The world’s urban population has rapidly increased from 0.8 to 4.2 billion between 1950 and 2018 and will continue to increase to an expected 5.7 billion in 2050 [1
]. The rapid urbanization and associated urban sprawl destroy or modify natural areas (e.g., forestry and grasslands) in the process of transformation to buildings and human infrastructure to accommodate the growing urban population [2
]. The decrease in natural areas in and near cities disconnects people from nature, something which has been found to contribute to stress-related health issues [3
]. Green spaces (e.g., parks, gardens, and green streetscapes) have widely been employed to improve the urban environment and mental health, as access to urban nature and going outdoors are associated with positive psychological well-being [4
Two theoretical perspectives are especially relevant to the positive psychological effect of green space on well-being [5
]. The Attention Restoration Theory (ART) emphasizes the perceptive response of humans to the natural world, aiming to understand how people perceive, understand, and explore natural settings [6
]. This theory inclines to stimulate people’s attention indirectly by characterizing feelings into four categories: being away, fascination, extent, and compatibility [7
]. ART has been widely used to assess the psychological benefits of green spaces, particularly on relieving anxiety [8
], reducing stress [9
], increasing happiness [10
], and restoring attention [3
]. Comparatively, the psycho-evolutionary Stress Recovery Theory (SRT) can evaluate human restorative responses to natural environments directly, often within minutes [11
]. This theory assumes that humans have two physiological responses to contact with nature: the preference for the natural environment and restoration after stressful events [12
]. Based on this theory, several physiological indicators have been proposed to measure the human response of accessing the natural environment, including heart rates [13
], blood pressure [8
], muscle tension [14
], and salivary cortisol level [13
Just understanding the theories of stress recovery and attention restoration is not enough to support the improvement of the urban environment in practice due to the limited details provided to guide the provision, design, and management of green space. Therefore, studies have attempted to identify the specific elements and configurations of green space that have significant impacts on mental health. For example, planting trees in barren areas leads to a significant increase in stress recovery, while the performance varies across the vegetation density and species [15
]. Hoyle et al. [16
] found that native plants with multiple layers were perceived as significantly more restorative than non-native plants with a single layer. The performance of stress recovery may further be improved by viewing pleasant rural scenes [17
]. Additionally, water bodies are a highly valuable element in stress recovery [18
]. White et al. [19
] pointed out that those with access to water bodies suffer lower levels of stress. However, the impacts of the water intervention (e.g., waterbody types and waterbody size) in green space on stress recovery are still unclear due to the limited amount of studies to date. Additionally, animals (e.g., birds) in green spaces help reduce levels of stress and anxiety since they can provide humans with a source of companionship, support, and entertainment [20
]. Other elements, including openness [22
], aesthetics [23
], fascination [24
], and accessibility [25
], have also been discussed to understand the impacts of green space on stress recovery and attention restoration.
Based on existing findings, a set of projects have been proposed by governments or organizations to improve and green urban environments. For example, URBACT introduced a Health and Greenspace action to link the green infrastructure design and management to urban health policies and practices in order to promote mental and physical health for communities [27
]. The World Health Organization (WHO) developed an urban green space indicator and toolkit to help policy-makers with evidence-based green space interventions for the health promotion of urban residents [28
]. Although these greening projects present useful suggestions and guidance for developing healthy cities, the realization and design of green spaces can be challenging for compact urban areas in practice due to limited available space and intense competition with other land use [29
]. Alternatively, greening the low-density urban areas can be an option for improving urban environments. In countries such as China, low-density areas have more easily available land for greening while still hosting a substantial number of people. This points to their feasibility for green space development in light of the need for mental health improvement. However, due to a lack of studies specifically focusing on green spaces in low-density urban areas and the different characteristics between compact and low-density areas, whether low-density residents’ exposure to green space can positively influence their mental health is still not clear. Therefore, there is a need to understand the impacts of green spaces on stress recovery and attention restoration in low-density urban areas.
Additionally, existing studies often assess stress recovery and attention restoration from a single perspective, which can be either physiological or psychological. On the one hand, psychological assessment based on surveys and investigation mainly emphasizes human perception. Grahn and Stigsdotter [30
] employed perceived sensory dimensions (PSD) to assess the stress restoration of green space via investigating 953 participants in Sweden. Wang, Zhao, Meitner, Hu, and Xu [23
] utilized the perceived restorative scale (PRS) to assess the restorative potential and aesthetic preference of green space. The Restoration Outcome Scale (ROS) is used by Mattila et al. [31
] to explore the restorative experiences in a virtual reality forest environment. The results of these studies strongly depend on people’s voluntary participation and suffer from subjectivity and bias. On the other hand, physiological assessment enables detecting human responses to green space more objectively. Physiological indicators such as heart rate variability (HRV), skin temperature (SKT), and electroencephalography (EEG) have been used to measure the stress recovery of green façade [32
]. Li et al. [33
] employed the skin conductance level (SCL) and blood volume pulse (BVP) to measure the restorative potential of participants viewing a virtual reality forest. However, physiological assessment fails to capture people’s feelings. Due to the complementary function of physiological and psychological assessment, they should be considered in a more integrated way, combining them into an assessment of restorative perceptions.
