Experimental study on the variations in human skin temperature under simulated weightlessness

Accepted Manuscript Experimental study on the variations in human skin temperature under simulated weightlessness Hui Zhu, Hanqing Wang, Zhiqiang Liu, Guangxiao Kou, Can Li, Duanru Li PII:

S0360-1323(17)30094-X

DOI:

10.1016/j.buildenv.2017.03.008

Reference:

BAE 4840

To appear in:

Building and Environment

Received Date: 15 December 2016 Revised Date:

3 March 2017

Accepted Date: 4 March 2017

Please cite this article as: Zhu H, Wang H, Liu Z, Kou G, Li C, Li D, Experimental study on the variations in human skin temperature under simulated weightlessness, Building and Environment (2017), doi: 10.1016/j.buildenv.2017.03.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Experimental study on the variations in human skin temperature under simulated weightlessness Zhu Hui a,b, Wang Hanqing a,b,c,*, Liu Zhiqiang a, Kou Guangxiao c, Li Can c and Li Duanru c a

School of Energy Science and Engineering, Central South University, Changsha, 410083, China; b

School of Civil Engineering, University of South China, Hengyang, 421001, China

Collaborative Innovation Center for Energy-conservation in Buildings and Environment Control, Zhuzhou, 412001, China

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Abstract: To study the variations in skin temperature in weightlessness, the skin temperatures of 6 male volunteers were measured under simulated weightlessness by head down bed rest (HDBR) experiments. The effects of the air temperature, relative humidity and air speed on the mean skin temperature were scrutinized, and the regional skin temperatures of the subjects under comfortable conditions were investigated. The results showed that the mean skin temperature increased with the air temperature both before and

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after HDBR in low and neutral air temperatures and that the mean skin temperature was found to be higher before HDBR. However, a higher mean skin temperature was observed after HDBR when the air temperature was high. Moreover, the mean skin temperature in low and neutral air temperatures was found to increase with the relative humidity both before and after HDBR, and a higher mean skin temperature was also observed before HDBR. Additionally, a negative correlation between the mean skin temperature and air

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speed was observed both before and after HDBR. However, different patterns of the mean skin temperature were observed when the environment changed from low air temperature and humidity to a neutral environment and finally to a high air temperature and humidity. Finally, the regional skin temperature after HDBR showed a different distribution compared with that before HDBR, such as a higher skin temperature in the thorax, forehead and back and a lower skin temperature in the thigh, calf and hand. This indicated that the skin temperature distribution changed greatly under simulated weightlessness by HDBR, which might suggest an altered thermal regulatory mechanism in humans experiencing weightlessness.

1 Introduction

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Key words: skin temperature; weightlessness; air temperature, relative humidity, air speed, HDBR

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Human skin temperature is an important physiological parameter reflecting human responses to the thermal environment, and it is a direct reflection of the thermal comfort and thermal sensation of the human body. As stated in ASHREA 55-2013, thermal comfort is the condition of mind in which satisfaction is expressed with the thermal environment [1]. This definition of thermal comfort consists of both its physiological and psychological aspects, where skin temperature often plays an important role [2]. Therefore, the skin temperature is often measured as an essential physiological parameter for the evaluation of human thermal comfort. Human thermal comfort in buildings in terrestrial conditions has been studied for decades, and many sophisticated theories and research models have been advanced. Usually, air temperature, air humidity, air speed, mean radiant temperature, clothing resistance and metabolism are thought to be the 6 major factors influencing human thermal comfort on earth [3]. Additionally, it is easy to judge the comfort level of humans simply by examining their physiological parameters, such as the mean skin temperature. However, what will happen to human skin temperatures when they live and work in a weightless environment? Will the mean skin temperature still be effective to judge thermal comfort? Findings from the recent studies on thermal comfort in terrestrial conditions may not answer these questions because weightlessness was not taken in account in these studies, and there is not a thermal comfort standard applicable to the weightless environment. Therefore, the research in this paper is actually an extension of the studies on human thermal comfort. Human skin temperature is influenced directly by the skin blood flow and its distribution. The cardiovascular system is the first one to respond to weightlessness when a man is exposed to a weightless environment [4]. Several adaptive changes of the cardiovascular system occur, including but not limited to blood redistribution, altered cardiovascular function and reduced plasma volume, among which blood redistribution has conspicuous effects on skin temperature. The blood flows up to the torso and head from the lower body due to the absence of

