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Reference topic 27: The Green Prescription

Nature and mental health: what green space does to stress

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.44 sources8 graded recommendationsTopic hub: Stress and the mind

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A three-story wing of a suburban Pennsylvania hospital held rows of nearly identical double rooms. Each had a window low enough for a patient in bed to see out. On one side of the wing the windows faced a small stand of deciduous trees. On the other they faced a brown brick wall.1

Roger Ulrich, an environmental psychologist, went through nine years of the hospital's gallbladder-surgery records from the months when the trees were in leaf. He matched patients in pairs by sex, age, smoking, weight, floor and year. One of each pair had looked at trees, the other at brick. A nurse read every chart without knowing which was which.1

Mismatch card

The system

The stress axis, directed attention and the cardiovascular system, tuned by a living landscape.

What it expects

Daylight hours among plants, water and animals, with shade and people nearby.

What it gets

Most of life spent indoors, a view of a wall, and a city hotter and louder than the land it replaced.

What happens in the gap

Slower stress recovery, tired attention, more psychiatric diagnoses among children raised in the least green places, and extra heart and lung deaths where the trees go.

The handrail

Two hours a week outdoors, visits of 20 to 30 minutes, a window onto something alive, trees on your street (●●○)

Strength of evidence

●●○ Natural experiments, twin and mover designs and a randomized trial point the same way, but individual effects are modest and exposure crudely measured.

What this topic covers

  • ·The Habitat You Left
  • ·The Beetle That Counted the Dead
  • ·Denmark, Twins, and the People Who Moved
  • ·The Lots of Philadelphia
  • ·How Much, and of What

The practical version is in the book: Chapter 10, Green and Quiet.

The handrail

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. About two hours a week in green or blue space (plants or water), in any pattern that suits you

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Parks, woodland, riversides and beaches all counted in the English survey behind the figure, whose design cannot prove direction. Canada's PaRx program prescribes the same two hours, in stretches of at least 20 minutes.2236

  2. Make each visit 20 to 30 minutes; 10 minutes still registers

    EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

    Leave the phone in your pocket, as the cortisol protocol did.2337

  3. If your mood is low, join something structured outdoors for 8 to 12 weeks

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Sixteen randomized trials pooled to better depressive mood and anxiety, with standardized effect sizes of -0.64 and -0.94, in sessions of 20 to 90 minutes. Gardening delivered in its own trial.3821 This complements treatment for depression, anxiety or ADHD rather than replacing it; never stop a prescribed treatment for a walk.

  4. Ask a clinician about a nature prescription

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Across 92 studies, mostly at moderate to high risk of bias, prescriptions were followed by systolic (upper) blood pressure about 4.8 mmHg lower and around 900 more steps a day (Topic 42).39

  5. Put your desk where you can see something alive

    EmergingEvidence grade: Emerging. Early, small, animal-only, or actively contested.

    A 40-second view of a flowering green roof left 150 students making fewer errors than a view of bare concrete.40 Surgical patients given live plants rated pain, anxiety and fatigue lower.41 Plants do not clean the air: matching ordinary ventilation would take 10 to 1,000 pots per square meter.42

  6. For a street or a city, plant trees and green the vacant lots

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    The one recommendation backed by natural experiments and a citywide randomized trial, and the only one that helps people who never read the advice (Topic 45).101218

  7. Enjoy forest bathing as relaxation , not as immune therapy

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Twenty trials put systolic pressure 3.15 mmHg lower in forest settings, mostly on the same day.43

  8. Safety

    Ticks live in grassy, brushy and wooded areas. The Centers for Disease Control and Prevention (CDC) advises an insect repellent registered with the Environmental Protection Agency, or EPA (DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol or 2-undecanone). It also advises 0.5% permethrin-treated clothing, a full body check on returning and a shower within two hours.44 Sun (Topic 19) and heat (Topic 25) apply, and one gardener in the trial had an allergic reaction.21

The skeptic's corner

The case against this topic, from the research itself.