To help fill the gaps in research mentioned above, this study integrates physiological and psychological assessments to understand the contributions of urban green spaces to stress recovery and attention restoration in low-density residential areas. Specifically, the objectives of this study were (1) to measure and compare the physiological and psychological responses to accessible green spaces, and (2) to identify the key elements of green spaces that have significant influences on stress recovery and attention restoration in low-density areas. This study aimed to broader our understanding of the impact of green spaces on mental health improvement, thus facilitating the planning and design of green spaces in low-density residential areas. The remainder of this paper is structured as follows. Section 2
introduces the study area, participant groups, assessment, and statistical methods with details. Section 3
explains the physiological or psychological assessment results. Section 4
discusses the implication and limitations of this study, followed by a conclusion in Section 5
2.1. Study Sites
This study was conducted in Fuzhou, Fujian Province, China. Fuzhou is a coastal city with 7.8 million people in 2019. The city experienced rapid urbanization in the past decade. According to the Fuzhou Municipal Bureau of Statistics (FMBS), the urban population was increased from 2.97 million in 2011 to 5.5 million in 2019 [34
]. The population expansion accompanies increased demand for green spaces that can provide numerous benefits such as leisure purposes and stress recovery. However, the growth of land for accommodation purposes in Fuzhou affects the available land for urban green spaces. As a result, the development areas of urban green spaces was significantly decreased from 710 ha in 2010 to 166.86 ha in 2019 [34
]. Alternatively, intensive green spaces have been dispersedly developed outside the compact urban area in the urban fringe, collectively called ‘low-density residential areas’ (or small–medium-sized cities) [35
Linpu has almost 7000 people and an area of 154.72 ha, which is a typical low-density residential suburb (45.24 people/ha) in the southeast of Fuzhou city. This area has longitude and latitude by 119°21′45″ E–119°22′16″ E and 26°01′07″ N–26°01′28″ N, respectively. Linpu was selected as a case study for three reasons. First, there are several green spaces and water bodies in the area, which can provide an opening view and a thermal comfort environment. The high-quality landscape provides a tremendous restorative opportunity for children, elders, and workers, which is highly appropriate for investigating the impacts of green spaces on stress recovery and attention restoration. Second, Linpu has a large number of well-preserved historical buildings and cultures. Due to its historical values, the area was nominated to the seventh batch of ‘Chinese historical and cultural villages’ in 2019. The well-preserved heritage can represent the traditional culture of Fuzhou city. Finally, Linpu is close to several industrial parks (e.g., Strait International Conference and Exhibition Center), which implies that there is a need to improve the mental health of office workers.
This study selected 13 typical sites of Linpu as experimental areas. The selected sites are further characterized based on plant richness, water landscape, openness, topography, road network, and cultural landscape to understand the impact of landscape elements on stress recovery. The details of these case areas and site attribution are illustrated in Figure 1
and Table 1
The participants identified in the experiment were recruited by using two methods: advertisements through social media apps and face-to-face at the campus of Fujian Agriculture and Forestry University. The following inclusion criteria were used:
People who have a mental illness history and undergo treatment for any disease are not included as they may feel depression and anxiety [32
Participants should have normal or corrected-to-normal vision since the loss of vision experiences continuous mental stress [32
People in the menstrual period are not selected since they may experience significant emotional changes caused by fluctuating hormone levels during this period [32
People with smoking or drinking habits are excluded because the habits can change feelings of depression, anxiety, and mood [37
To avoid some interventions such as political stance, all selected participants were neither involved in the projects nor conflicted with interest. As a result, a total number of 33 participants consisting of 14 males and 19 females are identified for the experiment. The age of all participants ranged from 22 to 28 years old. All participants were students and visited all selected green spaces. The first perceptions of participants were employed to measure the effectiveness of each site on stress recovery and attention restoration.
2.3. Measurement of Physiological and Psychological Responses
Due to the different characteristics of physiological and psychological responses, the ways of measuring these are different. The following sections present the respective assessment methods used.
