The claim that being outdoors is good for you has accumulated a genuine body of research, and it is worth separating what has been shown from what has been built on top of it.
What the studies find
Observational studies consistently find associations between residential green space and better mental health, lower mortality and better birth outcomes.
Experimental studies find that brief exposure to natural environments improves mood and reduces physiological stress markers relative to urban environments.
The effect sizes are modest, the findings are reasonably consistent, and the literature is now large.
The dose research
Attempts to quantify how much is needed have produced a commonly cited figure of around two hours weekly, from a large survey study.
That study was cross-sectional, meaning it cannot establish causation, and it found the association plateaued rather than continuing to increase.
Which is a useful figure and not the threshold it is frequently presented as.
The proposed mechanisms
Attention restoration theory proposes that natural environments allow recovery of directed attention because they engage attention effortlessly.
Stress reduction theory proposes an evolved preference for environments that historically signalled safety and resources.
Both have experimental support and neither is fully established.
The more mundane explanations are also plausible and frequently overlooked.
The confounding candidates
Being outdoors generally involves physical activity, which has its own strong evidence base.
It generally involves light exposure, which affects circadian regulation and mood.
It frequently involves social contact.
It involves being away from work and screens.
And access to green space correlates with income, housing quality and neighbourhood characteristics, which affect health through many routes.
Studies attempt to adjust for these, and disentangling them fully is genuinely difficult.
Which means the specific contribution of nature itself, beyond activity, light and absence of demands, is less established than the field's framing implies.
The forest bathing literature
A specific practice with its own research, much of it conducted in Japan where the concept originated as a public health initiative.
Studies have reported effects on blood pressure, cortisol, mood and immune markers.
Sample sizes are frequently small, control conditions vary, and the immune findings in particular — involving compounds released by trees — have not been well replicated outside the original research groups.
Which does not mean it is wrong. It means the confidence with which specific mechanisms are asserted exceeds the evidence.
Where it has been applied
Nature-based interventions have been introduced in several health systems, including green prescribing schemes referring patients to outdoor activities.
Evaluation has been mixed, with the practical problems being uptake, retention and the difficulty of measuring outcomes in a heterogeneous intervention.
The low cost and low risk make them attractive regardless, which is a reasonable basis for provision even where the evidence is modest.
The equity dimension
Access to quality green space varies substantially by area and by income in most countries, and the variation tracks other health inequalities.
Which means the individual advice to spend more time outdoors is easier for some people than others, and the policy question about provision is arguably more important than the individual one.
The practical conclusion
Going outside regularly is free, low risk, and supported by a reasonable body of evidence whether or not the specific mechanism is nature itself.
It is not a treatment for a mental health condition, and anyone with persistent difficulties should see a doctor rather than adding walks and hoping.
Light specifically
Worth separating out, because it may account for a substantial share of the effect.
Outdoor light levels are far higher than indoor levels, by more than an order of magnitude even on an overcast day.
Morning light exposure entrains the circadian system, which affects sleep timing and quality, which affects mood.
Which means part of what is attributed to nature may be light, and light exposure can be obtained in an urban street as readily as in a wood.
Light therapy has established evidence for seasonal affective disorder specifically, which is the strongest evidence in this general area.
Water
Some research distinguishes blue space from green space, finding associations with coastal and waterside environments that are at least as strong.
The literature is smaller and the mechanisms proposed are similar.
Which suggests the relevant quality is something about the environment generally rather than vegetation specifically, and nobody has pinned down what.
Urban application
Studies find effects from relatively small interventions — street trees, small parks, views of vegetation from windows.
Which matters because it means the finding is actionable in cities rather than requiring access to countryside.
Window views specifically have been studied since an early and influential hospital study, and the literature since has been mixed.
Gardening
Studied separately with reasonably favourable results, and it combines several plausible mechanisms.
Physical activity, light exposure, purposeful task, visible progress, and in allotment settings a social component.
Which makes it difficult to attribute the effect to any single element, and it is a reasonable thing to do regardless.