As long as human beings roamed the earth, nature and greenness were omnipresent parts of life. With human evolution came industrialization, advancing technology and sedentary behavior. From an evolutionary perspective, these general advancements, as well as grey colored facades, industrial areas and man-made pollution came to existence just about a blink of an eye ago. According to the biophilia-hypothesis, humans have developed and maintained an affinity for nature throughout their evolution [
1,
2], which makes positive outcomes from exposure to green spaces on the human body and mind stand to reason. Correspondingly, “greenness” is linked to restorative effects on cognition and stress [
3,
4], meaning that green space improves mental recovery after stress and reduces factors associated with decreased well-being.
Neighborhood greenness and mental health
The presence and quality of nature as well as access to it, measured by vegetation indices or subjective perceptions, is referred to as neighborhood greenness. It is associated with better mental health outcomes such as a reduced risk of stress, propensity to psychiatric morbidity, psychological distress, depressive symptoms, clinical anxiety, depression and reduced need for mood disorder treatment in adults [
5‐
13]. Despite this general evidence, a systematic review of the current literature on neighborhood greenness and mental health identified a lack of research in specific settings or for subgroups [
14]. Additionally, the reported outcomes may not exclusively be limited to objective greenness measures, since there is research showing that perceiving the neighborhood as highly green is accompanied by better mental health outcomes compared to perceiving it less green [
15]. Objective greenness measures were not included in this last study; thus the link between objective and perceived greenness could not be assessed.
Two main mechanisms have been proposed. First, there seems to be a preference for natural over built environments among humans, even when only photographs are presented (e.g. [
16]). Accordingly, it was theorized that just having a view of nature is enough to improve affective restoration to a greater extent than built environments [
17], meaning a reduction in the likelihood of emotional impulsive behavior and a reduced risk for mental illnesses. In line with this, a residents’ immediate home-environment was associated with higher satisfaction and well-being scores when there were natural elements in the view from their windows [
18]. This mechanism can be defined as greenness influencing humans through a mere visual exposure to greenness.
A second hypothesized mechanism is that one has to be physically exposed to green space so that it can have an influence. This means that positive effects on human health can only be obtained once individuals move or stay in natural environments over a given timeframe (e.g. [
1,
2,
19]). Most evidence for this hypothesis comes from cross-sectional studies that compared residents with high access to green in their neighborhood with residents with low access to green [
14]. Benefits on mental health and health in general from being physically present in a green space can be derived from a host of variables. Neighborhood greenness provides opportunities for being physically active, e.g. by increasing walking for recreation, or by promoting social contacts, e.g. through increased interaction with other people [
20,
21].
Most previous studies have assessed the relationship between green in the living environment and mental health. For instance, research focused on the amount of green around people’s residence or in the neighborhood, having a garden, the presence / number of green spaces within a certain distance or area, or the distance to the nearest green space (see: [
14]). However, people do not only spend time at home during the day. Many adults spend much of their time at work and many students at university or other educational facilities. The latter subgroup is known to have a high potential to suffer from stressors, which can be created by moving out from home in conjunction with academic, social and financial pressure [
22]. Similar to the findings related to greenness at home, we propose that green around university campuses has similar effects on mental health. This gets support from a study finding that students at universities with higher perceived greenness of a university campus are more likely to self-report higher quality of life compared to students with lower perceived greenness [
23]. It is, however, not expected that direct greenness exposure (being outdoors in the green environment for prolonged times) is the major driver of mental health benefits in this case. Home-environments are often chosen based on their natural surroundings and the “dose” of nature may depend on the actual time people use the green space. In line with this, there is support from a study that reported a decline in green space use with increasing distance from the home-environment [
24]. University-environments, where people are not as likely to spend much of their free time as at home, are usually not chosen with green space as criteria in mind. We suppose that possible positive effects from green space around students’ campuses will be based more on the first mechanism, which is concerned with mental health outcomes by looking at nature. There is a more limited opportunity to direct exposure to it, e.g. when walking around on the campus during classes, compared to home environments. Furthermore, the relation of green around the study environment with mental health might be less influenced by social economic status or a person’s own preference for their living environment.
This study extends the current body of evidence by assessing the associations between perceived greenness and mental health and, specifically, includes the perceived greenness both at home and at university.