Comparison of adaptive thermal comfort with face masks in library building in Guangzhou, China

Abstract

During the COVID-19 pandemic, wearing masks in public spaces has become a protective strategy. Field tests and questionnaire surveys were carried out at a university library in Guangzhou, China, during June 2021 and January 2022. The indoor environmental parameters were observed, thermal sensation votes of students on various environmental parameters were collected, symptoms of students wearing masks were quantified, and the appropriate amount of time to wear masks was established. To identify acceptable and comfortable temperature ranges, the relationship between thermal sensation and thermal index was investigated. During summer and winter, people wearing masks are symptomatic for a certain duration. The most frequently voted symptom was facial heat (62.7 % and 54.6 % during summer and winter, respectively), followed by dyspnea. During summer, more than 80 % of the participants subjects were uncomfortable and showed some symptoms after wearing masks for more than 2 h (3 h during winter). In the summer air conditioning environment in Guangzhou, the neutral T_op was 26.4 °C, and the comfortable T_op range was 25.1–27.7 °C. Under the natural ventilation environment in winter, the neutral T_op was 20.5 °C, and the comfortable T_op range was 18.5–22.5 °C. This study may provide guidance for indoor office work and learning to wear masks in Guangzhou.

Introduction

The World Health Organization (WHO) designated the COVID-19 outbreak a pandemic on March 11, 2020 [1]. In the United States, SARS-CoV-2 had infected more than 8 million kids as of January 2022 [2]. The COVID-19 burden among children and adolescents has increased due to new SARS-CoV-2 variant strains [3], [4], [5]. Other personal protective techniques, like as mask wearing, quarantine, vaccination, and hand cleanliness, played a significant part in epidemic mitigation in addition to public containment and closure laws [6], [7], [8], [9], [10], [11], [12], [13]. The most efficient and affordable way to stop human-to-human virus transmission and control the COVID-19 outbreak is to wear masks in public [14]. It has become customary to wear masks while traveling, at work, and while in school [15]. Some scholars also emphasize the discomfort caused by masks [16], [17], [18], [19]. Therefore, it is necessary to thoroughly investigate the effect of masks on human thermal comfort.

Literature review

Despite their recognized benefits in protecting and insulating against toxins and viruses, masks can have side effects due to the microclimate of hot and thick humid air they cause [20], [21], [22]; additionally, wearing a mask causes significant discomfort and breathing difficulties in most people [23]. Furthermore, the temperature of the air within the mask has a significant impact on human thermal sensation [24]. Davey et al. [25] described heat-related illness symptoms in healthcare workers (e.g., 40.2 % dizziness, 63.4 % fatigue, 79.0 % headache, and 54.5 % profuse sweating) and heat stress, which impairs cognitive and physical performance. Tang et al. [19] described symptoms among students in summer air conditioning conditions; 62.7 % and 25.4 % of the subjects voted for facial heat and dyspnea as the most commonly observed symptoms, respectively. Some subjects who wore masks for a long time experienced rapid heartbeats (9.1 %) and nausea (4.1 %). Peres et al. [26] established that masks were associated with discomfort (26.8 %), and affected task performance (18.9 %) and communication (40.9 %). Another research examined at how N95 masks and medical masks create significantly different microclimates, which have a significant impact on heart rate, thermal stress, and subjective perception of discomfort. [27]. Therefore, while people should pay attention to their own protection, they should strengthen their personal health.

Several previous studies have already been conducted to improve the indoor and outdoor environment and human thermal comfort [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. Since these investigations were carried out before the pandemic, the impact of masks was not examined. But ever since the pandemic was announced, it is now necessary to wear a mask in public to avoid contracting COVID-19 [6], [39], [40], [41], [42]. During summer and winter, the comfort range of the environment varies [30], [37]. Consequently, further attention and research on mask-wearing adaptability are needed.

Research objective

A library survey was conducted during summer and winter. The study’ objectives were as follows:

(1) The physical discomfort caused by wearing masks during summer and winter was analyzed.

(2) The correlation between the duration of wearing masks and human physiological symptoms was analyzed, and suggestions were made regarding the duration of wearing masks.

(3) The thermal index and comfort range when there are two situations simultaneously were calculated (with masks or without masks).

