Economic growth and urbanization can bring better living conditions, various opportunities (e.g., rapid development of the tertiary industry, employment opportunities, education and health care opportunities), and challenges (e.g., resource destruction, environmental pollution and frequent disasters). In the process, however, environmental problems are becoming increasingly serious, including a dramatic decrease in greenspace. Many urban dwellers today do not have easy access to and contact with various forms of greenspace (e.g., parks, green corridors, and functional green structures), including natural and artificial greenspace, which has negative impacts on human health and sustainable development in urban areas [1
]. A report by the World Health Organization (WHO) showed that nearly 25% of the world’s diseases were caused by environmental factors. With the faster-than-ever pace of modern life, an increasing number of urban residents experience unfavorable environmental exposures that adversely affect their mental health and often lead to negative emotions [2
]. In addition, the modern living environment often leads to the separation between individuals and families, as well as the reduction in social cohesion and interaction. Therefore, countries around the world have formulated the National Environment and Health Action Plan (NEHAP) [4
], which emphasizes environmental benefits that help counteract these urban threats and improve people’s health outcomes [6
]. Coincidentally, China has put forward the “Healthy China” initiative and highlighted the role of the environment in promoting national health and quality of life.
Urban greenspace has been associated with a wide range of health benefits. Decades of research has examined the direct effects of greenspace on people’s physical and mental health based on fixed contextual or areal units (e.g., census tracts, postcode areas and street network buffers). Some researchers suggested that residential green environment can help to regulate microclimate [8
], purify the air [9
], reduce noise pollution [10
], and promote the quality of the residential environment [11
]. All of these ecological benefits contributed to reducing the risk of obesity [13
] and high blood pressure and diabetes [14
], thereby improving physical health in general [15
]. Meanwhile, a rapidly expanding literature showed that exposures to greenspace help to strengthen individual attention [17
], enhance intelligence and inspiration [18
], and promote self-awareness and ability to reinvent oneself [20
]. Availability of ample greenspace has been found to have restorative [18
] and stress-relieving qualities [22
], and is recommended as an effective way to decrease violence and crime [24
]. Wood et al. [25
] and Akpinar et al. [26
] indicated that better psychosocial status was not only associated with the quantity and accessibility of greenspace, but also with the functions and types of the greenspace people are exposed to. Berg et al. [27
] found that the time spent on visits to greenspace should be considered individually, since it is a mediator in the relationship between greenspace and mental health.
Previous research mainly focuses on the direct effects of greenspace on physical and mental health. Recently, the literature on greenspace and individual health has expanded to consider its effects on people’s physical activity. Although urban inhabitants typically benefit from superior access to medical technology, health care, and other services, these benefits are offset by their sedentary lifestyle and lack of physical activity [28
]. Inadequate physical activity has been identified as a major risk factor of human health. Urban greenspace is now recognized as a suitable setting for physical activity and for its potential for promoting health outcomes. Some studies sought to examine the association between objectively measured greenspace, physical activity, and physical health. The results suggested that the provision of abundant urban greenspace may reduce the risk of obesity and promote physical health by increasing people’s physical activity level [30
]. A series of studies on the relationship between greenspace and individual mental health outcomes showed that physical activity is likely to be a mediating factor in this relationship: Namely, residents with higher levels of access to greenspace, and thus more opportunities for physical activity, reported better stress-relieving effects, mental health, and well-being [30
However, previous studies have rarely paid attention to the influence of daily greenspace exposure on social health and social interaction. Generally, the above studies analyzed the effects of greenspace on one or two dimension(s) of health (e.g., physical health, mental health, or social health) through the mediator of physical activity, but the different effects of greenspace on different dimensions of health were ignored. Thus, this paper focuses on the direct and indirect effects of objectively measured greenspace on three dimensions of health (physical health, mental health, and social health), and compares the different effects of greenspace on these health dimensions. Considering the definition of “greenspace” in previous studies that only includes “vegetation coverage” as too narrow, this paper extends the concept of “greenspace” to the broader notion of “greenspace exposure” by adding an activity dimension according to the concept of “environmental exposure science.” Greenspace exposure in this study encompasses the quantity, quality, and accessibility of greenspace for a person to meet the needs for a better urban environment and recreational activities in the actual geographic areas of his or her daily life. Among the indicators of greenspace selected in this paper, green vegetation contributes to improving the natural environment. Physical activity sites can provide more structured environments for social interactions and physical activity [35
]. In addition, accessibility to greenspace also has an impact on physical activity and social interactions.