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the gravity, which may result in a higher skin temperature in the upper body and a lower one in the lower body. Data from an early spaceflight suggested that the skin temperatures in the thorax, back and head increase, while the temperatures in the feet, hands, calves and even thighs are reduced to a lower level [5,6]. Findings by Lakota [7] and Qiu[8] regarding an increased chest-foot skin temperature gradient either in simulated or real weightlessness confirmed this result. In addition, research from the Chinese Aerospace Medical Engineering Research Institute revealed that changes of the circadian rhythms of the subjects occurred during the early stages of the terrestrial simulation of weightlessness. In addition, the concentration of heat stress proteins in lymphocyte cytoplasm increased during the simulation, as did the skin temperature in the head and neck[9，10]. Additionally, during head down bed rest, it was observed that there was a significant reduction in the skin temperature of the extremities, which was thought to be primarily ascribed to the hypovolemia caused by weightlessness [11, 12]. As for research from a manned spaceflight, the skin temperature data of 3 astronauts from Salyut-6 were analyzed by Russian scientist Ludvki Novak. The results suggested that the skin temperature in the thorax increased by 3℃ compared with that in a terrestrial condition, and the skin temperatures in the opisthenar and the foot were found to decrease by approximately 2 ℃ and 3℃, respectively[13，14]. Russian astronaut Polyakov analyzed and summarized the skin temperature data of another 12 astronauts during a 6-month spaceflight, and results similar to Novak’s were obtained [15]. In addition, his findings from the skin temperatures of 2 astronauts onboard the Mir station also agreed with the results of Novak [16]. For changes in mean skin temperature, Yang compared the mean skin temperature of 7 male subjects in a 29-day head down bed rest experiment in 2 different ventilation modes (constant air flow (CAF) and simulated natural air flow (SNAF)). The results showed that the mean skin temperature decreased under both CAF and SNAF on the 29th day of head down bed rest [17]. However, the results of another experiment under simulated weightlessness with horizontal bed rest indicated that the mean skin temperature increased insignificantly while the rectal temperature remained stable (horizontal bed rest, 11 subjects) [18]. The review outlined above highlights previous research studying the skin temperature in real and simulated weightlessness. However, these studies were conducted at steady state. For example, the data from the spaceflight were acquired in the comfortable environment of the spaceship, and the terrestrial simulation of weightlessness was usually carried out in a neutral air temperature, humidity and air speed. As a result, these results cannot be taken as evidence of the skin temperature variations in weightlessness. In this study, the skin temperature of 6 male volunteers were measured under 5 different air temperatures, 4 relative humidities and 3 air speeds in a -6°head down bed rest (HDBR) experiment. In addition, the skin temperatures were also measured under the same conditions before HDBR to provide a basis for comparison. The experimental results yielded a full description of the variations of human skin temperature under dynamic conditions.

2 Methodology 2.1 Subjects

Six healthy males volunteered to participate in the experiment. Their ages ranged from 19 to 22, with heights ranging from 166 to 174 cm and weights ranging from 53 to 61 kg. The subjects wore only pirate shorts of the same material to eliminate the influence of the thermal resistance of the clothes. In addition, during the experiment, they were instructed to keep still and calm. Physical examination of heart and lung functions were also carried out before the experiment to avoid risks in the HDBR experiment. Volunteers with latent or dominant heart disease were not allowed to take part in the experiment. All of the subjects were sophomores at a university majoring in the field of heating, ventilation and air-conditioning and were thus familiar with experiments in human thermal comfort. Even so, a brief training on the behavior to adopt during the experiment was conducted before the experiment. Table 1 summarizes the subjects’information.

ACCEPTED MANUSCRIPT Tab. 1 Brief information of the subjects Gender

Age

Height (cm)

Weight (kg)

BMI (kg/m2)

