Tip · Stress & mind
Forest bathing (Shinrin-Yoku)
Time in green spaces is associated with better well-being in large cohorts. The immune findings come from small Japanese studies.
Forest bathing has a usable dose figure, and it does not come from Japan but from an English population survey: 120 minutes per week. This page separates that part from the immune narrative, which rests on twelve men from Tokyo.
In short
Time in green spaces is consistently associated with better health and better well-being in large datasets. From 120 minutes of nature contact per week, the odds ratio for good health was 1.59 and for high well-being 1.23, with a maximum between 200 and 300 minutes. Acutely, a stay in the forest lowers heart rate (−4.00), tension and low mood. The catch lies in the immune findings: the well-known NK cell studies comprise 12 to 13 people from one research group.
What lies behind it
Two pathways are discussed, and they are supported to different degrees. One is the stress axis: in the meta-analysis of 143 studies, more contact with green space went along with lower salivary cortisol (−0.05), lower heart rate (−2.57), lower diastolic blood pressure (−1.97) and a shift in heart rate variability towards parasympathetic activity.
The other is the phytoncide hypothesis: trees release volatile organic compounds that are said to increase the activity of natural killer cells. This was tested in twelve healthy men aged 37 to 60 who spent three nights in a hotel room in which stem oil of the hinoki cypress was vaporized: NK activity rose, and adrenaline and noradrenaline in urine fell. The experiment had no control group.
The dose that comes from the data
The usable figure comes from a national English survey with 19,806 participants, controlled for the share of green space at the place of residence. Below 120 minutes per week, the association was not significant; from 120 minutes it became significant, in the range of 120 to 179 minutes with an odds ratio of 1.59 for good health and 1.23 for high well-being. The maximum lay between 200 and 300 minutes per week, with no further gain beyond that.
How the 120 minutes were accumulated made no difference, and the pattern was consistent across age groups. Something is measurable acutely as well: across 11 studies, heart rate fell by 4.00, tension and anxiety by 0.79, depression and low mood by 0.68.
The immune narrative and its scale
The first study in this series included twelve healthy men aged 37 to 55 who spent three days and two nights in forest areas; 11 of 12 showed higher NK activity afterwards, increased by about 50 percent on average. The only study with a genuine comparison condition compared the same twelve men on a forest trip and a city trip with matched activity levels: the increase occurred after the forest trip and persisted over 7 and 30 days, but not after the city trip.
Outside Japan the picture is mixed. In Taiwan, the proportion of natural killer cells was higher in 90 people working in the forest than in 110 office workers. In Korea, by contrast, a controlled study with 37 cancer survivors found no difference in NK cells and a significant increase in cortisol.
What is well supported
Two things hold up. First, the dose relationship from the English survey with 19,806 participants: from 120 minutes of nature contact per week, an odds ratio of 1.59 for good health and 1.23 for high well-being. Second, the physiological direction across 103 observational and 40 intervention studies, with consistent risk estimates: all-cause mortality 0.69, cardiovascular mortality 0.84, type 2 diabetes 0.72.
What the studies show
The 120-minute threshold
Cross-sectional analysis of a national English survey, n = 19,806, weighted for national representativeness, controlled for the share of green space at the place of residence. Compared with no nature contact at all in the previous week, the odds ratio at 120 to 179 minutes was 1.59 (1.31–1.92) for good health and 1.23 (1.08–1.40) for high well-being, with a maximum between 200 and 300 minutes per week.
The broadest summary on green spaces
103 observational and 40 intervention studies on around 100 endpoints. Pooled: salivary cortisol −0.05, heart rate −2.57, diastolic blood pressure −1.97, high-frequency heart rate variability +91.87. Risks: all-cause mortality 0.69 (0.55–0.87), cardiovascular mortality 0.84, type 2 diabetes 0.72. These are observational data, and not everything in them can be adjusted for.
What a single stay in the forest does acutely
Systematic review and meta-analysis according to PRISMA; of 718 records found, 11 studies met the criteria. Mean differences before and after the intervention: heart rate −4.00 (−6.90 to −1.09), tension and anxiety −0.79, depression and low mood −0.68. Heterogeneity I² = 49–87 %, GRADE rating very low.
Forest versus city, at the same activity level
The only study in the Japanese series with a genuine comparison condition: the same twelve men aged 35 to 56 went on a forest trip and a city trip with matched activity levels. The increase in NK activity occurred after the forest trip and persisted over 7 and 30 days, but not after the city trip.
The replication attempt that found nothing
Controlled study in Korea with 37 cancer survivors, 19 in the intervention group and 18 in the control group, a forest therapy camp over two nights and three days. No significant difference in natural killer cells, and, contrary to expectation, a significant increase in cortisol (p = 0.016). Stress level, total mood disturbance and serotonin changed in the expected direction.
