This method was applied to collect experts' thoughts to extract environmental factors because it was effective on the qualitative information of contextualized and complicated content. Eight experts with healthcare design backgrounds and IDNU experiences joined the interview and provided the information and opinions as to the environmental factors. Their detailed perspectives on the various topics were also explored to provide a holistic comprehension. The group size was not large in order to enable enough in-depth discussion. This approach would enable the experts to provide comprehensive information of environmental factors based on a specific case. The results of the interview were analyzed by two investigators and then synthesized into a list of environmental factors, which constituted the basis for the written survey.

The focus group interview followed three topics prepared by interviewers which comprised of: (1) The environmental factors and intervention in the IDNU from design. (2) The role of environmental design in affecting nurses in COVID-19. (3) The promising design measures and strategies to reduce the fatigue. For each topic, we prepared a list of discussion questions in order to guide the experts round the topics. The interviews were recorded, analyzed, and coded by the other two investigators. All the investigators and interviewers were trained to ensure the quality of interview. The answers and discussion of topics were divided into sub-topics, interpreted, compared, and sorted out. The environmental factors were then categorized by counting the frequencies of experts' agreements on them.

A written survey was adopted to collect the nurses' perceptions of environmental factors on the fatigue. The survey was conducted in the form of online questionnaires due to the restrictions in the pandemic. These questionnaires were based on the results of the focus group interview and designed as semi-structural. The questionnaires consisted of three parts (Appendix A): The first part was about their social-demographic information on gender, age, length of practice, titles, and education level (Table 1). The second part was the rating of the environmental factors regarding its importance on their psychological and physical fatigue (Likert approach, 1–5). The third part was the locations in the IDNU where nurses often worked and stayed. The questionnaires were distributed in the Wechat group of the qualified nurses, and they could answer the survey on their worksites in a given time. The questionnaires were developed with the tool of Questionnaire Star, which was a plug-in based on the Wechat platform (https://www.wjx.cn). This method followed the regulations of the hospital and could reduce the disturbance of the nurses of IDNU. The main environmental factors on the fatigue would be determined based on the results of the survey. The questionnaires were designed and reviewed without the violation of personal privacy. They were anonymous, and the participants were allowed to terminate the survey any time they desired. The focus group interview was conducted from 15 to 18 October, 2020.

In the focus group interview, the participants were consisted of eight healthcare design experts who were recruited from the Shenzhen Architectural Design Group, which was also the design contractor of the targeted IDNU. They were recruited through the team's extensive professional network and joined the interview voluntarily. The recruiting criteria were: (1) They were required to have experiences of three IDNU projects at least. (2) They were required to have professional experiences for at least 10 years in the healthcare sector. (3) They should fully understand the purpose and expected results of the interview.

The nurses of the written survey were recruited from the department of IDNU in Shenzhen 3rd People Hospital. In the pandemic, it was impossible to initiate the massive survey to the nurses in the IDNU due to restrictions on the contact with them. The number of nurses in the IDNU was also much less that of other nursing units. Hence, we decided to focus on one typical and large IDNU to ensure the quality of the survey. We distributed the requirements of the survey to all the nurses in that department with the assistance of hospitals. They were all trained and managed in the similar system with these from other public hospitals, which accounted for approximately 3/4 of all in-patient beds nationwide. The selection criteria included: (1) They had to work full-time in the IDNU; (2) They had the frontline experiences with COVID-19 cases, which meant they engaged in the direct treatment and care of confirmed cases; (3) They worked in the IDNU for a year at least. Qualified respondents would be identified as volunteer participants and invited in the survey group, after obtaining their verbal consents. Eventually, we received 64 valid feedbacks out of 72 qualified nurses. The written survey was conducted from 11 to 30 November, 2020.

The IDNU of Shenzhen 3rd People Hospital was selected in this study because it was the only designated COVID-19 treatment hospital in Shenzhen for its capacity and specialty (Figure 2). In the pandemic, the administration requested all the moderate-to-critical-condition COVID-19 cases to be transferred to this hospital for medication. The environmental design of that IDNU is very representative in China, which is featured with “three zones and three corridors” (Figure 3). As a special type of nursing unit, IDNU is more complicated than normal ones because it was designed to accommodate highly contagious patients and followed rather rigorous infection control protocol (Figure 4). Since all the IDNU were owned by the public hospitals, they were all designed and built under the same codes and guidelines.

Expect for conventional descriptive analysis, one-way ANOVA, and t-test were used to compare the environmental factors on the physical and psychological fatigue respectively, by age, gender, technical titles, and education. To find out the correlations of these factors on the fatigue, Pearson's correlation analysis was performed. The reliability of questionnaires was tested with the Cronbach's coefficient which was 0.792, indicating good reliability. The validity of the data was tested by KMO and Bartlett test and proved to be suitable of research with KMO of 0.771. The statistical analysis was conducted on the SPSS V26.0 (IBM Corp, Armonk, NY, USA). The significance level was set at p = 0.05.

