/s; (C) 40°C water on the body surface and with the wind speed of 1.0 m/s; (D) 40°C water on the body surface and with the wind speed of 2.5 m/s.
The three particle counters placed on both sides of the doll and at the exit of the wind tunnel did not register any particles. In all four patterns, no significant particle generation was noted in either the visualization images or the particle counting during EPCC. The temperature transitions of the doll's surface after the initiation of cooling are shown in Figure 5.
FIGURE 5.
Temperature transition on the doll surface after the start of cooling. The graph shows the temperature evolution from the doll's surface from the start of evaporative plus convective cooling. (a) 20°C water on the body surface and with the wind speed of 1.0 m/s; (b) 20°C water on the body surface and with the wind speed of 2.5 m/s; (c) 40°C water on the body surface and with the wind speed of 1.0 m/s; and (d) 40°C water on the body surface and with the wind speed of 2.5 m/s.
The cooling effect of the 2.5 m/s wind speed was greater than that of the 1.0 m/s wind speed.
DISCUSSION
The novelty of this study is that the simulation revealed that aerosol is not actually generated, although the danger of EPCC had been speculated based on the theory that aerosol is generated when water evaporates.
Four different combinations of water temperature and wind speed in the EPCC method, observed over a 1‐min cumulative still image, produced no particles larger than 0.5 μm. 7 Since previous studies have shown that particles smaller than 0.5 μm are not infectious, we believe that with standard precautions, there is no risk of viruses being trapped in the aerosol and spread over a wide area by fan blast from the patient's body surface.
Moreover, we found that at a relatively high temperature of 80°C, particles were generated as a result of water evaporation, whereas at a relatively low temperature of 40°C, which is close to human body temperature, almost no particles were generated. Therefore, we believe that the theory that aerosols are generated when water evaporates is limited only to high temperatures near 100°C (80°C) and that this theory should be ruled out for temperatures near human body temperature (40°C). This result is consistent with the experimental results of the observation of no particle formation from the doll surface when EPCC is implemented. Moreover, it may also be useful in studying aerosol generation at public bathhouses and shower washing workstations, as we verified the safety of heat stroke cooling methods in this study.
In this experiment, we also confirmed that the temperature of the doll's surface was lowered by EPCC. This may be due to the effect of moisture on the surface, which had become water vapor, thereby decreasing the heat of vaporization. These findings support the use of EPCC as an active cooling technique widely used for heat stroke. Though the effect of temperature of the liquid that moistens the body surface is small, the effect of wind speed is likely to be significant. In the EPCC, either a manual fan or an electric fan is used; however, an electric fan should be used because it is difficult to maintain consistent air speed manually. 2 , 3
EPCC is already recognized as one of the most common active cooling methods and considered as a simpler alternative to cold water immersion or advanced temperature management devices. However, the results of this study suggest that a strong wind speed is required for maximum effectiveness. In disaster and prehospital settings, adequate preparations could increase the effectiveness of early treatment for heat stroke patients and enable EPCCs to play a more significant role in these scenarios. 3
This study showed that performing EPCC is as safe as performing other methods by taking standard precautions. Since the cooling effect may vary depending on the wind speed for EPCC, in actual clinical practice, it is necessary to make reasonable preparations concerning electric fans to achieve the most effective wind speed.
Limitations
This study has some limitations. First, experiments observing particle formation from the water surface at 40°C and 80°C were limited to simple observations, indicating the need for further theory development. Moreover, to consider how water evaporates when applied to various fields, it is necessary to make observations such as those in this study, assuming the conditions of each field.
The simulation experiment was repeated for the four patterns of EPCC. The body surface was moistened using a wet towel, just as practiced at the researcher's institution. For clinical application, it may be advisable to consider the method of applying moisture (wet towel or misting spray) and the amount of moisture. However, since this study examined the risk of infection associated with aerosol generation, the method of evaporation should be considered rather than the method of applying the moisture. Additionally, at this stage, we do not believe that this concern will significantly compromise the study results. Nevertheless, further simulations and clinical studies are necessary to demonstrate a more robust safety profile.
The study did not show that the EPCC is completely safe because even though there was no risk of infection from aerosols, there was still a risk of contact and droplet infection during the COVID‐19 pandemic. Therefore, we should continue to treat patients with COVID‐19 with standard precautions.
Our findings suggest that wind speed is associated with the cooling effect; however, this needs to be verified in actual clinical practice. Therefore, we need to reevaluate which method is optimal in actual clinical practice. Specifically, the temperature of the water used to moisten the surface and the speed of the fan‐blown air needs to be verified keeping in consideration the actual cooling effect and the discomfort of the patient.
Finally, we have shown the safety of EPCC concerning aerosol generation, but we did not compare it with other cooling methods. Therefore, when choosing which active cooling method should be implemented for patients with heat stroke in each hospital, the decision should be made after examining the facilities present in each hospital as well as the results of other studies.
Despite the limitations described above, we believe that the results of this study will be valuable in utilizing EPCC for treating heat stroke.
CONCLUSIONS
This study showed that there was no risk of aerosol‐related infection specifically due to the convection and evaporation in EPCC. Moreover, our findings suggested that the effect of wind force is likely to be significant. Electric fans should thus be used to maintain a consistent wind speed. This study does not rule out the risk of contact or droplet infection. Standard precautionary measures should be continued when performing the EPCC method in treating patients with heat stroke.
FUNDING INFORMATION
This work was supported by the Subsidy for Health, Labour and Welfare Administration Promotion Research Project to Yasufumi Miyake (grant number 21LA2004) and by the Grants‐in‐Aid for Scientific Research KAKENHI grant to Jun Kanda (grant number 19K18365). The funding agencies provided grants to the authors without intervening in the study and had no role in study design; in the collection, analysis, and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.
CONFLICT OF INTEREST STATEMENT
Authors declare no conflicts of interest for this article.
ETHICS STATEMENT
Approval of the research protocol: The study protocol was reviewed, and the need for approval was waived by the Teikyo University Ethical Review Board for Medical and Health Research, as this study did not include humans.
Informed consent: N/A.
Registry and the registration number of the study/trial: N/A.
Animal studies: N/A.
ACKNOWLEDGMENTS
We thank Shin Nippon Air Technologies Company Limited (Tokyo, Japan) for their cooperation. We would like to thank Editage (www.editage.com) for English language editing and journal submission support.
Kanda J, Miyake Y, Sakamoto Y, Yoshiike S, Takeuchi T, Kanda T, et al. Evaluation of aerosol generation and cooling effects of evaporative plus convective cooling in heat stroke treatment: A simulation study. Acute Med Surg. 2025;12:e70023. 10.1002/ams2.70023
DATA AVAILABILITY STATEMENT
The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.