Methods

Research environment

Guangzhou is located in South China. This research was conducted at the library of Guangzhou University (Fig. 1 ). The study was done in June 2021 and January 2022. It was conducted for 10 days during the summer and 8 days during the winter (Table 1 ). The weather information for the days is shown in Fig. 2 , based on data from the Guangzhou Weather Station. During the investigation, the relative humidity ranged from 78 % to 95 % and 30 % and 56 % in June 2021 and January 2022, respectively. The average outdoor air temperature in Guangzhou reached 30 °C and 12 °C in June 2021 and January 2022, respectively.

Fig. 1.

Location of the testing site.

Table 1.

Data of outdoor environment.

Area	Date	Test days	Outdoor temperature range (°C)	Outdoor relative humidity range (%)
Guangzhou	June 2021	10	24.5 – 36.5	78 – 95
Guangzhou	January 2022	8	3.0 – 24.5	30 – 56

Fig. 2.

The outdoor environment data in Guangzhou.

Subjects and questionnaire

These experiments were carried out in accordance with the Helsinki Declaration’s ethical standards, and informed consent was obtained from all the participants. We asked them about their health status and screened them. In addition, all subjects were requested to stay in the library for more than 30 min to ensure that the subjects were in thermal equilibrium [43], [44], [45], [46], [47]. This survey’s participants were health students. Except for any discomfort with the mask, they had to indicate their health status before answering the questionnaire.

In the first section, the questionnaire used investigated subjective votes on the thermal sensation, air movement sensation, and humidity sensation. The subjective vote scales were based on the thermal environment comfort levels described in the ASHRAE Handbook [48] and ISO 7726 [49]. As shown in Fig. 3 , the thermal sensation scale was as follows: −3, cold; −2, cool; −1, slightly cool; 0, neutral; 1, slightly warm; 2, warm; and 3, hot. The second section of the questionnaire investigated whether students wore masks and any symptoms of discomfort, as well as the duration of mask use. Meanwhile, we suggested that subjects consider mask-wearing as the only factor causing discomfort when filling in the questionnaire. The questionnaire content is presented in the Appendix A.

Fig. 3.

Subjective feeling voting scale (a) and students wore masks and any symptoms of discomfort (b).

As shown in Table 2 , a total of 2,643 valid questionnaires were collected for this study, 1,602 in the summer and 1,041 in the winter. The average ages of the subjects were 20.7 years during the summer and 20.2 years during the winter. The clothing insulation values were calculated based on ASHRAE standard 55 [50]. the average total clothing insulation (I _cl) was 0.39 clo in summer and 0.88 clo in winter.

Table 2.

Subject anthropometric data (SD: standard deviation).

Sex	Number	Age (years) (SD)	Height (m) (SD)	Weight (kg) (SD)	Body surface area (m2) (SD)	Icl (clo) (SD)
Male	Summer: 550	20.9 (1.51)	1.73 (0.06)	62.7 (7.95)	1.74 (0.12)	0.35 (0.11)
	Winter: 399	20.4 (1.93)	1.73 (0.06)	62.4 (8.69)	1.73 (0.13)	0.83 (0.21)
Female	Summer: 1,052	20.6 (1.57)	1.61 (0.05)	50.2 (6.02)	1.50 (0.10)	0.41 (0.12)
	Winter: 642	20.0 (2.43)	1.61 (0.05)	49.9 (6.01)	1.49 (0.10)	0.91 (0.19)
Total	Summer: 1,602	20.7 (1.56)	1.65 (0.08)	54.5 (9.00)	1.58 (0.15)	0.39 (0.12)
	Winter: 1,041	20.2 (2.25)	1.66 (0.08)	54.7 (9.43)	1.59 (0.16)	0.88 (0.22)

Body surface area (A): A = 0.202 w^0.424 h^0.725[38].

Measured parameters and instruments

According to the ISO 7726 guidelines [49], test instruments were placed near the seats at a height of 1.1 m [51], [52], [53], [54], [55], [56], [57], [58], [59], [60]. A matt black standard globe thermometer with a diameter (D) of 0.15 m (globe emissivity, ε_g = 0.95) was used to measure the globe bulb temperature. The Mean radiant temperature (T_mrt) was calculated using Eq. (1) [49]:

$$T_{\text{mrt}}={\left\{{(T_g+273)}^4+\left[\frac{(1.1\times {10}^8\times V_a^{0.6})}{({\epsilon }_g\times D^{0.4})}\right]\times (T_g-T_a)\right\}}^{1/4}-273$$ (1)

where T_a is the air temperature, T_g is the globe temperature, and V_a is the air velocity.

