5.1. Measures of Greenspace Exposure
This study has used different greenspace exposure measures to be tested for their association with health-related outcome measures. NDVI, as a measure of the degree of greenness and one of the greenspace exposure measures, was calculated at different catchment area radii (200 m, 300 m, 1000 m, and 2000 m as Euclidean distances and 500 m as network distance), which match the walking distances set by Natural England’s AGS to different sizes of greenspaces (Natural England, 2023). These radii are different than the set chosen by [
18] for their NDVI calculations (100 m, 300 m, 500 m, 1000 m, and 3000 m as Euclidean distances). Still, both studies, despite being conducted in different places (West of England and Utrecht, Netherlands), show an increase in the mean NDVI as the radius increases. In other words, more greenspaces are found as the analysis captures a bigger buffer area from the resident address. That is why future research needs to be critically aware of defining the catchment areas of the study, as well as the logic of choosing these buffers in relation to the residents and the chosen health-related outcomes, because the degree of zooming in on (or out of) an area impacts the output measures of proximity to greenspaces or degree of vegetation which in return would show (or not) associations to health measures.
A major strength of this study is the use of the precise distances from the residents’ addresses to the greenspace access points, as a measure of greenspace exposure, similar to the method adopted by [
36] but conducted in this study on a larger urban scale and with a larger sample size. This method is chosen for its accurate representation of travel distances covered by residents from their home addresses to enter a greenspace, unlike [
37] who measured green space distances toward the population weighted centroid (not addresses). This study’s method also further improves on [
17] (that was chosen due to computational limitations), which created service areas of accessible streets within a network distance from the greenspace access points, thus, being limited to defined categories for greenspace proximity (≤100 m, 101–200 m, 201–300 m, 301–400 m, 401–500 m, 501–1000 m, and >1000 m), whereas this study yields precise distance figures (a continuous variable) for each residential address. The high attention given to the calculated distances is meant to minimise the margin of error for greenspace proximity measures, to match the highly accurate calculations of NDVI (degree of greenness measures), thus enabling a fair comparison of the different measures of greenspace exposure.
5.2. Efficacy of Different Measures of Greenspace Exposure
In common with other studies carried out over the COVID-19 lockdowns we found that wellbeing was lower during lockdown [
38,
39], but physical activity and greenspace visitations increased [
38,
40].
The first major finding in this study is that the relationship between exposure to greenspaces and the measured health outcomes changes for the periods before and during the first 2020 COVID-19 lockdown, shown by the differences in the results of the regression models for both periods and more importantly the significant differences in the Paired Sample
T-tests of the outcome health variables. Given that the greenspaces exposure measures did not change between those periods (i.e., no extensive work on creating or removing greenspaces), it is argued that what has changed is the participants patterns of increased visits to greenspaces and increased physical activity yet decreased wellbeing, in relation to the changes in the lifestyle during the lockdown when most activities (other than exercising and visiting greenspaces) were heavily restricted to limit the spread of COVID-19 [
41]. Thus, the proximity to greenspaces or the greenness of neighbourhoods, as measures of greenspace exposure, could be argued as one of the factors that impacted those changes in residents’ behaviours during lockdown. This corresponds to studies that have previously discussed the change in people’s behaviours during the COVID-19 pandemic [
42], with several studies reporting changes in greenspace use, as being related to health outcomes during the lockdowns, including anxiety and depression in China [
40] and Denver [
39] and wellbeing in England [
38].
Regarding the period before lockdown and considering the regression models, there are certain findings that reflect the positive relationship between exposure to greenspaces and health outcomes. First, the multivariate analyses of four regression models suggest a statistically significant relationship between the increase in the number of residents’ visits to greenspaces in respect to the decrease in Euclidean or network distance to the nearest greenspace or the decrease in distance toward the smaller greenspaces (0.5 ha and 2 ha). This is complemented with the results from another regression model that illustrates the significant increase in physical activity in relation to the increase in number of visits to greenspaces. In simple words, the closer the greenspace, the more often the resident visits that space which in return could enable more physical activity; thus, a spatial measure of greenspace exposure is positively related to a health-related outcome.
Moreover, despite the calculated NDVI of all radii (as a measure of greenness of the neighbourhood) not showing any relation to the number of visits to greenspaces, the NDVI of smaller radii (200 m and 300 m) has been significantly and positively related to the residents’ connectedness to nature before lockdown. In other words, the greener the immediate context of the home (NDVI radii 200 m and 300 m), the more the residents feel connected to nature. However, larger radii NDVI (1 km and 2 km—indicating the greenness of the larger context) appears to have no relationship to the residents’ connectedness to nature nor with frequency of visits to greenspaces. This could suggest that residents are more aware of their immediate smaller context and how green it is, which impacts their connection to nature and their use of close-by greenspaces. This is similar to findings reported for England, where the immediate neighbourhood greenspace exposure was found to be associated with wellbeing during the lockdown [
38].
