New 2026 Review Strengthens Evidence That Nature Can Support Mental Health

NEWS

A 2026 review finds nature-based activities can ease anxiety and depression, but scientists say there is no universal 20-minute dose.

 

TAYBAD, IRAN.— A major 2026 review of decades of research has strengthened evidence that activities involving natural environments can improve several measures of mental health, including anxiety, depressive symptoms and relaxation. But the findings also challenge a popular simplification: scientists have not established that exactly 20 minutes outdoors is a universal or optimal “dose” of nature for health.

The new analysis, published in Nature Human Behaviour on May 28, examined 116 systematic reviews covering 3,870 primary-study inclusions and an estimated population of more than 10 million people. Thirty reviews contained data suitable for a second-order meta-analysis.

Researchers found that nature-based interventions were associated with reductions in anxiety, depressive symptoms, heart rate and negative affect, as well as improvements in positive affect and relaxation. However, the authors cautioned that inconsistent definitions of nature-based interventions, uneven study quality and heavy reliance on passive control groups make it difficult to determine precisely which activities work best or what duration should be prescribed.

What the latest evidence says about nature and mental health

The 2026 analysis found an overall reduction in negative psychological and physiological outcomes among people taking part in nature-based interventions compared with control groups.

The strongest statistically significant findings included lower anxiety, fewer depressive symptoms, lower heart rate and reduced negative affect. Positive outcomes also improved overall, with particularly strong findings for relaxation and positive mood.

The review covered a broad range of interventions rather than a single activity. These included forest bathing, ordinary exposure to nature, gardening and horticultural programs, outdoor exercise, nature-based therapy and activities around blue spaces.

That breadth is both a strength and a limitation. A quiet park visit, a gardening program, a forest-bathing session and a group walk can all involve nature, but they may also introduce different combinations of physical activity, social interaction, mindfulness, daylight and relief from urban noise.

The researchers therefore warned against treating all forms of nature exposure as interchangeable. Only 27% of the systematic reviews assessed in the overview were rated high or moderate quality, according to the study's methodological evaluation, while many lacked active comparison groups.

Where the 20-minute nature claim comes from

The widely cited idea that roughly 20 minutes in nature can reduce stress is supported primarily by a much smaller 2019 study published in Frontiers in Psychology.

That experiment followed 36 urban adults for eight weeks. Participants were asked to spend at least 10 minutes in an outdoor location where they felt in contact with nature on at least three occasions each week. Researchers collected saliva before and after selected outings to measure cortisol and alpha-amylase, two biological indicators associated with stress.

The researchers calculated that nature exposure was associated with an additional 21.3% per hour decline in salivary cortisol beyond its normal daily decrease. The greatest stress-reduction efficiency per unit of time occurred between approximately 20 and 30 minutes; benefits continued beyond that point but accumulated more slowly.

That result is meaningful, but it does not demonstrate that 20 minutes is a universal maximum for all health benefits.

The study involved only 36 participants and examined specific stress biomarkers under particular conditions. The much larger 2026 review found that, when many studies were combined, effects on salivary cortisol and blood pressure were not among the outcomes showing statistically significant benefits, even though anxiety, depressive symptoms and heart rate did.

The evidence therefore supports a more careful conclusion: short periods in nature can be beneficial, and 20 to 30 minutes is a reasonable evidence-based duration for stress reduction in some settings, but science has not established a single optimal dose for every person or health outcome.

Two hours a week is associated with better self-reported health

Another influential study offers a different measure.

A 2019 Scientific Reports analysis examined 19,806 adults in England and found that people reporting at least 120 minutes of recreational contact with nature during the previous week were significantly more likely to report good health and high well-being than those reporting no nature contact.

The association appeared whether those two hours were accumulated during one longer outing or several shorter visits.

Researchers observed the strongest associations at roughly 200 to 300 minutes of nature contact per week, with no clear additional improvement beyond that range. The study was observational, however, meaning it identified an association rather than proving that nature exposure directly caused better health.

Together, the 20-to-30-minute stress study and the 120-minute weekly study answer different questions. One investigated short-term changes in biological stress markers; the other examined the relationship between weekly nature contact and self-reported health across a large population.

Forest studies suggest possible immune effects, but evidence is limited

Research from Japan has also examined whether forests can influence immune function.

Small studies involving multi-day forest trips reported increases in natural killer, or NK, cell activity and reductions in urinary adrenaline after participants spent time in forest environments. Some elevated immune measurements persisted for days after the trips.

Scientists have proposed that reduced stress and exposure to airborne plant compounds known as phytoncides may partly contribute to these effects.

Those findings should not be interpreted as proof that simply smelling trees will prevent viral infections or other diseases. The Japanese experiments involved small groups, specialized multi-day forest exposures and biological markers rather than clinical evidence showing that participants subsequently experienced fewer infections.

The distinction matters when translating laboratory or biomarker research into everyday health advice.

How contact with soil may interact with the human microbiome

There is also growing scientific interest in the relationship between environmental biodiversity and the microorganisms living on and inside the human body.

A major review in Nature Reviews Microbiology concluded that soil, plant, animal and human microbiomes are interconnected and that soil can act as a reservoir of both beneficial microorganisms and pathogens. Greater microbial diversity is generally associated with healthier ecological systems, but the biological pathways connecting environmental microbial exposure with specific human health outcomes remain an active research field.

This is more nuanced than saying that soil simply transfers “good bacteria” into the gut.

Environmental exposure may influence microbial diversity and immune regulation, particularly in childhood, but soil can also contain harmful organisms. The scientific evidence does not support deliberately ingesting soil as a health practice.

Nature-based care is moving into public health systems

The scientific debate has increasingly moved beyond individual wellness advice and into public-health policy.

England's National Health Service describes green social prescribing as connecting people with activities such as walking programs, community gardening, conservation projects, outdoor swimming and other nature-based opportunities to support physical and mental health.

A cross-government program recorded more than 8,500 referrals across seven pilot areas between 2021 and 2023, with Natural England reporting engagement among communities affected by health and social inequalities.

The British government continued a second phase of green social prescribing from April 2024 through March 2025 to gather more evidence on effectiveness and value for money. As of July 2026, work was still underway on evaluation and measurement of participation in health-related green and blue-space activities.

That approach reflects the current state of the science: nature-based activities can be useful components of health and well-being programs, but they should not be presented as substitutes for medical or psychological treatment when treatment is needed.

What researchers still need to determine

The newest research makes the broad case for nature exposure stronger while making simplistic prescriptions harder to defend.

The 2026 Nature Human Behaviour review called for better studies using standardized intervention definitions and active comparison groups. Researchers need to separate the effects of nature itself from factors such as exercise, social contact, mindfulness, sunlight and expectations.

Geography is another limitation. Much of the research included in existing reviews comes from developed countries, meaning conclusions may not transfer equally to communities with different climates, urban environments, access to safe green space or cultural relationships with nature.

For now, the evidence supports an accessible principle rather than an exact prescription: regular contact with parks, forests, gardens and other natural environments can contribute to mental and physical well-being, and even relatively short sessions may help. But “20 minutes” should be understood as a useful finding from one line of research—not a biological threshold that applies universally.

By Rachel Thompson | Health Correspondent | CRN Times

Editorial Supervision: María Quesada, Editor-in-Chief | CRN Times