1

male

20

171

55

18.81

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male

20

166

53

19.23

3

male

18

174

61

20.15

4

male

19

168

55

19.49

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male

19

167

58

20.80

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male

21

170

60

2.2 Experiment design

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A -6 ° head down bed rest (HDBR) experiment is the most effective terrestrial model to simulate weightlessness for space medicine research [19]. During a HDBR experiment, the body fluid shifts and loss occurs in the first 7 days. Thereafter, vascular remodeling and adaptive changes in neural functions take place, as the human cardiovascular system starts to function according to the mechanism in weightlessness [20]. Taking this into consideration, our experiment was divided into 3 stages and lasted for 14 days. For the first stage (Day 1 to Day 3), a skin temperature measurement before HDBR was conducted to provide a basis for comparison. The second stage of the experiment was conducted from the 4th day to the 10th day. During this stage, the 7-day long -6°head down bed rest (HDBR) experiment was carried out, during which all subjects were maintained lying on their backs. No strenuous exercise or out-of-bed activities, except excretion, was allowed. In addition, no more than 10 minutes for excretion were allowed every day. The 7-day HDBR experiment was conducted in an air temperature of 26±0.5℃, a relative humidity of 60±5% and an air speed of 0.1-0.3 m/s, which is considered a comfortable or neutral environment. During the HDBR, skin temperature, core temperature, heart rate variability, blood pressure and other physiological parameters were recorded. The last stage started on the 11th day and lasted until the end of the experiment. During this stage, the data acquisition of the skin temperature of all subjects in the different environments was conducted and all subjects were still required to maintain a position of -6°head down and lie on their backs. The different environments were created rapidly by the walk-in environmental chamber (Guangzhou Hongzhan, Inc., customized equipment; accuracy: 0.1℃ temperature, 0.1% relative humidity, 0.1 m/s air speed) that has a volume of 13.2 m3. The air temperatures, humidities and speeds it can create are -20-100℃, 0-95% and 0.1-1.5 m/s, respectively. Five different air temperatures (22℃, 24℃, 26℃, 28℃ and 30℃, in order) were chosen for this experiment, which included lower temperatures (cool), neutral temperatures and higher temperatures (hot). In addition, 4 different relative humidities (30%, 45%, 60% and 80%) were chosen, which included lower humidities, neutral humidities and higher humidities. Three air speeds were also selected, including low speed (0.2 m/s), neutral speed (0.5 m/s) and high speed (0.8 m/s). In total, there were 60 different combined environmental conditions. During the experiment, all subjects were kept blind to the exposure temperature, humidity and air speed.

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(a) The front view (b) The left view Fig. 1 The schematic of the walk-in environment chamber In the field experiment, the air temperature was selected as 22-30℃ to be consistent with the cabin temperature of spacecraft, as recommended by NASA (23±4℃) [21]. However, an increased cold sensitivity [22] and an attenuated sweating [23] under real or simulated weightlessness have been reported in previous studies. Therefore, when designing the air temperature, a greater lower limit (22℃) and upper limit (30℃) were selected. A medium temperature interval of 2℃ was selected to realize the various environmental temperatures and shorten the experiment time. In addition, the relativity humidity was selected as 30%-80% to be in agreement with the humidity requirement of the spacecraft cabin, as recommended by NASA (25%-75%) [21]. Finally, air speed was selected to be 0.2 m/s to be consistent with the forced ventilation through the fans in spacecraft cabins [21]. Air speeds of 0.5 and 0.8 m/s were also selected to further investigate the influences of stronger air flow under simulated weightlessness. According to the results of a previous study, the human body spends at least 15 minutes to adapt to a new environment [24]. Therefore, the experiment for each environment condition lasted 20 minutes, including 15 minutes for adaption and 5 minutes for effective data acquisition. During the measurements of the skin temperature, an 8-point method was adopted according to ISO 9886:2004[25]. The 8 measuring points were the forehead, the right scapula (back), the left upper chest (thorax), the right arm in upper location (right forearm), the left arm in lower location (left lower arm), the left hand (left opisthenar), the right anterior thigh and the left calf. The enhanced medical tape was adopted to fix the PT100 thermistors (Shangyi Group, Inc., WZP-PT100 (Class A); accuracy: 0.1℃). All of the thermistors were calibrated and had an accuracy of 0.1℃. The thermistors affixed to each subject were connected to an 8-channel recorder (Yuyao Tenghui, Inc., THTZ802R; accuracy: 0.2% FS) to record all of the skin temperatures in the different environmental conditions continuously and in real time. Before HDBR, all subjects were required to seat themselves stilly and calmly in a chair. During HDBR, the bedsheet that was in direct contact with the skin of the subject’s back was composed of textilene to ensure good air-permeability. In addition, all subjects had to be kept emotionally stable to avoid inaccuracy induced by psychical sweating. Finally, the field experiment started 1 hour after a meal to avoid the inaccuracy induced by the metabolic change due to food intake. The experiments were not conducted in the early morning or evening to minimize the influence of circadian rhythms. 2.3 Experiment procedure A strict schedule was required during the experiment to avoid the abnormality of the physiological parameters due to irregular schedules. All measurements were performed from 8:30 to 11:30 in the morning and 14:30 to 17:30 in the afternoon, during which the subjects had to keep calm and emotionally stable. In the first stage of the experiment, the subjects were asked to seat themselves stilly, while in the second and third stages, the subjects were required to lie on their backs.