According to the focus group interview, the environmental factors of IDNU could be sorted into four main categories which are Nursing Distance (ND), Spatial Crowdness (SC), Natural Ventilation, and Light (NVL), and Spatial Privacy (SP) (Table 2). By counting the agreements of experts, ND was thought to be the most important factor with total counts of 21. It would affect how much distance nurses would walk every day, which was considered as a prominent indicator of physical exhaustion (37). It could be determined by the plan layout (8), location of nursing station (6), and number of wards (7). In this case, the plan layout of IDNU was linear and featured with “three zone and three corridors,” which was the most common type of IDNU. The nurse station was located in the middle of the nursing unit. SC received total counts of 11. It could affect the nurses' perceptions of crowdness by spatial layout and design (30). The factors were consisted of size of nurse station (3), ratio of supportive rooms (4), and width of corridor (4). In this IDNU, the size of nurse station was 2.4 m × 7.2 m (depth and length), which was standard on every IDNU floor. The proportion of supportive rooms (rooms except for wards and medical rooms) was 32.2%, indicating a compact layout. The width of the main corridor was 2.4 m in this case. Natural light and ventilation were identified with the total counts of 12. It was reported to be associated with the stress and anxiety reduction (38), and mainly determined by the arrangements and design of windows. It included the factors of daylight around the nursing station (5), openable windows in the end of corridor and break area. In this case, there were openable windows at the end of corridor. The last factor was SP that received the total counts of 17, just behind the nursing distance. It referred to the perceptions of privacy in a spatial setting, and could be profoundly affected by environmental design. It included the sub-factors of independent break areas (8), safety of workplace (6), and typology of nurse (3). In this IDNU, there were no independent break areas for nurses. The access of their workplace was restricted to the visitors but connected to the patients. Their nurse station was open to the semi-contaminated area, where the exposure to the infection was high.

The environmental factors were rated by the nurses to determine their influence on the fatigue. The mean value of scores on the physical fatigue was higher than that on the psychological fatigue generally, indicating that these factors could significantly influence the physical fatigue (Table 3). Among them, ND was rated the highest on physical fatigue, with a mean value of (3.92), while SP was rated the highest (3.55) on the psychological fatigue. Regarding the frequent locations of nurses, it was apparent that senior nurses were mostly staying in the clean and semi-contaminated zone, while nurse assistants and practical nurses were often in the semi-contaminated zone and contaminated zone with frequent commuting from wards to nursing station (Figure 5). One-way ANOVA was conducted among clusters of nurses to compare these factors on the physical or psychological fatigue. The approximate normal distributions and equal variances were tested and satisfied by Shapiro–Wilk test of normality and Levene's test for homogeneity. The results revealed that technical titles could affect the nurses' perceptions of the fatigue from the environmental factors. The ratings from senior nurses were lower than practical nurses and nurse assistants, indicating that environment had less influence on the senior nurses than others, and were more evident on the low-ranking nurses.

This study demonstrated that the physical environment of IDNU could affect the fatigue of nurses, which is consistent with the previous studies on the other healthcare settings (9, 26). This study further showed that the environmental factors could be sorted into four main categories of ND, NVL, SP, and SC, which were identified as the basic parameters affecting the fatigue from the environmental design. In the nursing unit, ND is associated with the daily walking of nurses, and the larger ND is, the more distances nurses have to walk (37). ND could be affected by the plan layout, the location, and the size of the nursing station, indicating that fatigue could be interfered by the spatial design. SC and SP are thought to affect the perceptions of the nurses about the working environment. A crowded and intense working environment could make nurses nervous and stressed, especially during COVID-19. Similarly, the lack of SP could reduce the sense of security of the nurses, hindering their recovery from the heavy workload. The sense of privacy is thought to be associated with the psychological restoration spatially. NVL is supposed to consist of daylighting and fresh air, as well as the restorative effect natural resources by enhancing the perceptions of the nurses (45). Besides, it could provide nurses' sense of bio-safety and escaping from exposure in IDNU, which is often filled with the highly contagious pathogens.

These factors are thought to have more influence on the physical fatigue than the psychological fatigue, generally. The mean value of the environmental factors on the physical fatigue is all higher than that of psychological fatigue (Table 3), especially for ND (3.92). The advantage on the physical fatigue, may attribute to the surging amount of workload due to COVID-19. In another side, NVL was not found to be significantly associated with the fatigue (Table 3), which could be different from the previous studies (26, 41). The possible reason is that, in COVID-19, nurses in IDNU were always overwhelmed by work and not able to rest for long, which discourage them to benefit from restoration from natural resources. The prior studies showed that increasing the proportion of single-bed wards would reduce the fatigue and stress of nurses (9, 26, 46). Such results are not found in this case because double-bed and triple-bed wards are still the primary configuration in the nursing unit of developing countries due to limited healthcare resources. To illustrate, increasing the proportion of single-bed wards would barely alleviate the fatigue because it would increase the nursing distance by enlarging the size of nursing unit. Anyhow, this study reveals the dilemma of the hospitals in the developing countries, where there is a large demand of healthcare service but limited resources. These studies may be different from the studies in the developed countries.

