The first stage of our analysis was to examine the relationship between individual-level experiences of nature and four health outcomes in a population sample of 1538 residents of Brisbane City, Australia. These health outcomes included whether the respondent scored as having mild or worse depression determined from an established 7 item questionnaire28, whether the respondent reported being under treatment for high blood pressure, perceptions of social cohesion derived from three survey questions29,30,31, and the self-reported number of days on which physical exercise occurred for more than 30 minutes during the survey week.
We measured experiences of nature across three components, including the usual frequency of outdoor green space visits across a year, the average duration of visits to green space across a week, and the intensity of nature (measured as the highest level of vegetation complexity within any of the green spaces that a respondent visited, following a hypothesis that higher levels of vegetation lead to greater health outcomes; Table 1, Fig. 2). Multivariate analyses revealed that a longer duration of individual nature experiences was significantly linked to a lower prevalence of depression and of high blood pressure, and increased physical activity. A higher frequency of green space visitation was an important predictor for increased social cohesion, and both duration and frequency showed a significant positive relationship with higher levels of physical activity (Table 1). These multivariate analyses accounted for key covariates including age, gender, Body Mass Index (BMI; weight in kilograms/square of height in meters), and socio-economic indicators including the income, education, and neighborhood socio-economic disadvantage (Index of Socio-economic Disadvantage, IRSD; Table 1)32. We also found that people with a stronger self-reported connection to nature (measured using the Nature Relatedness scale33) had greater levels of social cohesion and physical activity, but did not show a reduced prevalence of depression or high blood pressure (Table 1).
Table 1 The relationship between four health outcomes (the response variables), socio-demographic covariates and nature experience predictor variables.
The bivariate relationships between health responses (A–D) and nature experiences, comprising (i) the average duration of visits to green space; (ii) the normal reported frequency of visits to green space; and (iii) the nature intensity, measured as vegetation complexity within the best visited public green space
We examined the dose-response relationship between the odds of a respondent being recorded as having high blood pressure or depression and incremental increases in the duration of nature experiences, while accounting for covariates (Fig. 3, Table 2). We found that the odds were significantly lower than the null model for depression when reported green space visits were an average of 30 minutes or more (i.e. the confidence interval did not overlap with an odds ratio of one; Fig. 3A), with a slight increase in mean gains until a duration of 1 hour 15 minutes. For high blood pressure, there was also a significant health improvement after 30 minutes of exposure, though the dose-response curve showed high variability at higher exposure levels (Fig. 3B). The power of the test for high blood pressure and depression was reduced at higher durations (indicated by wider 95% confidence intervals).
Figure 3: Dose-response graphs showing the adjusted odds ratio from logistic regression for incrementally increasing average duration of green space visits.
95% confidence intervals are shown. An odds ratio above one indicates an individual is more likely to have the disease where the threshold of green space visitation is not met.
Table 2 The odds ratios for a person having depression or high blood pressure where specific risk factors are present (the result for each variable was calculated while accounting for all their other risk factors; i.e. multivariate analyses), and the proportion of disease cases in the study population attributable to various risk factors (average population attributable fraction).
We found that the proportion of cases of depression and high blood pressure in the population that can be attributed to city residents failing to spend an average of 30 minutes or more during a green space visit across the course of their week (the ‘population attributable fraction’) was 0.07 for depression, and 0.09 for high blood pressure (Table 2); that is, there could be up to 7% fewer cases of depression and 9% fewer cases of high blood pressure if the entire sampled population met the minimum duration criteria of 30 minutes or more.
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).