Data processing

The Standard Effective Temperature (SET*) was calculated using the CBE Thermal Comfort Tool (https://comfort.cbe.berkeley.edu/). In this study, using SET* can help to reduce the impact of the inability to control clothing [61], [62].

The operative temperature (T_op) was calculated according to the conditions presented in ASHRAE Standard 55 normative [50]. The T _op was calculated using Eq. (2) [50] as follows:

$$T_{\mathit{op}}=A{T}_a+\left(1-A\right)T_{\mathit{mrt}},$$ (2)

where A is determined according to Table 3 as a function of relative air speed [50].

Table 3.

Relationship between A and V_a.

Va	<0.2 m/s	0.2–0.6 m/s	0.6–1.0 m/s
A	0.5	0.6	0.7

Linear regression was used to analyze the relationship between the environmental parameters and responses to the subjective questionnaire to determine the neutral temperature (when MTSV = 0) and comfort temperature range (when MTSV = ±0.5). All statistical analyses were performed using IBM SPSS Statistics 25 (IBM Inc., Armonk, NY, USA), and Origin 2021 (Origin Lab Corporation, Northampton, MA, USA), including the fitting of linear regression equations and the calculation of linear regression correlation index R² and the independent sample t-tests.

Results

Thermal parameters

The measured indoor thermal parameters (T_a, RH, V_a, and T_mrt) are summarized in Table 4 . The mean T_a, RH, and V_a were 27.7 °C, 79.2 %, and 0.17 m/s during the summer, respectively, and 22.5 °C, 66.3 %, and 0.04 m/s during the winter, respectively. Based on the calculations, the mean values of T_op and SET* were 27.6 °C and 20.2 °C during the summer and 26.6 °C and 21.1 °C during the winter, respectively. Because the average V_a was less than 0.2 m/s, there was almost no blowing sense for the human body, which satisfies the advised value of V_a in the ASHRAE Standard 55 [50].

Table 4.

Indoor thermal parameters (SD: standard deviation).

Seasons	Variables (units)	Mean	Minimum	Maximum	SD
Summer	Ta (°C)	27.7	25.9	31.1	1.15
	Tg (°C)	27.5	25.6	31.3	1.05
	RH (%)	79.2	74.1	85.2	1.98
	Va (m/s)	0.17	0.01	0.80	0.16
	Tmrt (°C)	27.6	25.8	31.4	1.04
	Top (°C)	27.6	25.9	31.2	1.10
	SET* (°C)	26.6	21.8	33.9	1.78
Winter	Ta (°C)	20.5	16.3	22.3	0.94
	Tg (°C)	20.1	16.4	21.5	0.54
	RH (%)	66.3	58.0	71.5	3.85
	Va (m/s)	0.04	0.01	0.44	0.08
	Tmrt (°C)	20.2	15.7	22.6	0.97
	Top (°C)	20.2	16.3	22.4	0.93
	SET* (°C)	21.1	16.4	25.3	1.24

Effect of wearing masks on human comfort

Among the 2,643 questionnaires collected, subjects of 1,683 questionnaires who wore masks were included (1,112 in summer and 571 in winter). Of which, wearing masks made 1,239 (73.6 %) of the participants uncomfortable. The most frequently voted symptom was facial heat (62.7 % in summer, and 54.6 % in winter) as shown in Fig. 4 , followed by dyspnea (25.4 % in summer, and 35.2 % in winter). All other symptoms were observed in <10 % of patients. When compared to summer, the percentage of subjects experiencing dyspnea increased while the percentage of subjects experiencing facial heat decreased during the winter However, wearing masks still caused more than 50 % of the subjects to feel facial heat. Wearing masks affects the comfort of the human face. In addition, compared to summer, the percentage of subjects wearing masks with dyspnea in winter increased. One of the possible reasons was that the wind speed in the indoor environment in winter is lower than that in summer, and the gas exhaled by the human body may stay in the mask for a long time.

