Effect of long-term indoor thermal history on human physiological and psychological responses: A pilot study in university dormitory buildings

Building and Environment 166 (2019) 106425

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Effect of long-term indoor thermal history on human physiological and psychological responses: A pilot study in university dormitory buildings Zhibin Wu a, b, Nianping Li a, b, *, Jinqing Peng a, b, Jingming Li a, b a b

College of Civil Engineering, Hunan University, Changsha, 410081, China Key Laboratory of Building Safety and Energy Efficiency (Hunan University), Ministry of Education, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Thermal adaptation Thermal sensation Indoor thermal history Physiological response Psychological response

The present study aimed to explore the effect of long-term indoor thermal history on the psychological and physiological responses of occupants. Based on a field study, 465 and 345 data sets were obtained from healthy students in naturally ventilated (NV) and split air-conditioned (SAC) dormitory buildings, respectively. The physiological and psychological responses were explored. Physical variables were measured by calibrated in­ struments; the psychological response was rated by occupants through questionnaires; physiological measure­ ment included four upper extremity skin temperatures (i.e., finger, wrist, hand and forearm), heart rate, systolic blood pressure, and diastolic blood pressure. The results indicated that indoor thermal history had no significant effect on physiological response (i.e., upper extremity skin temperatures, blood pressure, and heart rate), and induced psychological adaptation. The neutral temperature of the NV group was 26.2 � C, 0.7 � C higher than that of the SAC group (25.7 � C). The upper limit of 90% acceptable temperature range was 28 � C for the NV group, 0.7 � C higher than that of the SAC group (27.3 � C). Compared to the SAC group, a warm long-term indoor thermal history of the NV group produced a shift to higher neutral temperature and higher acceptable temperature. The clothing adjustment of NV group was more sensitive to indoor temperature than the SAC group.

1. Introduction The adaptive comfort theory [1] indicates that occupants play an active role in interacting with thermal environment, rather than passive objects. Thermal adaptation has been thought as crucial evidence sup­ porting adaptive thermal comfort [2]. Occupants can adapt to the thermal environment through physiological adaptation, psychological adaptation, and behavioral adaptation. Existing chamber experiment and field studies have demonstrated that past thermal history can in­ fluence thermal perception [3–8]. People with different thermal history may have different neutral temperature and acceptable temperature range [9–11]. Indoor thermal environment and outdoor climate with different time scale can shape occupant’s past thermal history. For the outdoor climate, people living in different climate zone have different past thermal history. Several studies [11–13] have revealed that the neutral temperatures of occupants in different climate zone are signifi­ cantly different. For instance, Zhang et al. [3,14] compared several climate chamber studies conducted with occupants in different climate and found that the neutral temperature of the naturally ventilated (NV)

group in the hot-humid climate zone was significantly higher than that of the tropical, temperate and subtropical climate zones. Wu et al. [15] compared many thermal comfort field studies conducted in different climates, and found that the warmer climate, the higher neutral tem­ perature. Wang et al. [16] investigated NV residential buildings in Harbin and found the neutral temperature in the cold area was lower than that of warm area. For the indoor environment, many studies have been conducted to determine the effect of indoor thermal history [17–20]. For example, Yu et al. [21] conducted an experiment with two groups living in NV and AC environment for a long period; it was found that indoor thermal history ^ndido et al. [22] affects occupants’ physiological acclimatization. Ca examined whether occupants’ previous indoor thermal history affects thermal perception in naturally ventilated buildings. There is a signifi­ cant discrepancy between the actual thermal comfort and those pre­ dicted by Fanger’s PMV model [23]. These results demonstrate that people have adapted to the indoor thermal environment, which shapes occupants’ thermal history. Based on related studies, the applicability of adaptive comfort model has been proved. The essential method of the

* Corresponding author. College of Civil Engineering, Hunan University, Changsha, 410081, China. E-mail address: [email protected] (N. Li). https://doi.org/10.1016/j.buildenv.2019.106425 Received 4 July 2019; Received in revised form 21 September 2019; Accepted 23 September 2019 Available online 24 September 2019 0360-1323/© 2019 Elsevier Ltd. All rights reserved.