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2.4 Data analysis

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The experiment was conducted according to the following procedures. In the first stage, all subjects were asked to assemble in a thermally comfortable room 30 minutes before the experiment. Meanwhile, the environment chamber was switched on, and the parameters (air temperature, humidity and speed) of the first environmental condition were set up. The thermistors were then adjusted to the measuring points. Thereafter, a portable temperature and humidity measuring device (Testo. Inc, Testo435; accuracy: 0.2℃ in temperature, 2% in relative humidity, 0.01 m/s in air speed) was utilized to check whether the environment in the chamber reached the set values and were stable. When the air temperature, humidity and speed were stable, all subjects went into the environment chamber and sat still. After 15 minutes, the paperless recorders were switched on, and the skin temperatures data were recorded automatically. The experiment on each environmental condition (each combination of air temperature, humidity and speed) lasted for 20 minutes, during which other physiological parameters were recorded, including but not limited to core temperature, blood pressure and heart rate variability. When the measurement of the first environmental condition was completed, the air temperature, humidity and speed values were adjusted to the next environmental condition, and the same procedure was followed until the end of the measurement. At the second stage, all subjects were required to lie on their backs to generate the adaptive changes in weightlessness. During this stage, the skin temperature was still recorded but was not analyzed in this paper. The third stage of the experiment aimed to study the skin temperature changes in simulated weightlessness. During this stage, the same experimental procedure in stage 1 was adopted, but all subjects had to maintain a -6° head down bed rest during the entire stage, except during excretion. In addition, only 3 subjects could be accommodated during HDBR due to the limited space of the environment chamber. Therefore, subjects had to be divided into 2 groups. Three subjects were assessed in the morning, and the other 3 subjects were assessed in the afternoon. The skin temperature of each group member was measured at a fixed time. Specifically, subjects 1, 4 and 5 were assessed in the morning, while subjects 2, 3 and 6 were assessed in the afternoon.

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The sampling period of the paperless recorder was set to 10 seconds. Therefore, 30 skin temperatures for each measuring point were acquired in the last 5 minutes of each environmental condition. The arithmetic mean of the 30 temperatures was then calculated and used as the skin temperature at the measuring point in the environmental condition. The mean skin temperature (Tsk) of the whole body was calculated according to ISO9886. The calculation method is presented in the equation below:

Tsk = 0.07Tforehead + 0.175Tback + 0.175Tthorax + 0.07Tright forearm + 0.07Tleft lower arm

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+ 0.05Tleft opisthenar + 0.19Tright thig h + 0.2Tleft calf where Tsk is the mean skin temperature of the whole body, and Tback, Tthorax, Tright forearm, Tleft lower arm, thigh

Tleft opisthennar, Tright

and Tleft calf represent the calculated skin temperature of each measuring point, respectively.

The data analysis toolkit GraphPad Prism 6, broadly used in many research fields, including human thermal comfort [26, 27], was utilized to perform the statistical analysis in this study, with the significance level set at less than 0.05. As stated in a previous study, a positive relationship between human skin temperature and the air temperature combined with humidity before sweating has been reported, after which the skin temperature decreases gradually due to the evaporative cooling of the sweat [24]. In addition, a negative relationship between skin temperature and air speed has also been reported [24]. Taking this into consideration, linear regression and polynomial regression were employed to determine the relationship between the different variables during the data analysis. To find the best model, the Akaike Information Criterion (AIC) was utilized. The Akaike information criterion (AIC) is now widely used in the studies of human thermal comfort [28, 29]. It is usually used in a multivariate regression model to select which variables are chosen for the best fitting model and to select which is the best fitting model using the same

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3 Results

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3.1 The effects of air temperature on mean skin temperature