Fig. 4.

The distribution of symptoms among participants wearing masks.

During the test, the subjects evaluated the environmental air quality. A total of 85.7 % and 91.6 % of the participants believed that the air quality in the library was acceptable during summer and winter, respectively (as shown in Fig. 5 ). For participants wearing masks, discomfort and symptoms caused by air quality problems were avoided, which was confused with the impact of masks. In addition, 35.5 % of the subjects (21.2 % wearing masks) felt stuffy indoors in summer, indicating that the airflow speed in the environment was occasionally low during the test.

Fig. 5.

Evaluation of the indoor air quality.

Acceptable duration of wearing masks

More than 75 % (78 %) of participants were anticipated to wear masks for 2.0 h or less throughout the summer (winter), based on the voting statistical distribution of respondents' acceptable time of wearing masks. As shown in Fig. 6 (Left–Y), only a few subjects could tolerate wearing masks for more than 3 h. The acceptable duration of wearing masks increased in the winter when compared to the summer. Within a certain period, the mask can keep the face warm and reduce the heat loss caused by the ambient cold air. The longer the mask was worn, the wetter the face became, reducing the comfort of the human body. After exceeding their “acceptable duration,” the majority of participants experienced increased physical discomfort, impacting their work and learning efficiency. As shown in Fig. 6 (Right–Y), the percentage of no symptoms and the duration of wearing the mask have a good linear relationship. More than 80 % of the subjects were uncomfortable and showed some symptoms after wearing masks for more than 2 h in summer and 3 h in winter.

Fig. 6.

Distribution of percent “acceptable duration” (Left -Y) and the relationship between the “percentage of no symptom” and duration (Right -Y) for wearing masks.

Distribution of TSV

As shown in Fig. 7 (a–b), the thermal sensation vote (TSV) results for the whole body and the face indicated that wearing a mask had some effect on the human. The percentage of subjects wearing masks who reported a TSV greater than zero was approximately 6.7 % (7.2 %) greater than the proportion of subjects without masks during the summer (winter). For the face, the percentage of TSV greater than 0 also increased (13.4 % in summer and 9.4 % in summer). During the winter, wearing masks can improve the comfort of the human face to some extent, with the percentage of TSV less than 0 reduced by 2.4 % and 1.4 % during summer and winter (without masks vs with masks), respectively. There was no obvious effects observed during the summer. The percentage of subjects wearing masks decreased at TSV = 0 (vs without masks). There was increased impact on face comfort during summer (10.9 % less vs winter 7.9 %). During the summer, wearing a mask had a certain effect on thermal sensation in the head and chest, with thermal sensation shifting to greater than 0 (Fig. 7c–d), whereas wearing a mask had no obvious effect during winter. In addition, wearing masks did not have a significant effect on the human back and limbs (Fig. 7e–f).

Fig. 7.

Percentage distribution of TSV in the library: (a) whole; (b) face; (c) head; (d) chest; (e) back; (f) limbs.

Comparison of the whole thermal sensation and local thermal sensation

The results of the regression analyses of local and thermal sensations are shown in Table 5 . TSV on the head and face had a significant impact on the whole TSV. There is a significant relationship between the whole and local thermal sensations because the entire thermal sensation can be seen as the integration of each local thermal sensation signal via brain control [38]. These models are shown in Eq. (3) to Eq. (8). As can be seen from Eq. (5) and Eq. (8), wearing masks had an effect on the thermal comfort of the face, head, and chest in summer and on the thermal comfort of the face in winter. During colder seasons, masks may increase comfort to some extent and protect the face from the cold air.

Table 5.

Analysis of whole thermal sensations and local thermal sensation.