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Table 1 The information of the investigated occupants. Type NV

SAC

a b

Gender

N

Age b

Height (cm)

Weight (kg)

BMIa(kg/m2)

INSUAL (clo)

Met (met)

Male

275

20.8�2.6

173.1�6:0

63.5�9:0

21.1�2:6

0.32�0:14

1.2�0:3

Female

190

20.5�1:9

161.4�4:9

50.4�6:8

19.7�2:2

0.40�0:10

1.2�0:6

All

465

20.7�2:4

58.5�10:1

20.6�2:5

0.35�0:13

1.2�0:5

Male

291

22.4�3.4c

168.3�8:0 172.8�5:6

64.8�9:2

21.7�2:8

0.34�0:10

1.2�0:3

Female

54

23.0�1:8

160.3�3:9

48.0�3:7

18.7�1:4

0.40�0:10

1.2�0:3

All

345

22.5�3:2

170.9�7:0

62.1�10:5

21.2�2:9

0.35�0:13

1.2�0:3

2

Body mass index, BMI ¼ weight/(height 2), normally between 18 and 25 kg/m . Standard deviation. ^

adaptive theory is relating outdoor temperature to indoor comfort temperature. Many international standards have described the adaptive comfort model and use outdoor temperature indicators to evaluate in­ door thermal comfort. The ASHRAE Standard 55 [24] uses prevailing mean outdoor temperature, which is calculated from the outdoor air temperature. The EN15251 [25] uses running mean outdoor air tem­ perature. Both indicators reflect the past thermal history with time scale, climate, and location. The adaptive theory emphasizes on using outdoor temperature to describe indoor comfort temperature, which does not combine the effect of indoor thermal experience. However, the indoor environment also plays an essential role in shaping the occupants’ past thermal history. As stated above, many studies have been conducted to explore the indoor thermal history. A few climate chamber experiments focused on the effect of the long-term indoor thermal history on potential physio­ logical adaptation. No field study has ever examined whether long-term thermal history influences physiological adaptation. The potential psy­ chological difference between the NV and SAC buildings may be induced by physiological adaptation or psychological adaptation. Therefore, the purpose of this study is to explore the mechanism of long-term indoor thermal history effect on physiological and psychological response. Several physiological parameters (the upper extremity skin tempera­ tures, heart rate (HR), systolic blood pressure (SBP), and diastolic blood pressure (DBP)) were included to examine the potential physiological adaptation. The psychological adaptation was determined through subjective survey between the NV group and the SAC group. The specific objectives of this study were listed as follows:

Fig. 1. Physical measurements.

11 NV buildings were investigated from July 17 to September 30, 2016. 465 healthy respondents in NV buildings and 345 healthy respondents in SAC buildings voluntarily participated in the field study. All the re­ spondents were college students and performed sedentary activities, such as reading or rest when surveyed. Also, the respondents have been living in the investigated building more than one year, which ensures their natural acclimatization to local climate and long-term indoor thermal history with the investigated buildings. Each respondent was investigated only once. Table 1 presents the information of occupants.

(1) To investigate actual physiological and psychological responses in real buildings through field study. (2) To determine the potential physiological and psychological adaptation between the NV group and the SAC group. (3) To identify the neutral temperature and acceptable temperature for occupants in the NV and SAC buildings. (4) To explore the difference of behavioral adjustment between the NV and SAC buildings. 2. Research methods

2.2. Physical and physiological measurements

2.1. Buildings and samples

The “right now right here” method was used in this study. The re­ spondents filled in the questionnaires, and the physical measurements (Fig. 1) were conducted simultaneously. Physical measurements con­ sisted of indoor air (Ta), relative humidity (RH), globe temperature (Ta),

This field study was conducted in split air-conditioned (SAC) build­ ings and naturally ventilated (NV) buildings. The NV buildings were dormitory buildings of college student located at the Hunan Normal University in Changsha. No mechanical cooling system was installed in the room of NV buildings, and occupants usually adjust clothing, use fans, and open windows to make them comfortable. The SAC buildings were dormitory buildings of college student located at the Hunan Uni­ versity in Changsha. All rooms in SAC buildings were equipped with split air conditioner. Occupants in SAC buildings can easily adjust the setting temperature of the split air conditioner. In both buildings, four occupants share one room. In this study, a total of 13 SAC buildings and

Table 2a Detail information of physical instruments.