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Due to space constraints, not all of the data analysis results for the 60 environmental conditions are described here, and only certain typical conditions are shown. Specifically, when investigating the influence of air temperature, the relationship between mean skin temperature and air temperature was analyzed at 4 levels of relative humidity and an air speed of 0.2 m/s. The relative humidities used represented lower, neutral and higher levels, and the lower and upper limits were close to those of the humidity requirements in spacecraft. In addition, an air speed of 0.2 m/s is the usual speed of the ventilation in spacecraft [21]. Similarly, when studying the influence of air humidity, air temperatures, including lower, neutral and higher levels, and an air speed of 0.2 m/s were selected, which are close to the air temperature and speed requirements in spacecraft[21]. Finally, when investigating the influence of air speed, a neutral environment, environments with a lower air temperature and humidity, and environments with a higher air temperature and humidity were selected to explore the influence of air speed on mean skin temperature under different air temperatures and humidities.

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As for the analysis of the effects of air temperature on mean skin temperature, conditions with an air speed of 0.2 m/s were selected. The results are presented in Figure 2. Fig. 2 (a) to 2(d) illustrate the relationships between mean skin temperature and air temperature under the relative humidities of 30%, 45%, 60% and 80%, successively. The dash lines represent the mean skin temperatures before HDBR, while the solid lines indicate the mean skin temperatures after HDBR (under simulated weightlessness). The colors represent different subjects. From Fig. 2, it can be observed that the air temperature significantly influenced the subjects' mean skin temperatures. It was observed that increased air temperature resulted in a higher mean skin temperature, both before and after HDBR. However, there were some differences in the variation tendency.

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Little difference between the mean skin temperatures before and after HDBR was observed when the air temperature was low (22℃). However, the mean skin temperature before HDBR was found to be higher than that after HDBR, when the air temperature increased from 24℃ to 28℃ and the relative humidity was below 80% (see Fig 2(a), Fig 2(b) and Fig 2(c)). Furthermore, when the relative humidity reached 80% (see Fig 2(d)), the mean skin temperature of each subject before HDBR reached its maximal value at approximately 26℃, thereafter decreasing with the air temperature. However, the mean skin temperature of the subjects after HDBR still increased with the air temperature. The mean skin temperature of the subjects after HDBR increased somewhat slower when the air temperature reached 30℃ and the relative humidity was 80%, but it was still higher than that before HDBR (Fig 2(d)).

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Figure 2 Relationship between the mean skin temperature and air temperature 3.2 The effects of air humidity on mean skin temperature

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As for the analysis of the effects of air humidity on mean skin temperature, the conditions of air speed of 0.2 m/s and temperatures of 22℃, 26℃and 30℃ were selected. The results are presented in Figure 3. Fig. 3(a) to 3(c) illustrate the relationships between mean skin temperature and air humidity at 22℃, 26℃ and 30℃, successively. The dash lines represent the mean skin temperatures before HDBR, while the solid lines indicate the mean skin temperatures after HDBR (under simulated weightlessness). The colors represent different subjects (S1-S6).

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Fig. 3(a) illustrates that the mean skin temperature of the subjects increased slowly with the air relative humidity both before and after HDBR when there was a lower air temperature (22℃). The mean skin temperature of the subjects (except subject 1) had a lower value after HDBR. This phenomenon became more significant when there was a neutral temperature (Fig. 3 (b)). More complicated changes are presented in Fig. 3(c), when there was a higher air temperature (30℃). Fig. 3(c) illustrates that the mean skin temperature of the subjects both before and after HDBR increased when the relative humidity was below 60%. The subjects after HDBR had a higher mean skin temperature than that before HDBR. However, the mean skin temperature of the subjects both before and after HDBR dropped rapidly when the relative humidity was higher than 60%. However, the subjects after HDBR had a higher mean skin temperature than that before HDBR. Therefore, it is a high possibility that the subjects after HDBR had a lower sweat rate in the air temperature of 30℃ and relative humidity above 60%, although it was not certain whether they had detectable perspiration.

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Figure 3 Relationship between mean skin temperature and air humidity 3.3 The effects of air speed on mean skin temperature

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As for the analysis of the effects of air speed on mean skin temperature, 3 typical conditions were analyzed: a lower air temperature and humidity condition (22℃, 30%), a neutral environmental condition (26℃, 60%) and a higher air temperature and humidity condition (30℃, 80%). The results are presented in Figure 4. Fig. 4(a) to 4(c) illustrate the relationships between mean skin temperature and air speed in the 3 conditions, successively. The dash lines represent the mean skin temperatures before HDBR, while the solid lines indicate the mean skin temperatures after HDBR (under simulated weightlessness). The colors represent different subjects (S1-S6).

