Over the years, it's become increasingly clear that spending time in nature is somehow linked to healthier and happier lives. But while some physicians have literally started 'prescribing' doses of the natural world, like they would exercise, such practices are far from established.
Researchers in the United Kingdom have now taken a small but important step towards one of the most crucial, unanswered questions: how much time outdoors is enough?
Drawing on a nationwide survey of nearly 20,000 British adults from 2014 to 2016, the team thinks they might have found a weekly 'sweet spot' for nature exposure.
"Compared to no nature contact last week, the likelihood of reporting good health or high well-being became significantly greater with contact ≥120 mins," the authors conclude.
The findings are supported by past research, which has found that living in greener areas is associated with lower risks of cardiovascular disease, obesity, diabetes, asthma emergencies, mental distress, mortality and even myopia in children. Some have even landed on this 120 minute threshold before.
Nevertheless, these results are still in their infancy, and it remains unclear how much exposure we humans truly need to reap the benefits.
Exploring this idea further, participants in the most recent study were asked how much nature they had experienced in the last seven days. Randomly selecting just one of these "nature diary entries", the interviewer then asked for more details, including how long the visit was, who they went with, how they got there, and what they got up to.
Last but not least, each individual was asked how their health was in general, and also how satisfied they were with their lives.
Bringing together these responses, the authors found that individuals who spent less than two hours in nature across the week - including visits to woodlands, beaches and parks - reported similar health and well-being to those who experienced no nature whatsoever.
On the flip side, however, those who spent more than two hours in nature consistently reported higher levels of health and well-being; while those who spent more than three hours showed only gradual further gains and sometimes even experienced losses.
"We tentatively suggest, therefore," the authors write, "that 120 minutes contact with nature per week may reflect a kind of "threshold", below which there is insufficient contact to produce significant benefits to health and well-being, but above which such benefits become manifest."
While it's still far too early to make any evidence-based recommendations on these results, the authors think their work is a good starting point for further discussion and investigation.
Their findings, for example, suggest that it doesn't matter how those minutes of exposure are achieved each week, just that they are. In other words, if it takes multiple short walks in the woods to achieve two hours of nature exposure then that appears to be just as useful as one long picnic in the park.
What's more, this pattern was consistent across a broad range of British adults, regardless of long-term illnesses, disabilities, age, gender, wealth or urbanity.
In terms of sheer magnitude, the authors claim that the threshold they've identified is similar to recommended levels of physical activity or nutritional advice.
"Given the widely stated importance of all these factors for health and wellbeing, we interpret the size of the nature relationship to be meaningful in terms of potential public health implications," the authors conclude.
While many previous studies have measured nature contact through proximity to green spaces alone, the new research suggests that this is a major flaw. Instead, it appears that the "threshold" for natural exposure is present even for those who live in areas with few natural settings.
"Impoverished local opportunities need not be a barrier to nature exposure," the authors suggest.
As interesting as these results are, the team has also acknowledged the limitations of their study. Not only is the data subjective and self-reported, there are several explanations that can't be ruled out just yet.
For instance, it's unclear if the association between happiness, healthiness and exposure to nature is due to healthier and happier people simply spending more time in nature.
"One explanation for our findings might be that time spent in nature is a proxy for physical activity, and it is this which is driving the relationship, not nature contact per se," the authors admit.
"In England, for instance, over 3 million adults achieve recommended activity levels fully, or in part, in natural settings."
This latter explanation, however, is probably not as likely. The authors note that the threshold they found applied even to those who did not meet physical activity guidelines. Besides, activities like Japanese "forest bathing", which simply require sitting in nature, have also been linked to various psychological and physiological benefits.
In the same way that physical activity guidelines were initially created, the authors are insisting on further long-term studies to explore the true nature of this phenomenon. The potential value of spending time outside is simply too great to ignore.
The research has been published in Scientific Reports.