2

Instrument

Variables

Range

Accuracy

Thermo recorder TR-72U

Air temperature

10-60 C

�0.3� C

HEART INDEX CHECKER 8778

Relative humidity Globe temperature

10-95%RH 0–50� C

�5%RH �0.6� C

WBGT-2009

Globe temperature

0–80� C

�0.6� C

Testo 425

Air velocity

0–20 m/s

�0.03 m/s

�

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Fig. 2. Physiological measurements. Table 2b Detail information of physiological instruments. Instrument

Variables

905-T2, Testo, USA

Skin temperature

Omron J12

Heart rate Systolic blood pressure Diastolic blood pressure

Table 3 Rating scales of psychological response. Range 50-350� C 40-180 bpm

Accuracy �1� C �5% �5 mmHg �5 mmHg

Dependent variable

Response scale

Thermal sensation

3 Cold, 2 Cool, 1 Slightly cool, 0 Neutral, þ1 Slightly warm, þ2 Warm, þ3 Hot 3 Very uncomfortable, 2 Uncomfortable, 1 Slightly uncomfortable, 0 Neutral, þ1 Slightly comfortable, þ2 Comfortable, þ3 Very comfortable 3 Much cooler, 2 Cooler, 1 Slightly cooler, 0 No change, þ1 Slightly warmer, þ2 Warmer, þ3 Much warmer 2 Clearly unacceptable, 1 Unacceptable, 0.01 Just unacceptable, þ0.01 Just acceptable, 1 Acceptable, 2 Clearly acceptable 3 Very dry, 2 dry, 1 Slightly dry, 0 Neutral, þ1 Slightly humid, þ2 Humid, þ3 Very humid 3 Much dryer, 2 dryer, 1 Slightly dryer, 0 No change, þ1 Slightly warmer, þ2 Warmer, þ3 Much warmer 2 Clearly unacceptable, 1 Unacceptable, 0.01 Just unacceptable, þ0.01 Just acceptable, 1 Acceptable, 2 Clearly acceptable 3 Very low, 2 low, 1 Slightly low, 0 Neutral, þ1 Slightly high, þ2 High, þ3 Very high 3 Much lower, 2 Lower, 1 Slightly lower, 0 No change, þ1 Slightly higher, þ2 Higher, þ3 Much higher 2 Clearly unacceptable, 1 Unacceptable, 0.01 Just unacceptable, þ0.01 Just acceptable, 1 Acceptable, 2 Clearly acceptable

Thermal comfort Thermal preference Thermal acceptability

and air velocity (Va). The air temperature and relative humidity were measured at 0.1 m, 0.6 m, 1.1 m heights above the floor. The globe temperature was measured by black spheres at 0.6 m height. The air velocity was measured at 0.1 m, 0.6 m, 1.1 m heights above the floor. The measurement instrument and accuracy are shown in Table 2a. Based on the ISO 7726 [26], operative temperature (To) were calculated. All these physiological variables were measured after the re­ spondents filled in the questionnaires. The physiological measurements consisted of the upper extremity skin temperatures, heart rate (HR), systolic blood pressure (SBP), and diastolic blood pressure (DBP). The upper extremity skin temperatures were measured at four local skin location, namely the 3rd finger of left hand (Tfinger), the wrist of the left hand (Twrist), the dorsal side of the left hand (Thand), and the left lower arm (Tlowerarm). Skin temperatures were measured using Testo surface temperature measurement instrument (905-T2, Testo, USA). The measuring head with sprung thermocouple strip was attached to the four local skin surface location, and measured the upper extremity skin temperatures one by one (Fig. 2b). All local skin temperatures of re­ spondents were measure only once, and the reading value in display was recorded when the instrument became stable within 2 min. After this, the heart rate, systolic blood pressure, and diastolic blood pressure were also measured once using blood pressure monitor (Fig. 2b), and then the presented value was recorded on paper. The detail information of the instrument is presented in Table 2b.