Season	TSV (Whole)		Face	Head	Back	Chest	Limbs	Constant
Summer	With masks	Coef.	0.351	0.436	0.224	0.164	0.022	–0.048
		P	0.001	0.001	0.001	0.001	0.01
	Without masks	Coef.	0.301	0.384	0.229	0.109	0.028	–0.086
		P	0.001	0.001	0.001	0.001	0.01
	ΔTSV	Coef.	0.05	0.042	–0.005	0.055	–0.006	0.038
Winter	With masks	Coef.	0.266	0.342	0.188	0.163	0.132	–0.053
		P	0.001	0.001	0.001	0.001	0.01
	Without masks	Coef.	0.205	0.339	0.184	0.166	0.138	–0.084
		P	0.001	0.001	0.001	0.001	0.01
	ΔTSV	Coef.	0.061	0.003	0.004	–0.003	–0.006	0.031

Summer:

With masks:

$$\mathit{TS}{V}_{\mathit{whole}}=0.351TS{V}_{\mathit{face}}+0.436TS{V}_{\mathit{head}}+0.224TS{V}_{\mathit{back}}+0.164TS{V}_{\mathit{chest}}+0.022TS{V}_{\mathit{libms}}-0.048$$ (3)

Without masks:

$$\mathit{TS}{V}_{\mathit{whole}}=0.301TS{V}_{\mathit{face}}+0.384TS{V}_{\mathit{head}}+0.229TS{V}_{\mathit{back}}+0.109TS{V}_{\mathit{chest}}+0.028TS{V}_{\mathit{libms}}-0.086$$ (4)

(3) - (4):

$$\mathrm{\Delta }TS{V}_{\mathit{whole}}=0.05TS{V}_{\mathit{face}}+0.022TS{V}_{\mathit{head}}+0.055TS{V}_{\mathit{chest}}+0.038$$ (5)

Winter:

With masks:

$$\mathit{TS}{V}_{\mathit{whole}}=0.266TS{V}_{\mathit{face}}+0.342TS{V}_{\mathit{head}}+0.188TS{V}_{\mathit{back}}+0.163TS{V}_{\mathit{chest}}+0.132TS{V}_{\mathit{libms}}-0.054$$ (6)

Without masks:

$$\mathit{TS}{V}_{\mathit{whole}}=0.205TS{V}_{\mathit{face}}+0.339TS{V}_{\mathit{head}}+0.184TS{V}_{\mathit{back}}+0.166TS{V}_{\mathit{chest}}+0.138TS{V}_{\mathit{libms}}-0.084$$ (7)

(6) - (7):

$$\mathrm{\Delta }TS{V}_{\mathit{whole}}=0.061TS{V}_{\mathit{face}}+0.031$$ (8)

Effect of wearing mask on thermal preference

As shown in Table 4, the mean V_a in the library was relatively low during the summer and winter. However, the subjects expected more V_a during the summer. The humidity level was deemed acceptable by the majority of participants. The subjects' ability to adapt to high relative humidity in South China was the primary reason [63], [64], [65]. The thermal preference of the participants for environmental factors is shown in Fig. 8 for wearing masks and without masks. In the summer, more than half of the subjects preferred to lower the operative temperature to improve thermal comfort, especially those wearing masks. Thus, the effects of masks on thermal comfort are significant. During winter, approximately 40 % of the subjects expected the T_op to rise. Therefore, to improve the thermal comfort, the T_op must be reduced in summer and increased in winter.

Fig. 8.

Percentage thermal preference distribution for environmental parameters (−1, lower; 0, no change; and + 1, higher). (a) summer; (b) winter; (A) without masks; (B) wearing masks.

Correlation analysis of MTSV and T_op/SET*

Within 1 °C intervals from T _op/SET*, the mean thermal sensation vote (MTSV) was computed. The neutral T _op/SET* was determined by using a regression equation. As shown in Table 6 and Fig. 9 , during the summer, for subjects without masks, when MTSV = 0, T _op = 26.5 °C and SET* = 25.3 °C. For subjects with masks, when MTSV = 0, T _op = 26.2 °C and SET* = 25.0 °C. For subjects with masks, the neutral T _op/SET* of the environment was 0.3 °C lower than it was for subjects without mask. With little T _op/SET* difference between the two conditions, the mean value of 26.4/25.3 °C was used as the neutral T _op/SET*. Similarly, the MTSV distributions of subjects with and without masks during winter were calculated. Neutral T _op (SET*) was 20.5 °C (23.4 °C). The blue box shows the proportion of respondents with no symptoms at each temperature gradient. During the summer (winter), when T _op/SET* was 26.4/24.2 °C (22.3/20.0 °C), the proportion of respondents with no symptoms reached 50 %. Therefore, subjects wearing masks preferred a lower temperature environment. This temperature difference is extremely small; therefore, it can be overlooked. However, wearing masks for a long period can cause symptoms in humans. However, it has little impact on the neutral temperature of the environment and whole-body heat balance, which is consistent with some previous studies [24], [66]. Consequently, more consideration needs to be given to the duration of wearing masks and health problems.