The central objection has never gone away: healthier, wealthier people live in greener places and visit parks more. The designs built to beat it only half succeed. Identical twins kept the depression association but lost stress and anxiety.16 Danish adjustment shrank the extra ADHD risk by nearly two-thirds.15 Movers gained, yet a panel study of more than 10,000 people called individual effects small.17 Believe the direction; stay modest about the size.

Exposure is measured badly too. Most studies use a satellite greenness index that cannot tell a grand elm from a weedy verge. In Portland, sites with identical values looked nothing alike.29 Scale can flip the sign. Across the largest US cities, the greener ones had higher death rates, blamed partly on sprawl and car dependence.30

Attention restoration replicates unevenly. A review of about thirty studies found support on three measures and nothing marked on ten others,31 and a repeat of a much-cited experiment failed.32 Forest bathing is the weakest link. Its immune studies run to a dozen people each, unrandomized, from one group whose leader sits on forest-therapy boards.33 Forest groups had lower cortisol before the walk as well as after, a hint that expectation may do some of the work.34 In Berlin, volunteers were randomized to an hour in a forest or on a busy shopping street. After the forest walk, activity fell in the amygdala, a brain region that reacts to threat. Yet their stress, felt or measured, did not differ between the groups.35 A brain signal moved; the person did not feel it.