2.3.1. Physiological Assessment
Several indicators have been proposed to measure physiological responses, e.g., to green space use. In the present study, five widely used indicators, including electrodermal activity (EDA), facial electromyography (EMG), respiration sensor (RESP), skin temperature (SKT), and photoplethysmography (PPG), were selected to measure human emotional experiences. The values of indicators were measured by the ErgoLab data platform Version 2.0, which is a set of biometric wearable sensing devices, as shown in Figure 2
EDA aims to measure stress levels by monitoring skin conductance. Skin conductance caused by a stimulus of the sweat glands’ secretion was monitored by a wearable sensor with two reusable electrodes attached to two fingers of one hand [32
]. High skin conductance leads to a high EDA, indicating the high stress levels of human bodies and negative physiological responses.
EMG reveals the emotional response by monitoring facial muscle activity. To ensure the accuracy and comprehensiveness of measurement, multiple sensors were employed to detect and amplify the small electrical impulses generated by specific facial muscles (e.g., corrugator and zygomatic muscles). The electrical impulses were then converted into EMG signals to measure facial muscle activity. A high value of the EMG signal means a strong reaction and high stress.
RESP aims to reveal the level of relaxation. A respiration sensor can measure the respiration waveform, amplitude, and frequency of the human body by detecting chest or abdominal expansion and contraction. The real-time information recorded by the sensor was inputted into a respiration belt place around the abdomen or chest for the calculation of RESP. A high RESP represents tension or nervousness, while low RESP represents relaxation [43
SKT measures human stress by monitoring the skin temperature of fingers. Wireless skin temperature sensors attached to fingers were used to measure the SKT values. In general, low SKT can be monitored in a relaxed environment.
PPG reflects an emotional response by measuring heart rate variability. Heart rate variability caused by the surrounding environment was measured by a portable, wireless photoplethysmography sensor attached to the earlobe. High PPG commonly accompanies tension or nervousness.
2.3.2. Psychological Assessment
Since PRS supports our understanding of stress reduction, the attraction of green space, the motivation for visiting, and the compatibility between people and nature, it has been widely used to measure the restorative quality of physical environments. This study employed a version of the PRS derived from Hartig, Korpela, Evans, and Gärling [7
]. There are four scales and 26 items in the PRS, as shown in Table 2
. Most items are evaluated with a 5-point scale from 1 = Not at all to 5 = Completely. However, some reverse items are assessed with a 5-point scale from 5 = Not at all to 1 = Completely due to their negative relation with restoration. The scores of all items are summed up to calculate the value of PRS. In general, high values indicate the high effectiveness of attention restoration.
The experiments were conducted on sunny days from January 2018 to December 2018. The time of testing ranged from 8:00 to 11:00 and 14:00 to 17:00 in each experiment day. Figure 3
illustrates the process and time distribution of measurement. The measurement started with a brief introduction to explain the goals and specific procedures of the experiment. Then, the electrodes and sensors were installed on the participants. After a 2-min adjustment period, participants were asked to sit and rest for 3 min, followed by recording the physiological and psychological responses of participates in the village entrance as a baseline. Subsequently, participants visited the case areas from S1 to S13 in sequence. At each site, participants were given 5 min to fill a questionnaire (PRS) to test their attention level. Meanwhile, the electronic data were continually recorded during this period. Finally, the electrodes were stripped if data collection was finished, followed by reporting data.
2.5. Statistical Analysis
To ensure the reliability of electronic data, the Kruskal–Wallis (K–W) test was used to analyze the significance of EDA, EMG, RESP, SKT, and PPG. Meanwhile, the scores of PRS before and after the recovery stage were analyzed by Paired sample t-tests to keep the internal consistency. The level of significance was set at p < 0.05. The statistical analysis in this study was processed in IBM SPSS Statistics 22.
Understanding the impacts of green spaces on mental health improvement in urban areas, particularly in low-density residential areas, helps develop healthy cities. This study investigated the physiological and psychological responses of participants aged between 22 and 28 visiting green spaces in low-density areas. The results showed the significant impacts (p < 0.05) of green spaces on participants’ PRS, EDA, EMG, RESP, and PPG, despite the negligible influence on SKT. The psychological and physiological responses of participants were highly consistent and correlated (R < 0.8). Additionally, the typology of green spaces is discussed to provide suggestions for planning green spaces in low-density areas. Based on the results, this study recommended increasing plant species richness, water landscape, bumpy ground, and culture landscape, and reducing roadways to improve the stress recovery and attention restoration potential of green spaces. However, no significant correlation was found between the mental health benefits and openness of green spaces in low-density areas. The findings can demonstrate the positive impacts of green spaces on stress recovery and attention restoration and help design high-performance green spaces. Overall, this study provides planners and landscape designers with specific information on specific configurations and characteristics of green spaces in low-density areas that promote mental health, supporting their decision-making.