In addition, some qualitative and quantitative studies examined the relationship between the environment and health using fixed geographic or contextual units based on buffer areas around individuals’ residences [36
] or administrative units, such as census tracts, postcode areas and street network buffers [39
]. These studies presupposed that the most relevant areas affecting health were residential neighborhoods or residence-based buffer zones delimited in a variety of ways. This presupposition entails the view that people who live in the same contextual unit experience the same environmental impacts, regardless of where they actually work or undertake their daily activities. However, it is inappropriate to use static geographic units like census tracts to represent people’s true activity space that exerts contextual influence on their health, since there are considerable differences in people’s daily spatiotemporal behaviors, which may lead to their exposures to different areas beyond their residential neighborhoods [42
]. Static geographic units cannot accurately represent people’s activity space, since they ignore human mobility and daily spatiotemporal behaviors [45
]. Thus, human mobility cannot be neglected and it is essential to look beyond residential neighborhoods to take into account people’s environmental exposure in their daily activity space. Recently, Kwan [45
] called our attention to the crucial role of human mobility and daily activity space in accurately assessing people’s environmental exposure through the notion of the uncertain geographic context problem (UGCoP). Several studies have provided important evidence on how the UGCoP may affect research findings in environmental health studies and the need to use geographic units or methods that capture people’s spatiotemporal activities [49
]. For example, Zhao et al. [47
] suggested that researchers should try to estimate the influence of various environmental exposure on individual health more accurately using contextual units that can capture people’s daily activities and travel. Therefore, this study seeks to identify and delineate residents’ activity space during the 24 h of a weekday to capture the real contextual areas that people are exposed to and interact with, in order to advance the analysis of how true environmental exposure level in people’s daily activity space affect their health. The reason for choosing only weekdays to study is that participants in this research are between 19 and 59 years of age and are employed (e.g., employees, employers) (note that students are excluded from the study), which may lead to routine and similar activities during weekdays. However, there may be considerable variations and irregularity in their activities in weekend days. Therefore, we selected to focus on participants’ daily activities in a weekday as their usual behaviors in this research, since they have more regular and frequent activities in weekdays, which took up most of their individual life when compared with weekends. In addition, given that where and when people spend their time differ from individual to individual, the spatiotemporal features of daily activities and their cumulative effects are also considered in this article. These features include activity types, activity locations and actual time spent in different areas.
Based on such considerations, this study takes advantage of the methods of human mobility research and constructs a conceptual framework for modeling the health benefit of individual daily greenspace exposure. A structural equation model is used to analyze the causal mechanisms among daily greenspace exposure, physical activity, and individual health based on questionnaire survey data and objectively measured data (e.g., deriving vegetation coverage using remote sensing images). Meanwhile, individual activity space is considered when analyzing how greenspace exposure influence people’s health behaviors and outcomes. Specifically, the article seeks to address the following questions: Is there any relationship among daily greenspace exposure, physical activity and individual health from the perspective of human mobility and the uncertain geographic context problem? Does individual daily greenspace exposure directly affect health, and does it also indirectly affect health through the mediating role of physical activity? How do specific elements of daily greenspace exposure and physical activity influence the different dimensions of individual health? This paper seeks to enrich and deepen our knowledge of health geography and spatiotemporal behaviors. Moreover, it has theoretical and practical value for urban planning, informing greenspace construction programs and strategies for achieving global environmental health.