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Fig. 4 illustrates that the mean skin temperature of all subjects decreased linearly with the air speed, both before and after HDBR. However, the temperature reduction process for the subjects before and after HDBR had different characteristics. As shown in Fig. 4(a), the mean skin temperature of the subjects before and after HDBR showed no significant difference when there was a lower air temperature and humidity. However, when the environmental condition was a neutral one, the mean skin temperature of the subjects before HDBR was found to be significantly higher than that after HDBR, which was similar to that in Fig. 3(b). In contrast, the mean skin temperature of the subjects after HDBR was found to be higher than that before HDBR when there was a relatively higher air temperature and humidity.

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(c) Figure 4 Relationship between mean skin temperature and air speed 3.4 The regional skin temperature changes in a comfortable environment

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The research on the effects of air temperature, humidity and speed on mean skin temperature of the subjects proved that the variations in the mean skin temperature showed different patterns in simulated weightlessness, which suggested an altered thermal regulation of the subjects. Thus, how would the regional skin temperature of the subjects be affected in simulated weightlessness in a comfortable environmental condition? Generally speaking, an environment with an air temperature of 26℃, relative humidity of 60% and an air speed of 0.2 m/s is considered to be comfortable in a terrestrial condition[30]. An experiment on the changes in the regional skin temperature of the subjects before and after HDBR was carried out under such a condition, and the results are presented in Figure 5. A stacked bar graph is used in Fig. 5 to describe the skin temperatures at the 8 measuring points of each subject. The bars with colors represent the skin temperature of the subjects after HDBR (in simulated weightlessness), and the clear ones represent the skin temperatures before HDBR. Fig. 5 illustrates that great changes occurred in the skin temperatures of the subjects after HDBR. The skin temperatures of the forehead, thorax and back after HDBR were found to be higher than those before HDBR. Meanwhile, lower skin temperatures in the right forearm, left lower arm, left opisthenar, right thigh and left calf were observed. Furthermore, as shown in Fig. 5, a larger vertical difference in the skin temperature after HDBR was observed. There was a larger temperature difference, ranging from 2 to 4℃ between the thorax and calf for all subjects, which might induce thermal discomfort.

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Figure 5 Difference in the regional skin temperatures before and after HDBR Figure 6 compares the skin temperature distribution of the subjects before and after HDBR. The photos were shot by a thermal infrared imager (Flir., Inc., Flir T420; thermal sensitivity<0.04°C, accuracy: 2% FS). Fig. 6 (a) to Fig. 6 (f) represent the 6 subjects successively. In each photo, the left half represents the skin temperature of the subject before HDBR and the right half represents the skin temperature after HDBR. It can be observed that all subjects after HDBR had higher skin temperatures in the thorax than those before HDBR. Fig. 6 provides a more clear description of the difference in skin temperature distribution of the subjects before and after HDBR.