Humidity sensation Humidity preference Humidity acceptability Velocity sensation Velocity preference Velocity acceptability

The seven-point scales were used to collect several kinds of subjective perceptions. The first part of the questionnaire asked about thermal perception, including thermal sensation votes, thermal comfort votes, thermal preference votes, and thermal acceptability votes; the second part was about humidity [27], including humidity sensation, humidity preference, and humidity acceptability; the third part collected velocity perception [28], such as velocity sensation votes, velocity preference votes, and velocity acceptability votes. The description of these scales is listed in Table 3. The clothing types of occupants were also collected by questionnaires. According to ASHRAE Standard 55, all these clothing pieces were estimated by their detailed type, and the overall clothing insulation was determined by summing the value of every clothing and a chair. Based on the responded activity type of respondents, the

2.3. Subjective survey Respondents in the present study were asked to fill in a paper questionnaire. They were required to provide personal information (age, gender, height, weight, activity level during the survey, and garments). 3

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Table 4 The distribution of physical variables. Type

Heights

Ta (� C)

RH (%)

Va (m/s)

Tg (� C)

Tr (� C)

To (� C)

NV

Mean

30.6�2.1

64.4�10.2

0.40�0.34

30.4�2.1

30.2�2.1

30.4�2.1

Non-uniformity

0.30�0.33

2.76�2.39

0.67�0.90

0.1 m

30.4�2.2

64.9�10.1

0.20�0.22

0.6 m

30.6�2.1

63.9�10.5

0.35�0.48

1.1 m

30.7�2.0

64.4�10.3

0.64�0.87

Mean

26.6�2.4

63.4�12.6

0.19�0.21

26.6�2.2

26.6�2.2

26.6�2.3

Non-uniformity

1.27�1.28

4.70�4:26

0.33�0.54

0.1 m

26.0�2.6

65.2�12.8

0.15�0.18

0.6 m

26.7�2.6

62.6�12.9

0.19�0.40

1.1 m

27.1�2.3

62.3�13.0

0.22�0.44

SAC

Fig. 4. The temperature.

Fig. 3. The relationship between air velocity and operative temperature.

metabolic rate of respondents was determined by referring to the metabolic rates list in ASHRAE Standard 55.

relationship

between

clothing

insulation

and

operative

buildings (1.27 � C) was extremely higher than that in NV buildings (0.30 � C). The vertical temperature difference in both buildings was lower than the upper limit defined by ASHRAE Standard 55–2017 [30] (�3 � C). The mean air velocity was 0.4 m/s of NV buildings was double of that in SAC buildings (0.19 m/s). The air velocity in both buildings was almost near or higher than the applicable upper limit of ASHRAE Standard 55–2017 (0.2 m/s).

2.4. Data analysis The physiological variables (i.e., upper extremity skin temperatures, systolic blood pressure, diastolic blood pressure, and heart rate) and behavioral response (i.e., air velocity and clothing insulation) were binned in every 0.5 � C of operative temperature to explore the potential physiological adaptation and behavioral adjustment, respectively. In section 3.4, Mann-Whitney test was used to determine the difference of physiological response between the NV and SAC group. Kruskal-Wallis test was adopted to determine whether thermal sensation was signifi­ cantly different from the physiological variables. The regression corre­ lations was determined with one-way ANOVA test. All the statistical analyses were made with the SPSS software, version 22.0 (SPSS Inc., Chicago, Illinois, USA), values of 0.05 (*), 0.01(**), and 0.001 (***) were accepted as significantly different.