Where the data stop
The NK cell findings rest on samples of 12 to 13 people, predominantly middle-aged men from Tokyo, collected by a single research group. The only controlled replication attempt outside Japan so far found no NK effect and a significant increase in cortisol in the forest group. The phytoncide hypothesis was tested in twelve men in a hotel room, without a control group.
Even the good-sounding numbers have their limits. The 120-minute threshold comes from a cross-sectional study, and the authors themselves name prospective longitudinal and intervention studies as the next step. The most recent Shinrin-Yoku meta-analysis rates its own results as very low according to GRADE. And the association between green space and all-cause mortality in older people is only marginal when pooled across 8 cohorts: 0.99 (0.97–1.00) per 0.1 NDVI units.
How to do it
The supported dose for general well-being is at least 120 minutes of nature contact per week, with the maximum at 200 to 300 minutes and no further gain beyond that. Whether this was accumulated in one go or over several visits made no difference. Two sessions of 20 minutes each per week, as often recommended, are well below this threshold.
The immune studies worked with considerably more: two hours of walking in the afternoon on the first day, then two hours each in the morning and afternoon on the second day, over three days and two nights. The randomized crossover study on depressive mood had participants walk 2.5 km in 90 minutes in the morning and the same route in the afternoon, that is, 5.0 km per day. In all intervention studies participants walked, and the comparison condition was the same route in the city.
Safety
Time in the forest is one of the lowest-risk measures of all; none of the studies evaluated reported adverse effects of the intervention. The real risks are those of being outdoors: ticks, relevant in Germany because of Lyme disease and tick-borne encephalitis (TBE), as well as sunburn, accidents on rough terrain and allergic reactions to pollen. Anyone who regularly goes into the forest in TBE risk areas should clarify the vaccination question with their family doctor. One point about expectations: in a controlled study in cancer survivors, cortisol rose in the forest group while mood and perceived stress improved. Forest bathing is not a cancer therapy and is not studied as such in any of the cited studies.
BK-Score Thin human evidence
| Human evidence | 4 | |
|---|---|---|
| Mechanism | 4 | |
| Safety data | 10 | |
| Hype gap | 4 | |
| Track record of use | 8 |
The well-known findings on natural killer cells come from small Japanese studies by a single research group and have hardly been replicated independently. The association between green spaces and well-being, by contrast, is consistent in large cohorts.
What is rated is the state of knowledge, not the effect. “Safety data 9” means well studied – not harmless.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)
Frequently asked questions about forest bathing (Shinrin-Yoku)
How long do I need to spend in the forest for it to make a difference?
For general well-being, the most robust figure is 120 minutes of nature contact per week. Below that, the association was not significant in the large English survey with just under 20,000 participants. The maximum lay between 200 and 300 minutes per week; above that there was no further gain. Whether you manage it in one go or spread out does not matter; the authors explicitly tested this.
Is it true that forest air strengthens the immune system?
There are findings on this, but they are narrow. The well-known studies on natural killer cells included 12 to 13 participants and largely come from one Japanese research group. In Taiwan a difference was found between forest and city workers; in Korea, by contrast, a controlled study with 37 cancer survivors found no difference in NK cells. This is not yet robust.
What are phytoncides and do they really work?
Phytoncides are volatile substances released by trees. This was tested in twelve men who spent three nights in a hotel room in which hinoki cypress oil was vaporized: NK activity rose, and adrenaline and noradrenaline in urine fell. The experiment had no control group; the comparison was with measurements on a normal working day beforehand. That is an indication, not proof.
Is a park in the city enough too?
The large English survey did not distinguish between forest and other green spaces; it asked about nature contact during leisure time in general, and control for the share of green space at the place of residence was built in. The meta-analysis on green spaces also pools very different forms of green space. The specific effects on the immune system, by contrast, have only been studied for forests.
Does forest bathing lower my cortisol?
On average yes, but not reliably. In the large meta-analysis of 143 studies, the pooled effect on salivary cortisol was −0.05. In a controlled study with cancer survivors, by contrast, cortisol rose significantly in the forest group, while mood and perceived stress improved. Cortisol is not a reliable indicator of whether an experience does you good.
Is this more than just going for a walk?
That is the decisive question, and studies that pose it cleanly are rare. In the better Japanese studies, the forest route was compared with the same route in the city at a matched activity level, and the difference occurred there. The most recent meta-analysis on Shinrin-Yoku nevertheless rates the certainty of its own results as very low according to GRADE.
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Sources
- White et al., Scientific Reports 2019
- Twohig-Bennett and Jones, Environmental Research 2018
- Andrade et al., Frontiers in Psychology 2026
- Yuan et al., Aging Clin Exp Res 2021
- Li et al., Int J Immunopathol Pharmacol 2008 (forest versus city)
- Li et al., Int J Immunopathol Pharmacol 2009 (phytoncides)
- Chae et al., J Korean Biol Nurs Sci 2024
- Tsao et al., Oncotarget 2018
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Information only, not medical advice and not a usage recommendation. If you have pre-existing conditions, and before major changes, consult a physician. Last updated: 2026-09-13.