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4 Discussions

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It was observed that the subjects had a lower mean skin temperature under simulated weightlessness by HDBR when there were lower and neutral air temperatures. Our conclusion is in accordance with the findings from Yang, who conducted an HDBR experiment using the environmental conditions of 23±0.48℃, 55.2±5.7% and 0.2 m/s. Yang’s results showed a reduction in the mean skin temperature of 7 subjects from 33.54℃ to 32.9℃ during a 29-day HDBR experiment[17]. For comparison, the mean skin temperature in this study decreased from approximately 32.75℃ to 32.4℃ in Yang’s experimental conditions (Fig. 1(c)). The possible reason leading to this deviation may lie in the calculation method of mean skin temperature and the subjects’physique. Moreover, the reduction in mean skin temperature in simulated weightlessness (after HDBR) may suggest an impaired thermal regulation, as there is no evaporative cooling by sweat in lower and neutral air temperatures nor any other cooling devices in the experiment. Qiu’s[9] findings on an altered concentration in heat stress proteins (HSPs), which is closely related with thermal regulation of humans, also confirmed our prediction regarding an impaired thermal regulation after HDBR. It was also observed that the subjects after HDBR had a higher mean skin temperature than that before HDBR when there was a higher air temperature (e.g., 30℃). In contrast, the reverse was observed when there were lower and neutral air temperatures (Fig. 3 (a) and Fig. 3 (b)). The difference in mean skin temperature may result from a different sweat rate before and after HDBR at higher air temperatures. Subjects before HDBR may have sweat severely in a higher air temperature, causing a rapid drop in the mean skin temperature, while the subjects after HDBR may have just started sweating, with a sweat rate less than that before HDBR. In fact, sweat was observed in the subjects before HDBR at 30℃, while the sweat of the subjects after HDBR was not obvious. The possible reasons for the changes in sweating after HDBR are outlined as follows. First, the threshold air temperature for sweating in real or simulated weightlessness may have increased. The experiment by Michikami [23] indicated that attenuated sweating and cutaneous vasodilation were observed during a 14-day HDBR. Specifically, his results showed that HDBR reduced the sensitivity of cutaneous vasodilation by 40% in the chest and 31% in the forearm, while the threshold core temperature for sweating was increased by 0.31℃ in the chest and 0.32℃ in the forearm. This can explain the reason why the subjects, after HDBR at a higher air temperature, had a higher mean skin temperature than that before HDBR. The other factor that impacts sweating may be the body fluid loss induced by real or simulated weightlessness. Findings from Spacelab indicated that the plasma of the astronauts was reduced by approximately 17% during 9-day and 14-day missions [31]. In addition, the data from Gemini-4 and Skylab indicated that the plasma of the astronauts was found to be reduced by 10% during 30 days in spaceflight [32, 33]. Due to the loss of plasma, the osmotic pressure decreases. To strike a balance in the osmotic pressure, the intracellular fluid may be decreased, ultimately leading to fluid loss in the whole body. The body fluid loss may result in a reduction in sweat secretion, which explains the lower sweat rate of the subjects in simulated weightlessness by HDBR. In addition, the experimental results indicate that the regional skin temperature of the subjects was found to be similar to the findings of Novak [14], though the experimental conditions were different. In addition, the thorax-calf temperature gradient (2-4℃) observed in the experiment was determined to be close to the findings by Qiu (2-5℃)[8]. This deviation may be the result of the different experimental conditions (20.7±0.5℃, 50.7±9.3%, 0.5 m/s) and the numbers of subjects (5 subjects). Possible reasons for the changes in regional skin temperature are outlined as follows. The first reason may be the blood redistribution induced by simulated weightlessness by HDBR. The blood redistribution induced by real or simulated weightlessness brings about a fluid shift in the upper body, and therefore more blood circulates in the thorax and head and less in the arms and legs. As a result, a higher skin temperature in the thorax and head was observed, while the subject always felt cold in their lower body, especially in the extremities. This type of skin temperature distribution is more significant in a

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neutral environment and at high air temperatures. However, in a cold environment, a countercurrent heat transfer mechanism activates; thus, the heat transfer mainly occurs between the deep arteries and the venae comites, with less blood flow through the skin. This is also a good explanation for why the difference in mean skin temperature is inconspicuous at 22℃ before and after HDBR. Another reason responsible for the different skin temperature distribution may be the disuse-atrophy of the skeletal muscles, especially the muscles of the lower limb. Changes in the contraction characteristics of the skeletal muscle bring about changes in the components of the contractile proteins and regulated proteins. As a result, the consumption and synthesis of the materials involved in metabolism are inhibited or decreased, which may result in a reduction in heat production [34]. As for the short-term weightlessness and simulated weightlessness, the effects of blood redistribution are thought to be more significant [35]. One of the important characteristics of the weightless environment is the absence of the natural convection, or Rayleigh convection. The gravitational acceleration in weightlessness approximates 0, therefore, the Rayleigh number is not large enough to generate the buoyancy-driven convection. Instead, a surface tension-driven convection, the Marangoni convection, dominates in weightlessness [36]. A previous study has revealed that the human body surface has different regional sweat rates in the same ambient temperature [37]. As a result, a film of sweat on the surface of the skin may appear because of the Marangoni convection when a man sweats in weightlessness. The formation of the sweat film on the skin surface may increase thermal resistance and impair sweat evaporation [22]. Unfortunately, the simulation technique of weightlessness in a terrestrial condition cannot realize the absence of the natural convection and the dominance of the Marangoni convection during a field experiment, which is the reason why the skin temperature increase in the thorax and back is greater in Novak’s findings [13, 14]. Therefore, the results of this study may demonstrate some inaccuracy compared with that in real weightlessness, which represents a limitation of the experiment. Besides, as is stated in previous section, human thermal comfort is a mixed concept of physiological and psychological aspects. In our experiment, attention was paid to the physiological aspect. Meanwhile the psychological one was also fully considered. We did much work to keep the subjects emotionally or psychologically stable in the experiment. However, as is stated by Beysens D [36], a wide range of environmental factors would influence the emotion of humans in real weightlessness (space-flight), such as reduced and closed space, life-support restriction, far from civilization, lack of natural light and surrounding, lack of privacy, and monotony of daily life. The differences in psychology was not mimicked in the experiment. And this is another limitation of our research. Despite of the limitations, this study can still provide a useful reference for the study of the skin temperature variation in weightlessness and also demonstrate a new technique for the study of thermal comfort in weightlessness. Finally, the autonomic nervous system of human body, consisting of sympathetic nervous system and vagal nervous system, is involved in the control of thermoregulatory system [36]. For example, the sympathetic nerve system has close relationship with the sweating activity and vasodilation. In addition, the respiration and metabolism of human body are also controlled by autonomic nerve system [36], which is relevant to the human thermal balance. Therefore, the changes in skin temperature in weightlessness are affected intrinsically by autonomic nervous system. And future study on human skin temperature, or even thermal comfort and thermal sensation, may focus on this aspect.