3.2. Behavioral adjustment 3.2.1. Air velocity Fig. 3 shows the air velocity variation with the operative temperature of the SAC and NV groups. For the NV group, the air velocity was line­ arly correlated with the indoor temperature. For the SAC group, the quadratic relationship between the air velocity and operative tempera­ ture was determined. When the temperature was lower than 26 � C; the air velocity was negatively correlated with the operative temperature; the air velocity was positively correlated with the operative temperature when the temperature was higher than 26� C. The NV group had a wider air velocity range than that of the SAC group.

3. Results 3.1. Indoor thermal environment

3.2.2. Clothing adjustment The clothing variation with indoor operative temperature of the two groups was explored in this study. Fig. 4 shows that the clothing insu­ lation correlates linearly with indoor temperature. The clothing insu­ lation of the SAC group was almost lower than that of the NV group. The clothing adjustment of NV group was more sensitive to indoor temper­ ature than the SAC group. The relationship between the clothing insu­ lation and the operative temperature of the SAC group was weak, and the clothing insulation varied around 0.35 clo.

Table 4 shows the summarized statistic results of the investigated thermal environment. The mean indoor air temperature in NV buildings and SAC building were 26.6 � C and 30.6 � C, respectively. The mean indoor relative humidity in NV buildings and SAC building were 64.4% and 63.4%, respectively. The humidity and temperature in NV buildings were higher than those in SAC buildings. The non-uniformity [29] of the thermal environment was determined in the present study, which was the maximum difference among the values measured at three heights. There was evident non-uniformity of temperature and velocity in both types of building. The non-uniformity of air temperature in SAC 4

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Fig. 5. The distribution of thermal perception.

Fig. 6. A Humidity perception distribution. Fig. 6b Velocity perception distribution.

3.3. Psychological responses

was 0.34 of the SAC group and 1.38 of the NV group. Many respondents (70%) in NV buildings felt warm. About 70% of occupants felt thermally comfortable in SAC buildings, while less than 30% of person was comfortable in NV buildings. Fig. 5 indicates that over 90% of votes are located within the acceptable range in SAC buildings; about 70% of

The percentage of thermal neutrality vote (47%) was the highest proportion in SAC buildings, and the “slightly warm” vote of 28% was the largest proportion in NV buildings. The mean thermal sensation vote 5

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Table 5 The distribution of physiological variables. Type

SBP (mmHg)

DBP (mmHg)

HR (bpm)

Tfinger (� C)

Twrist (� C)

Thand (� C)

Tlowerarm (� C)

NV

109.6�12.0

66.7�9.8

74.9�10.6

35.3�1.3

35.1�1.0

34.9�0:8

34.9�0:8

SAC

114.6�13.2

69.4�9.4

72.0�10.6

33.7�2.3

34.1�1:2

33.8�1:3

33.8�1:3

Fig. 7. Physiological responses.

respondents accepted their surrounding thermal environment in NV buildings. The proportion of cooler preference votes was remarkably higher than the warmer preference votes in both types of building. Notably, over 80% of occupants preferred a cooler environment in NV buildings, while the highest percentage of occupants in SAC buildings preferred maintaining the present indoor temperature. Fig. 6a shows that both the NV group and the SAC group have similar distribution in humidity sensation votes, humidity acceptability votes, and humidity preference votes. In both groups, the highest proportion of

occupants had neutral humidity sensation, and the percentage of dry sensation was almost equal to humid sensation. About 80% of re­ spondents were satisfied with the surrounding humidity condition. The largest part of occupants preferred present humidity in both groups; more people preferred lower humidity than higher humidity. Fig. 6b presents the distribution of actual velocity sensation votes, velocity acceptability votes, and velocity preference votes. The greatest propor­ tion of occupants had neutral velocity sensation in both environments. Most of them felt low in velocity sensation. Over 70% of occupants 6

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Fig. 9. Acceptable temperature.

