Respondents were invited to report on any visit within the previous week to a place they considered ‘outdoor green space’, and were asked to name or describe the location. We manually geo-located these locations based on the descriptions where possible. Three aspects of nature dose were measured, encompassing the duration and frequency of experiences, and nature intensity, through a mixture of self-report and remote sensing analysis. Nature dose questions were asked in the survey before the health questions to avoid any potential priming effects of a person’s health status on self-reported nature dose (e.g. see49).
Duration of experiences of nature
Average duration of green space visits was estimated based on self-reported time spent during each visit across the survey week. We chose this timeframe as it provided a short and recent reference period to improve accuracy49. Note that this measure of duration is necessarily linked to frequency, as to achieve a duration measure the respondent must have visited a green space at least once during the survey week. Duration was selected from a time category (1–29 minutes; 30 minutes to one hour; one to two hours; two to three hours; three to four hours; four or more hours), and the mid-point of each selected category was summed (with four or more hours being treated as ‘four’), and this value was averaged across all visits.
Frequency of experiences of nature
Given that frequency of visitation would be highly correlated with duration if measured on the same time scale, here it was estimated based on the respondent’s self-reported frequency of visits to green spaces where their usual frequency of visits across a year was selected from the following categories: never; once a year; once every three months; two to three times a month; once a month; once or more per week. This approach also allowed us to account for people who use green spaces infrequently (i.e. less than once a week who were missed by the duration measure).
Here we generated one possible measure of nature intensity, the vegetation complexity within the most complex map-able green space each respondent visited (hypothesizing that more complex vegetation leads to better health outcomes by promoting attention restoration, and increasing the appeal of green spaces; Fig. 1; this measure also tends to correlate with plant and animal diversity56,57). Most (77%) of respondents only visited one or two green space locations so other measures such as the most common, or average complexity were not useful here. Analyses involving nature intensity were limited to respondents for whom the visited green space (a) could be geo-located, and (b) had established boundaries within the Brisbane City limits to ensure we vegetation was measured within the visited area. Complexity was measured using LiDAR-derived maps of vegetation cover at a 5 × 5 m resolution (details provided in the Supplementary material). Five separate vegetation strata were used that have relevance to the human experience of nature, including 0.15–1 m (likely to influence access and egress); 1–2 m (the line of sight may be affected); and three layers likely to provide varying levels of shade and visual vegetation complexity, 2–5 m; 5–10 m; 10 m+. For each of the vegetation strata we created a binary grid layer (where 1 indicated vegetation was present), and we summed all five of these layers for each 5 × 5 m pixel. We calculated the average summed measure across the entire green space. Higher values of vegetation complexity were achieved in green spaces with higher vegetation cover and more complex vegetation structure. This measure was calculated for 664 survey respondents who visited green spaces within the study area, and only these respondents were used in relevant analyses.
Health response measures
Respondents provided information on four health outcomes:
A measure of depression was generated based on the depression component of the Depression, Anxiety and Stress scale28. Scores were converted to a binary measure where 0 indicates no depression and 1 indicates mild or worse depression.
Respondents reported whether they were currently receiving treatment for high blood pressure, coded as a binary measure where 0 indicates no treatment and 1 indicates treatment.
Respondent’s perceptions of social cohesion were estimated based on three previously developed questions that measure trust, reciprocal exchange within communities, and general community cohesion29,30,31 (see Supplementary material for details). The scores across all three questions were averaged.
Respondents provided a self-report indication of physical activity, specifically the number of days they exercised for 30 minutes or more during the survey week (regardless of location; ‘green exercise’ and exercise in other locations were not differentiated). The resulting count variable was between 0 and 7.
How to cite this article: Shanahan, D. F. et al. Health Benefits from Nature Experiences Depend on Dose. Sci. Rep. 6, 28551; doi: 10.1038/srep28551 (2016).