Table 6.

Regression equation between T_op and MTSV.

Condition		Equation	Neutral Top (°C)	Comfort temperature range (°C)
Summer	Without masks	MTSV = 0.359Top − 9.51	26.5	25.1–27.8
	With masks	MTSV = 0.398Top − 10.49	26.2	25.0–27.7
	Total	MTSV = 0.378Top − 10.00	26.4	25.1–27.7
Winter	Without masks	MTSV = 0.242Top − 5.00	20.5	18.5–22.5
	With masks	MTSV = 0.245Top − 5.02	20.5	18.5–22.5
	Total	MTSV = 0.244Top − 5.01	20.5	18.5–22.5

Fig. 9.

Relationship between T_op/SET* and MTSV (Left -Y), Relationship between T_op/SET* and the percentage of no symptomatic subjects (Right -Y): (a) T_op; and (b) SET*.

Discussion

The effect of wearing masks on the human health

Adverse effects of mask use have been reported in both healthcare workers and the general public [67], including headaches [68], increased thermal discomfort [67], potential thermal physiological responses [69], and decreased quality of work [68], [70]. Masks can raise the temperature of the skin on the face as well as the heat or moisture of the inhaled air [71]. The inner layer of a long-wearing mask becomes wet due to condensation of water vapor generated by breathing and sweat evaporation [18], [24]. Such elements have been proposed to be responsible for the increased respiratory discomfort when masks are used [72], [73]. The masks were tightly attached around the participants’ face and may have collapsed, potentially increasing dyspnea [71]. Long-term mask use has been linked to an increase in subjective visual complaints [74]. According to an online survey [75], 18.3 % of respondents reported having dry eye issues, particularly women and people who wore glasses or contacts. Wearing masks for an extended period of time causes secondary complications, particularly in vulnerable populations, because airflow is restricted, resulting in a high concentration of CO₂ in the body [76], [77]. The amount of CO₂ in the air affects the blood’s pH, which can lead to numerous health risks like vertigo, dyspnea, headache and hypoxia when the level is elevated [78]. Therefore, the effect of masks on breathing cannot be overlooked. Masks should also be removed in time to eliminate adverse effects during learning. Masks typically influence the face and head of the human body, increasing the need for overall environment comfort, especially when wearing the mask for an extended period of time [19]. Long-term usage of a moist mask might also result in facial irritation and discomfort [19], [25], [26]. Therefore, the masks must be replaced regularly. This is one of the reasons for wearing a mask for no more than 2 h.

Discussion of the whole thermal sensation and local thermal sensation

There have been a number of studies that have conducted a series of experiments on the effect of local heat sensation on whole heat sensation [79], [80], [81], [82], [83]. Zhang [79] found in the study that as the gap between the local thermal sensation of the site and the thermal sensation of the whole body increased, the weight of the influence of the site on the sensation of the whole body increased linearly. Zhang et al. [80], [81], [82] developed several models to predict local and overall thermal sensation and thermal comfort in the human body. For body parts with the same skin temperature, local sensation is much warmer during the cold tests when the whole body is cold, and much colder during the warm tests when the whole body is warm [80]. Some body parts strongly influence overall thermal sensation [81]. The effect of thermal sensation on the overall thermal sensation of different parts is usually expressed in terms of the weight of the influence of thermal sensation in different parts on the sensation of heat in the whole body, and this coefficient is mostly obtained by linear regression [84]. Therefore, multiple linear regression was used to calculate the local coefficients. By comparing the two types of TSV models (without or with masks), the main effects of wearing masks in summer and winter were calculated, and Eq. (5) and Eq. (8) were obtained. Wearing masks had a significant effect on the head, face, and chest, similar to local thermal sensation vote (Section 3.4), and consistent with the mechanistic analysis in Section 4.1.