5 Conclusions In this study, the changes in the skin temperature of 6 male subjects in simulated weightlessness by HDBR were investigated. It was observed that the mean skin temperature increased with the air temperature and that a higher temperature was observed in cool and neutral environments before HDBR. However, a higher mean skin temperature was observed in the subjects after HDBR when there was a higher temperature and humidity likely

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due to the cooling effect of the sweat of the subjects before HDBR, while the sweat of the subjects after HDBR was not conspicuous. It was also observed that the mean skin temperature increased with the relative humidity both before and after HDBR when there was a cool or neutral environment, while the subjects before HDBR had a higher mean skin temperature than after HDBR. However, when the air temperature was relatively high, a relative humidity of 60% was found to be the critical point for the changes in the mean skin temperatures. When the relative humidity was below 60%, the mean skin temperature for the subjects both before and after HDBR was found to increase slowly with the air humidity, and a higher mean skin temperature was observed in the subjects before HDBR. However, when the relative humidity was above 60%, the mean skin temperature of the subjects decreased with the air humidity both before and after HDBR. A higher mean skin temperature was observed in the subjects after HDBR. Additionally, the effects of the air speed were also investigated. The results showed that the mean skin temperature of the subjects decreased with the air speed both before and after HDBR in the cool, neutral and hot environments. However, the difference in the mean skin temperature of the subjects before and after HDBR was found to be insignificant in the cool environment. In a neutral environment, the mean skin temperature of the subjects before HDBR was found to be higher than that after HDBR. By contrast, the mean skin temperature before HDBR was found to be lower than that after HDBR in high air temperature and humidity. The temperature drop of the subjects before HDBR may lie in the cooling effect of conspicuous sweat. Finally, the variations in the regional skin temperature of the subjects in a comfortable environment were scrutinized. Compared with the temperatures before HDBR, higher skin temperatures in the head and torso and lower skin temperatures in the legs and arms were observed. The regional changes in the skin temperature after HDBR may be the result of the blood redistribution and disuse-atrophy of the skeletal muscles induced by simulated weightlessness. The results also showed that the vertical temperature difference was enlarged, which can cause thermal discomfort. This also suggests that the criteria for thermal comfort in a terrestrial condition may not applicable to humans in a weightlessness environment.

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This research is supported by the National Natural Science Foundation of China (Grant number 51276057, and 51676209), and is also supported by Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2014B065). Great thanks to the colleges Li Chengjun, Zhou Zhenyu, Li Kunxiang and Wang Tingting for their contribution to the field experiment and survey. Special thanks to all the subjects who volunteered to participate in the experiment.

References

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Subjects were exposed to 60 environmental conditions -6°HDBR was used to study the skin temperature variations in weightlessness Changes in mean skin temperature in simulated weightlessness were presented Changes in regional skin temperature in simulated weightlessness was presented The results demonstrated a technique to study the thermal comfort in weightlessness.

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