Fig. 8. Neutral temperature.

accepted their surrounding velocity in both environments. The per­ centage of preferring higher velocity was higher of the NV group than the SAC group. The percentage of preferring lower velocity was less than 10% in both environments.

The thermal sensation of the SAC group was significantly different from that of the NV group. The neutral temperature of the NV group (26.2 � C) was 0.7 � C higher than that of the SAC group (25.5 � C). Both groups had the same thermal sensitivity to indoor temperature. The respondents of the NV group always felt about 0.3 scale warmer than the SAC group.

3.4. Physiological responses The upper extremity skin temperature distributions were summa­ rized in Table 5. The mean value of finger, hand, wrist, and lower arm were 35.3 � C, 35.1 � C, 34.9 � C, and 34.9 � C in NV buildings, respectively; the mean value of finger, hand, wrist, and lower arm were 33.7 � C, 34.1 � C, 33.8 � C, and 33.8 � C in SAC buildings, respectively. The comparisons of physiological responses between the NV group and the SAC group are shown in Fig. 7. For upper extremity skin temperatures, the NV group had significantly higher extremity skin temperatures than those of the SAC group of the same thermal sensation in thermal sensation condi­ tions. The heart rate (HR) between the NV and SAC groups is presented in Fig. 7e. The mean heart rate was 74.9 bpm of the NV group and 72.0 bpm of the SAC group. There was no significant difference between the NV group and the SAC group in all thermal sensation conditions. Fig. 7f and g shows the distribution of systolic blood pressure and diastolic blood pressure in terms of thermal sensation. The comparisons of sys­ tolic blood pressure (SBP) and diastolic blood pressure (DBP) were conducted between the NV group and SAC group. Both SBP and DBP were higher of the SAC group than that of the NV group in neutral and warm sensation, and vice versa in the cool sensation. The statistic test indicated there was no significant difference of DBP between the two groups in all sensation conditions except the cool and neutral sensation. No significant difference was determined for DBP within different thermal sensation. For SBP, significant difference was found from neutral to warm sensation between the two groups; also, significant difference was detected among different thermal sensation. These re­ sults indicated that DBP could not be used to assess thermal comfort.

3.5.2. Acceptable temperature The upper limit of 90% acceptable temperature range is adopted in ASHRAE Standard 55–2017 to assess the actual thermal environment. The acceptable temperature was determined by the actual percentage of dissatisfied (APD). In this study, the thermal acceptability votes were binned in every 0.5 � C of operative temperature. Fig. 9 presents the acceptable temperature ranges of the NV group and the SAC group. The APD was determined as a second-order function with operative tem­ perature. The regression equations of the NV group and the SAC group were listed: NV: APD ¼ 1.81–0.20Toþ0.005T2o, R2 ¼ 0.93 SAC:

APD ¼ 2.97–0.25Toþ0.005T2o,

2

R ¼ 0.71

(3) (4)

The upper limit of 90% acceptable temperature range were 28 � C for the NV group, 0.7 � C higher than that of the SAC group (27.3� C). The acceptable percentage of the SAC group was higher than that of the NV group in the cool and neutral sensation side and was similar in the warm sensation side. 4. Discussion 4.1. Thermal adaptation Thermal adaptation can be grouped into three processes, namely behavioral adjustment, physiological acclimatization, and psychological habituation. The present study explored whether the long-term indoor thermal history had a significant effect on physiological and psycho­ logical response. In the NV buildings, the indoor temperature and rela­ tive humidity were higher than that in SAC building. Therefore, the indoor thermal history of the NV group was warmer than the SAC group.