Terms this topic defines

Related topics

Sources cited above

  1. 1.Ulrich, R.S. (1984). "View through a window may influence recovery from surgery." Science, 224(4647), 420-421. https://doi.org/10.1126/science.6143402 (46 patients in 23 matched pairs, Paoli Memorial Hospital, Paoli, Pennsylvania; operations 1 May to 20 October, 1972-1981; funded through the Consortium for Environmental Forestry Studies, US Department of Agriculture Forest Service, grant 23170. Quotations from the full text.) (A three-story wing of a 200-bed hospital; fewer doses of strong narcotics between the second and fifth day after surgery; 1.13 negative nursing notes per patient in the tree-view rooms against 3.96 in the wall-view rooms, including "needs much encouragement." Ulrich added that "a lively city street might be more stimulating and hence more therapeutic than many natural views" for a chronically understimulated patient.)
  2. 16.Cohen-Cline, H., Turkheimer, E., and Duncan, G.E. (2015). "Access to green space, physical activity and mental health: a twin study." Journal of Epidemiology and Community Health, 69(6), 523-529. https://doi.org/10.1136/jech-2014-204667 (4,338 same-sex twins, University of Washington Twin Registry; no within-pair effect among identical twins for stress or anxiety after adjustment for income and physical activity.)
  3. 15.Thygesen, M., Engemann, K., Holst, G.J., Hansen, B., et al. (2020). "The association between residential green space in childhood and development of attention deficit hyperactivity disorder: a population-based cohort study." Environmental Health Perspectives, 128(12), 127011. https://doi.org/10.1289/EHP6729 (n = 814,689; IRR 1.55 before and 1.20 after adjustment.)
  4. 17.Alcock, I., White, M.P., Wheeler, B.W., Fleming, L.E., and Depledge, M.H. (2014). "Longitudinal effects on mental health of moving to greener and less green urban areas." Environmental Science & Technology, 48(2), 1247-1255. https://doi.org/10.1021/es403688w (1,064 movers in the British Household Panel Survey: 594 to greener, 470 to less green areas.) The related fixed-effects panel analysis of more than 10,000 people, which describes individual-level effects as small, is White, M.P., Alcock, I., Wheeler, B.W., and Depledge, M.H. (2013). "Would you be happier living in a greener urban area? A fixed-effects analysis of panel data." Psychological Science, 24(6), 920-928. https://doi.org/10.1177/0956797612464659 (The 470 who moved to less green areas were already worse off before the move, then returned to baseline.)
  5. 29.Donovan, G.H., Gatziolis, D., Derrien, M., Michael, Y.L., et al. (2022). "Shortcomings of the normalized difference vegetation index as an exposure metric." Nature Plants, 8(6), 617-622. https://doi.org/10.1038/s41477-022-01170-6
  6. 30.Richardson, E.A., Mitchell, R., Hartig, T., de Vries, S., et al. (2012). "Green cities and health: a question of scale?" Journal of Epidemiology and Community Health, 66(2), 160-165. https://doi.org/10.1136/jech.2011.137240 (49 largest US cities, about 43 million people.)
  7. 31.Ohly, H., White, M.P., Wheeler, B.W., Bethel, A., et al. (2016). "Attention restoration theory: a systematic review of the attention restoration potential of exposure to natural environments." Journal of Toxicology and Environmental Health, Part B, 19(7), 305-343. https://doi.org/10.1080/10937404.2016.1196155 (31 studies; support on Digit Span Forward, Digit Span Backward and Trail Making Test B.) A later review of 42 articles found low to moderate effects on working memory and cognitive flexibility: Stevenson, M.P., Schilhab, T., and Bentsen, P. (2018). "Attention restoration theory II." Journal of Toxicology and Environmental Health, Part B, 21(4), 227-268. https://doi.org/10.1080/10937404.2018.1505571
  8. 32.Neilson, B.N., Craig, C.M., Curiel, R.Y., and Klein, M.I. (2021). "Restoring attentional resources with nature: a replication study of Berto's (2005) paradigm including commentary from Dr. Rita Berto." Human Factors, 63(6), 1046-1060. https://doi.org/10.1177/0018720820909287
  9. 33.Li, Q. (2010). "Effect of forest bathing trips on human immune function." Environmental Health and Preventive Medicine, 15(1), 9-17. https://doi.org/10.1007/s12199-008-0068-3 ; Li, Q., Morimoto, K., Kobayashi, M., Inagaki, H., et al. (2008). "Visiting a forest, but not a city, increases human natural killer activity and expression of anti-cancer proteins." International Journal of Immunopathology and Pharmacology, 21(1), 117-127. https://doi.org/10.1177/039463200802100113 (12 men; the city comparison trip went to Nagoya in mid-May 2006 and the forest trip to Nagano in early September 2006.) The board memberships are declared in Li (2022), cited above.
