GradedHealth

Does walking in the woods help your immune system and mood?

Forest trips raised natural killer cell activity, an immune marker, in small studies; pooled trials find short-term relief from anxiety and depression.

Published October 2026

Short answer

Forest walks ease anxiety and depression in the short term in pooled studies, though participants couldn't be blinded. Small trials found immune-cell activity rising after forest trips, a marker rather than proof of fewer illnesses. Evidence grade C (Limited). Meta-analyses of forest-bathing trials (1,876-3,554 participants) show short-term mood benefits, downgraded for inconsistency and unblinded, self-reported outcomes; the immune evidence adds only small before-and-after studies of a lab marker.

Key takeaways

  • A 2026 meta-analysis of 25 forest-therapy studies (1,876 people) found anxiety fell by about 0.77 standard units and depression by 0.68, though variation between studies was high [1].
  • An earlier review of 36 studies (3,554 people) also found less depression and anxiety, with fewer benefits on body measures [2].
  • In studies of 12-13 healthy adults, a three-day forest trip raised natural killer (NK) cell activity; a matching city trip did not, and the rise lasted more than 7 days [3][4].
  • Tree chemicals called phytoncides are one proposed explanation: 12 men who slept three nights with hinoki cypress oil vaporised in their hotel room had higher NK activity and lower stress hormones [5].
  • NK activity is a lab marker: the studies did not test whether forest time prevents colds, infections or cancer [6].

The short version

Walks in woods and forests can genuinely improve mood: pooled studies of forest bathing — slow, mindful time among trees — find short-term reductions in anxiety and depression symptoms [1][2]. The immunity story is earlier and smaller: several studies of a dozen people found natural killer (NK) cell activity, a lab measure of immune cells, rose after forest trips and stayed up for about a week [3][7]. Tree chemicals called phytoncides are a leading explanation, but the studies measured immune markers, not whether anyone got fewer infections [5][6].

What the studies show

Mood and stress. A 2026 meta-analysis pooled 25 forest-therapy studies (1,876 people) and found reductions in psychological stress (SMD -0.71), depression (-0.68) and anxiety (-0.77), where SMD is a standardised score and negative means symptoms fell; between-study variation was high, and the authors rated the certainty moderate for stress and low for depression and anxiety [1]. An earlier review of 36 studies (3,554 people) also found significantly less depression and anxiety, but fewer benefits on body measures [2]. A 2026 review of 11 studies agreed on short-term drops in tension-anxiety and low mood but rated the certainty very low [8]. In 37 adults with major depression, a single 60-minute walk in nature lowered negative mood more than an urban walk [9], while a three-hour forest walk did not change burnout scores in 56 healthcare workers [10].

The immune-cell findings. In 12 healthy men, a three-day, two-night forest trip raised NK cell activity — 11 of 12 men were higher afterwards, about 50% higher on average — along with anti-cancer proteins inside the cells [3]. A follow-up compared forest and city trips in another 12 men: only the forest trip raised NK activity, and the increase lasted more than 7 days [4]. Thirteen nurses showed the same pattern after a three-day trip [11], and even a day trip with two two-hour walks raised NK activity for 7 days [7].

Do tree chemicals matter? Phytoncides such as alpha-pinene and beta-pinene were detected in the forest air during those studies and not in city air [4]. In one experiment, 12 men slept three nights in an urban hotel room while hinoki cypress oil was vaporised; their NK activity rose and urine stress hormones fell [5]. A randomised trial of 55 gynaecological cancer survivors found that hour-long phytoncide-scented rest sessions, five days a week for eight weeks, increased some NK cell measures and lowered stress [12]. In the lab, phytoncides increased the killing activity of a human NK cell line [13]. In a small randomised crossover trial, 30 men at risk of COPD who spent a day in a cypress forest had lower blood CRP and IL-6 than after a matched city walk [14].

What the reviews say. A 2021 systematic review of 13 studies found most reported improvements in NK cell numbers or activity after forest therapy, but the authors say more randomised trials are needed [6]. A second review of 33 studies found positive signals for immune measures but noted that weak study designs leave the size and duration of any effect uncertain [15].