3.5. Neutral and acceptable temperature 3.5.1. Neutral temperature Linear regression is classical method for determining neutral tem­ perature. Fanger [23] used the linear regression method to determine the neutral temperature of 256 Danish respondents. The present study used this method as done in many studies of thermal comfort area [31–34]. Fig. 8 shows the linear regression relationship between thermal sensation and indoor operative temperature. The regression equations of the NV group and the SAC group were listed: NV: TS ¼ -8.61-0.33To, R2 ¼ 0.31

(2)

SAC: TS ¼ -8.35-0.33To, R2 ¼ 0.35

4.1.1. Physiological adaptation Fig. 7 indicates that all the upper skin temperatures of the NV group was significantly higher than that of the SAC group. All these upper extremity skin temperatures were positively correlated with thermal sensation. These results showed that the applicability of using the upper

(1) 7

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Fig. 10. Comparisons between the SAC and NV groups for physiological indicators.

extremity skin temperatures to predict thermal sensation. This has been demonstrated by Wu et al. [35] and Wang et al. [36]. For HR, there was no significant difference for the two groups within the same thermal sensation scale, but the significant difference was found among different sensation. For SBP, a significant difference was found in neutral and warm sensation for the two groups; also, such a significant difference was detected among different thermal sensation. No significant differ­ ence was determined for DBP within different thermal sensation. The physiological difference between the two groups of the same thermal sensation may be caused by the different indoor thermal history of the two groups. The indoor thermal history could introduce physio­ logical adaptation or psychological adaptation. Fig. 10 shows the com­ parisons between the NV and SAC groups for physiological parameters with operative temperature. Second order polynomial functions were

established for relating the upper extremity skin temperatures with operative temperature. The regression line showed that all these upper extremity skin temperatures of the NV group were almost the same as those of the SAC group. To further determine whether there was sig­ nificant physiological difference between the two groups, the physio­ logical variables were binned in every 1 � C of operative temperature; the overlap operative temperature range (25–31 � C) of the NV group and the SAC group were selected (Fig. 11). The statistic results showed that the upper skin temperature of the NV group was consistent with those of the SAC group except the 28 � C of Tfinger and 25 � C of Twrist. For the blood pressure, there was no obvious tendency between the DBP and the operative temperature. Linear regression between the SBP and the operative temperature was determined for the NV and SAC groups. No statistic difference was found except the 27 � C and 28 � C of SBP. Also, 8

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Fig. 11. Boxplot between the SAC and NV groups for physiological indicators.

linear regression was developed for heart rate and operative tempera­ ture. Only in the 28 � C, the heart rate of the NV group was significantly higher than that of the SAC group. Above results demonstrated that there was almost no impact of in­ door long-term thermal history on the physiological process. Therefore, the indoor thermal history did not make the NV group, and the SAC group had a physiological adaptation to their surrounding environment. This evidence was consistent with what Zhang [1,3] found.

past various thermal history would provide a benchmark for future thermal expectation. Therefore, these results were consistent with the previous research and provided convincing evidence for the effect of indoor thermal history on psychological habituation [1]. 4.1.3. Behavioral adjustment For behavioral adjustment, section 3.2 illustrate the clothing adjustment and air velocity variation with the operative temperature. The clothing adjustment of the two groups was different. The clothing variation of the SAC group was less sensitive to the operative tempera­ ture than that of the NV group. Compared to the NV group, the SAC group had narrower clothing insulation variation range. The SAC group may prefer to adjust the set point of air conditioner rather than the clothing insulation, and the NV group had no choice rather than adjust their clothing insulation with the temperature. The difference of the air velocity adjustment was clear between the two groups. The air velocity was extremely higher of the NV group (0.4 m/s) than the SAC group (0.19 m/s). The strange phenomenon was that when the temperature was lower than 26 � C, the air velocity of the SAC group was negatively correlated with the operative temperature. The potential reason could be listed as follows: firstly, the SAC group preferred to have a cooler environment, and they turned down the set point of the AC and they increased the air velocity of the fans simultaneously; secondly, the re­ spondents decreased the set point of the split air conditioner, but the split air conditioner needed to automatically increase the air velocity for cooling the room. Unlike the AC building, the upper limit of air velocity is controlled under 0.2 m/s. Both the SAC and the NV buildings pro­ vided air velocity adjustment opportunity for occupants to satisfy themselves. However, there were still more than 50% of respondents in the NV group and 40% of respondents in the SAC group expecting higher air velocity. This indicated that the existing building operator should