Comparison of MTSV models

The ASHRAE Standard 55 [50] provides a graphical comfort zone method for indoor environments. In the comfort zone, the activity level was maintained between 1.0 and 1.3 m, and clothing insulation was set at 0.5 clo in the summer and 1.0 clo in the winter. As shown in Fig. 10 , all data in this study were completely outside the comfort zone of ASHRAE Standard 55 in summer. More than half of the data were within the comfort zone in winter.

Fig. 10.

Data plotted onto the PMV − PPD index-based chart.

Considering the adaptability to humidity in the Guangzhou area, combined with the comfort top interval in Table 5, the comfort areas during summer and winter are shown in Fig. 10. It is evident that most of the data during winter are in the comfort band. During summer, the air temperature on the fifth floor of the library was warmer than it was on the lower floors due to some spots receiving direct sunlight. By analyzing the regression equation, the upper limit of acceptable T _op in summer was 29.0 °C. Therefore, when considering the impact of radiation on indoor air temperature, increasing the wind speed or shading is necessary. Owing to the high temperature of the library and the small number of students, this greatly wastes space resources and even leads to further crowding on the lower floors.

Limitations

Many factors affect human health and comfort, such as the duration of wearing masks, environmental parameters, and air quality. However, because of the limitations of field investigations, these factors cannot be controlled. For example, during the test period of this study, the humidity range was small and the impact of humidity on the population wearing masks could not be considered. Therefore, the influence of each factor can be considered only while analyzing the human body. In addition, the mask material has a significant impact on breathing [17], [27], [85], which requires further research. Some of the current conclusions are obtained through field investigation. In winter, wearing a mask for a certain period of time will improve thermal comfort. The maske reduced the heat exchange between the face and around environment. However, from the field survey, some subjects still felt that their face had a fever, which was partly related to the temperature and the duration of wearing a mask. In winter, the indoor air temperature is affected by the outdoor environment significantly. The relevant tests will be conducted in a chamber. Therefore, future simulation experiments can be conducted in an experimental chamber, combining multiple factors for analyzing the impact of wearing masks on human health, and verifying and analyzing the results of this study.

Conclusions

Field tests and questionnaire surveys were carried out in a campus library in Guangzhou, China, in June 2021 and January 2022. By analyzing the relationship between the duration of wearing masks and symptoms and the relationship between the relevant thermal index and TSV, the following results were obtained:

• (1).People wearing masks show symptoms for a certain duration. Wearing masks for more than 2 h, and the percentage of symptoms is more than 50 %. The most frequently voted symptom was facial heat (62.7 % in summer, and 54.6 % in winter), followed by dyspnea (25.4 % in summer, and 35.2 % in winter). Timely replacement of masks and wearing a mask for a maximum of 2 h at once are recommended.
• (2).The mask significantly influences the facial thermal comfort, but it has no obvious influence on the whole body. Wearing a mask for a long time also has varying degrees of impact on the comfort of the head and chest. In summer: ΔTSV_whole = 0.05TSV_face + 0.022TSV_head + 0.055TSV_chest + 0.038; In winter: ΔTSV_whole = 0.061TSV_face + 0.031.
• (3).In the summer air conditioning environment in Guangzhou, the neutral T _op was 26.4 °C, and the comfortable T _op range was 25.1–27.7 °C. Under the natural ventilation environment during winter, the neutral T _op was 20.5 °C, and the comfortable T _op range was 18.5–22.5 °C.

CRediT authorship contribution statement

Tianwei Tang: Data curation, Writing – original draft. Xiaoqing Zhou: Supervision. Kunquan Dai: Data curation. Zhaosong Fang: Conceptualization, Writing – review & editing. Zhimin Zheng: Methodology, Data curation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A.

Appendix B.

Instruments used to measure the environmental parameters.

Equipment	Thermal comfort instrument		Universal wind speed recorder
Model	SSDZY-1		WFWZY-1
Parameter	Ta (°C)	RH (%)	Tg (°C)	Va (m/s)
Measuring range	−20–80 °C	0.01–99.9 %	−20–80 °C	0.05–5.00 m/s
Accuracy	± 0.3 °C	± 2 %	± 0.3 °C	5 % ± 0.05 m/s
Sampling rate	30 s	30 s	30 s	30 s

Data availability

The authors do not have permission to share data.