During the period of lockdown, there is a change in the residents’ behaviours in terms of visits to greenspaces and physical activity (as shown in the paired
T-tests), which accordingly affected the relationships between greenspace exposure measures and the health outcomes. For instance, there is a significant relationship between the number of visits to greenspaces and the greenness of the local area around the participants address (smaller NDVIs 200 m, 300 m and 500 m). This is further supported by another significant relationship between increasing the number of visits to greenspaces and decreasing the distance towards the doorstep green space (minimum of 0.5 ha), as well as another significant relationship between the more frequent visits to greenspaces and the increase in physical activity during lockdown. These findings, when seen together as a chain of inter-related decisions and daily choices, could reflect how residents of areas with locally high greenery were more likely to leave their home and either visit local greenspaces (one of few places they are allowed to go to) to engage in physical activity or to walk in their neighbourhood. This is suggestive of a causal pathway from proximity to greenery to greenspace use to health outcomes reported elsewhere (e.g., Refs. [
3,
23]).
From the above discussions, it could be recognised that, in this study, visits to greenspaces, physical activity, and connectedness to nature are the three health-related outcomes that showed a significant relationship to any of the greenspace exposure measures, and those relationships were in the expected direction; i.e., being within greener areas or being closer to greenspaces positively impact visits, physical activity, and nature connectedness. In contrast, wellbeing (measured through the ICECAP score) is one health-related outcome that did not show significant association to greenspace exposure except in an opposite to expected manner where wellbeing was shown to increase proportionally to the distance from a greenspace during the lockdown period (i.e., wellbeing is worse with more exposure to greenspace). Wellbeing (ICECAP) is also the only health-related outcome that is not significantly related to visits to greenspaces. It is not uncommon for a non-significant relationship between wellbeing and greenspace exposure to be reported in the literature once the modifying effect of sociodemographic variables are accounted for [
38]. It may be that during the COVID-19 pandemic wellbeing was so severely affected that greenspace exposure did not have a positive impact or even that going outdoors reduced wellbeing due to the risk of contact with others.
Finally, from all the significant regression models in this study, either for the period before or during the lockdown, it is noticed that the studied health-related outcomes are mostly associated to measures of greenspace exposure that capture the immediate context, whether expressed as (1) the smaller NDVI radii (200 m, 300 m and 500 m), (2) the proximity to smaller greenspaces (0.5 ha) which are commonly close-by, or (3) the nearest greenspaces (which are commonly of smaller areas). However, heath-related outcomes in this study do not relate to the larger context in terms of the larger NDVI radii (1000 m and 2000 m) nor the proximity to larger greenspaces (10 ha and 20 ha). This happens despite the mean distance to the smaller greenspaces not meeting Natural England’s AGS, i.e., being farther away than where they are expected to be.
This study demonstrates that it is important to use an appropriate measure of greenspace exposure. The findings here may go some way to explaining the inconsistent findings reported in systematic reviews examining the relationship between greenspace access and health outcomes (e.g., Refs. [
4,
5,
6,
7,
43]). Here we report that a measure of greenspace use appears to have a closer relationship to greenspace access and neighbourhood greenness as well as physical activity and nature connectedness, perhaps indicative of a causal pathway. It is therefore important that those examining the impact of greenspace on health and wellbeing consider including a measure of greenspace use in addition to, or instead of, greenspace access or neighbourhood greenness.
5.3. Limitations
There are a number of limitations to the study. First, this is a cross-sectional study in which we relied on the ability of respondents to recall their pre-lockdown greenspace behaviours, physical activity, nature connectedness, and wellbeing. Recall bias has been reported to be a limitation of self-reported wellbeing [
44] and physical activity questionnaires, such as GPAQ, leading to an overestimation of physical activity [
45]. Given that the before lockdown measure was further in past when the survey was completed, it is possible that this timepoint is subject to greater recall bias than the measure during lockdown. In common with all cross-sectional studies, we cannot attribute causality as those exhibiting greater levels of greenspace use may have chosen to live in areas with greater opportunity to visit local greenspaces, which they benefitted from during the lockdown. Second, although we considered Nature Connectedness during lockdown, this was recorded as a change from the before lockdown condition, so the two measures cannot be compared across the two timepoints. Third, our sample was not representative of the West of England, tending to be biased towards those who are white, female, more highly educated, affluent, and older. Those in more socially disadvantaged groups tend to have lower levels of wellbeing, physical activity, and/or greenspace use than the general population in the UK [
46], so our findings might not be representative of the population of the West of England.