4.1.2. Psychological habituation In the present study, it was found that the NV group with warmer thermal history had 0.7 � C higher neutral temperature than that of the SAC group. As indicated in Fig. 5, the upper limit of 90% acceptable temperature range of the NV group (28 � C) was 0.7 � C higher than that of the SAC group (27 � C). These results indicated that occu­ pants in NV buildings could endure, accept, and then accustom to a higher temperature. Because section 4.1.1 has examined the long-term indoor thermal history had no significant effect on physiological adap­ tation. These difference was induced by psychological adaptation be­ tween the NV group and the SAC group. As illustrated in section 3.1, the percentage of comfortable votes and acceptability votes in NV buildings were significantly lower than those of the SAC group. The relatively warm and uncomfortable thermal experience in NV building made the occupants have a lower expectation of the surrounding thermal envi­ ronment than the SAC group. This phenomenon was the same as pre­ vious studies. For example, Ning [4] found the neutral temperature was 1.9 � C higher in warm exposure than cool exposure sample. Zhang [3] showed that a warmer indoor thermal history in summer produced a higher neutral temperature while no physiological adaptation was found between the SAC group and the NV group in chamber experiment. The indoor thermal history induced psychological habituation because the 9

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pay more attention to the behavioral adjustment.

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4.2. Potential implication As presented above, the percentage of comfortable votes in NV buildings was less than 30%. This environment was worse than what the Chinese standard GB/T 50785-2012 [37] requires. The indoor thermal environment is important for occupants’ health, performance, and comfort [38–40]. It is urgent to improve the indoor thermal conditions in buildings like the investigated NV buildings. The Chinese economy has developed rapidly during these years. Many buildings are being or will be renovated to provide a better indoor environment. This study could provide a reference for guiding future buildings renovation. For example, when we renovate the NV buildings to AC or SAC buildings, the potential thermal adaptation should be considered. It is unreason­ able to apply the “one rule all set” in guiding the future mechanical system design in buildings. 4.3. Limitation This study has explored the potential effects of indoor thermal his­ tory on human physiological and psychological response. However, several points still need to be clarified in future studies. Firstly, only upper extremity skin temperatures, blood pressure, and heart rate were measured to examine the potential physiological adaptation; more po­ tential physiological variables and more accurate instrument can be used in further study. Secondly, the percentage of the female and male was not the same, which could have an effect on the physiological and psychological results; future study should ensure the equal sex ratio and the similar sample size in both types of buildings. Last, this study only explored the indoor thermal history in warm exposure, and the potential effects of indoor thermal history also should be determined in cool exposure. 5. Conclusions The present study explored the effect of long-term indoor thermal history on physiological and psychological responses through field study. 465 and 345 data sets were obtained from healthy students in naturally ventilated (NV) and split air-conditioned (SAC) dormitory buildings, respectively. Potential physiological adaptation and psycho­ logical adaptation were examined. The main conclusions could be drawn as follows: 1) The temperature in NV building was far higher than that in SAC buildings; there was a lower proportion of comfortable votes (30%) in NV buildings, and further steps must be token to improve the in­ door thermal environment in NV buildings. 2) The neutral temperature was 25.5 � C; and the upper limit of 90% acceptable temperature range was 27.3 � C for the occupants in SAC buildings, and these values for the NV group were 26.2 � C and 28� C, respectively. 3) No significant difference was found with physiological response be­ tween the NV group and the SAC group, and the indoor thermal history produced psychological adaptation. 4) The clothing adjustment of NV group was more sensitive to indoor temperature than the SAC group. Acknowledgement This work presented in this paper was financially supported by the National Natural Science Foundation of China (Project No. 51878255). Z. Wu would like to express his gratitude to the China Scholarship Council for the support at Technical University of Denmark (No. 201806130199). 10

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