Nurses did not consider these environmental factors equally. ND and SP were thought to be the principal factors that influence the physical and psychological fatigue, respectively (Table 3). In accordance to the previous study, ND is found to be the main factor on the physical fatigue because it is closely associated with the daily walking distance for nurses. Cruising between patient rooms and nursing station is a large stressor for the work-related fatigue (6). For a routine, nurses need to walk on almost every step of their work from reading the instructions of doctors at the nurse station, ensuring the instructions in the wards, and entering the records after returning to the nurse station. The increasing walking distance due to COVID-19 makes nurses more likely to be physically exhausted than normal. Hence, ND is supposed to be the main factor on the physical fatigue.

SP is thought to be the most prominent factor on the psychological fatigue. The previous studies paid more attention on the patients than nurses regarding the perceptions of privacy (26). This study showed that the perceptions of nurses on the fatigue could also be affected by the sense of spatial privacy, especially for low-ranking nurses. In China, nurse assistants and practical nurses often have to share their working and resting space with others, leading to the lack of privacy. The possible reason could be the limited spatial resources of nursing unit, and the priority is always given to the needs of patients, doctors for the medical function. There is little considerations of privacy on the low-ranking nurse, as a result, they could suffer from the fatigue due to the disruptions such as noise and wrong way finding (9). Moreover, increasing SP by separating nurses away from the public could enhance their sense of safety. Owing to frequent incidents of assaulting healthcare workers in China, nurses would feel safe if their private areas are ensured with restricted access. Therefore, for the sake of their safety and well-being, the issue of privacy should be improved by environmental design such as setting up private break areas, common space, and restricted access.

Nurses perceived the distinct influence of environmental factors on the fatigue. To clarify, a stronger influence on the physical fatigue than the psychological fatigue was found in this study, particularly for practical nurses and nurse assistants. Normally, they are more physically involved with the daily workload. It could also be proved by the ratings of ND, which was scored the highest on the physical fatigue by them. The possible reason is that COVID-19 makes the workload increase dramatically, and nurses have to walk more distances than normal, leading to the prevalence of physical exhaustion. Besides, the lifting and supporting activities during transferring patient could also contribute to the physical fatigue as the work becomes heavier. Report showed that the workload of nurses increased almost 1.5–2.0 times during the pandemic in some developing countries (14). Moreover, IDNU requires a complicated and rigorous infection-control protocol. There are two sanitary passes between three zones where they need to change gears (Figure 3). It could be very physically exhausting for nurses because they have to commute to wards for many times a day. Such studies are not evident for the senior nurses as they are less physically involved. Senior nurses are more vulnerable to psychological fatigue as their jobs are more about the management and instruction.

Technical titles showed significant differences for nurses on their perceptions of the environmental factors. Regarding the physical fatigue, practical nurse, and nurse assistant rated higher than senior nurses did. It indicates that low-ranking nurses are more likely to be affected by these environmental factors on the physical fatigue. In IDNU, the work of low-ranking nurses is more physically oriented than senior nurses. In the pandemic, they walked longer than normal, and would not stay in one place for long because they needed to keep commuting to ensure the condition of the patients of COVID-19. As a result, they were more vulnerable to physical fatigue than senior nurses. As the low-ranking nurses account for the majority of the nurse staff (87.5%) in this case, it highlights the demand to put more attention on the low-ranking nurses when conducting environmental interventions.

Senior nurses were found to be more affected on the psychological fatigue by these environmental factors, which aligned with the previous studies that they were more likely to suffer from the psychological fatigue. Their daily work is mainly about management, instruction, and supervision; and their frequent working places are often at the nursing station and office (Figure 5). The characteristics of their work are less physical exhausting but could produce more stress. As a result, their walking range is not as large as low-ranking nurses, and they are less affected by the physical fatigue than the psychological fatigue by the environment.

Environmental interventions and strategies should be presented to help designers and decision-makers to improve the working environment of frontline nurses. According to the findings of main environmental factors on the fatigue, the design recommendations on ND and SP would be emphasized.

Reducing ND could be the primary design advice. It is suggested to locate the nurse station in the middle of IDNU instead of in the end to reduce the maximum nursing distance for one direction. There are two types of IDNU plan: middle-mode and end-mode, depending on the location of the nursing station. The middle-mode could be more beneficial to nurses fatigue because the distance to wards would be balanced for both directions. For the large IDNU, setting up a secondary nursing station could be a solution to reduce the nursing distance (37).

As for SP, creating an independent break area for nurses could be beneficial to alleviate the fatigue. The design measures could also intensify this effect such as arranging the patio and balcony with access to nature. It could help nurses recover from the heavy workload and other disturbances. Improving privacy by setting up the independent areas could also promote their sense of safety by separating them from patients and visitors. With the access to nature, it could further provide nurses restoration and sense of escaping from the stress.

As a survey and interview-based study, it is limited in the sample size. Nonetheless, the IDNU of this study is representative for its environmental features and nurses, one hospital could limit the scope of the findings and dynamic behind it. After the restrictions of IDNU are removed as COVID-19 gradually under control, IDNU from more hospitals of different levels could be explored to find out the dynamic behind the environment and fatigue. In the future, with the development of wearable devices, mobile app and online instruments, more objective and detailed information could be collected.