The nuances

  • The immune studies are small — 12 or 13 people in the classic trips — and measure NK activity, a lab marker, not whether anyone stayed well [11][6]. Those trips were three days and two nights with two-hour walks [4]; a day trip with two two-hour walks also raised NK activity [7]. One review reported the effect could last more than 30 days and suggested a forest visit about once a month [16].
  • The mood findings agree across meta-analyses, but nature studies cannot blind participants, outcomes are self-reported, and effects were mostly measured right after sessions [1][8]. A 2017 review of the six available RCTs found every one had high risk of bias and judged the evidence insufficient for clinical guidelines [17].
  • Forest time also lowered blood pressure by about 3 mmHg in a pooled analysis of 21 studies, with wide variation and prediction intervals that included zero [18].
  • Practical safety: in a survey of 212 Scottish hill runners, 82.5% reported at least one tick bite in the past year, and wearing leg cover was linked to lower odds of a bite [19]; pollen can trigger allergy symptoms, and the level that does varies by person [20]. The Time in Nature page covers the rest.

What this means for you

For most people a forest walk is low-cost and low-risk, and the mood effects showed up quickly in the trials, sometimes after a single walk [9][1]. The immune studies that found changes used two- to three-day forest trips with a couple of hours of walking a day, or day trips with two longer walks [4][7]. Keep the size in mind: these were small studies measuring a lab marker, so treat the immune findings as early rather than settled [6]. As with any outdoor activity, tick and pollen precautions are worth knowing; see the Time in Nature page.

Go deeper

  • Time in Nature — Time in green spaces and nature is linked to lower stress and better short-term mood, forest visits lower blood pressure a little, and forest trips raised natu…

References

  1. Forest therapy for psychological stress and emotional disorders: a systematic review and meta-analysis.. BMC Public Health, 2026.
  2. The effects of forest bathing on psychological well-being: A systematic review and meta-analysis.. Int J Ment Health Nurs, 2023.
  3. Forest bathing enhances human natural killer activity and expression of anti-cancer proteins.. Int J Immunopathol Pharmacol, 2007.
  4. Visiting a forest, but not a city, increases human natural killer activity and expression of anti-cancer proteins.. Int J Immunopathol Pharmacol, 2008.
  5. Effect of phytoncide from trees on human natural killer cell function.. Int J Immunopathol Pharmacol, 2009.
  6. The Effects of Forest Therapy on Immune Function.. Int J Environ Res Public Health, 2021.
  7. A day trip to a forest park increases human natural killer activity and the expression of anti-cancer proteins in male subjects.. J Biol Regul Homeost Agents, 2010.
  8. Short-term cardiovascular and mental health responses to Shinrin-Yoku (forest bathing): a systematic review and meta-analysis.. Front Psychol, 2026.
  9. The effects of walking in nature on negative and positive affect in adult psychiatric outpatients with major depressive disorder: A randomized-controlled study.. J Affect Disord, 2022.
  10. Assessing the Impact of a Shinrin-Yoku (Forest Bathing) Intervention on Physician/Healthcare Professional Burnout: A Randomized, Controlled Trial.. Int J Environ Res Public Health, 2022.
  11. A forest bathing trip increases human natural killer activity and expression of anti-cancer proteins in female subjects.. J Biol Regul Homeost Agents, 2008.
  12. Effects of phytoncide on immune cells and psychological stress of gynecological cancer survivors: randomized controlled trials.. J Exerc Rehabil, 2023.
  13. Phytoncides (wood essential oils) induce human natural killer cell activity.. Immunopharmacol Immunotoxicol, 2006.
  14. Forest bathing improves inflammatory markers, SpO2, and subjective symptoms related to chronic obstructive pulmonary disease (COPD) in male subjects at risk of developing COPD.. J Occup Health, 2025.
  15. Nature Exposure and Its Effects on Immune System Functioning: A Systematic Review.. Int J Environ Res Public Health, 2021.
  16. Effect of forest environments on human natural killer (NK) activity.. Int J Immunopathol Pharmacol, 2011.
  17. Health and well-being benefits of spending time in forests: systematic review.. Environ Health Prev Med, 2017.
  18. The Effects of Forest Therapy on the Blood Pressure and Salivary Cortisol Levels of Urban Residents: A Meta-Analysis.. Int J Environ Res Public Health, 2022.
  19. Running into the Lyme-light: a retrospective cross-sectional study of tick bites and Lyme disease prevalence, incidence, and prevention in hill runners, Scotland, UK.. BMC Public Health, 2026.
  20. The Clinical Utility of Pollen Counts.. Clin Rev Allergy Immunol, 2019.