  10. 34.Antonelli, M., Barbieri, G., and Donelli, D. (2019). "Effects of forest bathing (shinrin-yoku) on levels of cortisol as a stress biomarker: a systematic review and meta-analysis." International Journal of Biometeorology, 63(8), 1117-1134. https://doi.org/10.1007/s00484-019-01717-x (Salivary cortisol was lower in forest groups after the intervention, MD -0.05 μg/dl, and before it, MD -0.08 μg/dl; the authors note that "anticipated placebo effects can play an important role".)
  11. 35.Sudimac, S., Sale, V., and Kühn, S. (2022). "How nature nurtures: amygdala activity decreases as the result of a one-hour walk in nature." Molecular Psychiatry, 27(11), 4446-4452. https://doi.org/10.1038/s41380-022-01720-6 (63 adults randomized to Berlin's Grunewald forest or Schloßstraße; no significant environment-by-time interaction in self-report, cognitive or physiological stress measures.)
  12. 22.White, M.P., Alcock, I., Grellier, J., Wheeler, B.W., et al. (2019). "Spending at least 120 minutes a week in nature is associated with good health and wellbeing." Scientific Reports, 9(1), 7730. https://doi.org/10.1038/s41598-019-44097-3 (19,806 adults, Monitor of Engagement with the Natural Environment, 2014/15-2015/16; 120-179 minutes, OR 1.59, 95% CI 1.31-1.92 for good health. The authors call the threshold and peak "tentative" and note they cannot rule out healthier, happier people spending more time in nature.) (Benefits peaked at 200-300 minutes a week; nature time explained about 1% of the variance.)
  13. 36.BC Parks Foundation. PaRx: A Prescription for Nature. https://bcparksfoundation.ca/initiatives/parx/ and https://www.parkprescriptions.ca/ (The program recommends two hours a week, at least 20 minutes at a time. Canadian Parks and Recreation Association, release of 10 March 2026, https://www.cpra.ca/parx_launches_national_initiative , reports about 20,000 registered practitioners.)
  14. 23.Hunter, M.R., Gillespie, B.W., and Chen, S.Y. (2019). "Urban nature experiences reduce stress in the context of daily life based on salivary biomarkers." Frontiers in Psychology, 10, 722. https://doi.org/10.3389/fpsyg.2019.00722 (44 recruited, 36 analyzed, 92% female, mean age 45.8; at least three experiences a week of at least 10 minutes over eight weeks; no control group. The first edition of this book reported the 20-to-30-minute window as the point of greatest drop; in the paper it is the point of greatest efficiency per minute spent.) (Salivary cortisol fell 21.3% per hour beyond the hormone's ordinary 11.7% diurnal decline.)
  15. 37.Meredith, G.R., Rakow, D.A., Eldermire, E.R.B., Madsen, C.G., et al. (2020). "Minimum time dose in nature to positively impact the mental health of college-aged students, and how to measure it: a scoping review." Frontiers in Psychology, 10, 2942. https://doi.org/10.3389/fpsyg.2019.02942 (14 studies, 10 of them in Japan.)
  16. 38.Coventry, P.A., Brown, J.E., Pervin, J., Brabyn, S., et al. (2021). "Nature-based outdoor activities for mental and physical health: systematic review and meta-analysis." SSM - Population Health, 16, 100934. https://doi.org/10.1016/j.ssmph.2021.100934 (50 studies, 16 of them randomized; programs of 8 to 12 weeks with sessions of 20 to 90 minutes were the most effective; less evidence of physical health benefit.)
  17. 21.Litt, J.S., Alaimo, K., Harrall, K.K., Hamman, R.F., et al. (2023). "Effects of a community gardening intervention on diet, physical activity, and anthropometry outcomes in the USA (CAPS): an observer-blind, randomised controlled trial." Lancet Planetary Health, 7(1), e23-e32. https://doi.org/10.1016/S2542-5196(22)00303-5 (291 adults in Denver and Aurora, Colorado, 2017-2019; fiber +1.41 g/day at harvest time; moderate-to-vigorous activity +5.80 min/day.)
  18. 39.Nguyen, P.Y., Astell-Burt, T., Rahimi-Ardabili, H., and Feng, X. (2023). "Effect of nature prescriptions on cardiometabolic and mental health, and physical activity: a systematic review." Lancet Planetary Health, 7(4), e313-e328. https://doi.org/10.1016/S2542-5196(23)00025-6 (92 studies, 28 in the meta-analyses; systolic blood pressure -4.82 mmHg, 95% CI -8.92 to -0.72; daily steps +900, 95% CI 790-1,010.)
  19. 40.Lee, K.E., Williams, K.J.H., Sargent, L.D., Williams, N.S.G., and Johnson, K.A. (2015). "40-second green roof views sustain attention: the role of micro-breaks in attention restoration." Journal of Environmental Psychology, 42, 182-189. https://doi.org/10.1016/j.jenvp.2015.04.003 (150 university students; reported here from the University of Melbourne media release of 26 May 2015.)
  20. 41.Park, S.H., and Mattson, R.H. (2009). "Ornamental indoor plants in hospital rooms enhanced health outcomes of patients recovering from surgery." Journal of Alternative and Complementary Medicine, 15(9), 975-980. https://doi.org/10.1089/acm.2009.0075 (90 patients recovering from hemorrhoidectomy, randomly assigned to rooms with or without live plants; patients could not be blinded.)
  21. 42.Cummings, B.E., and Waring, M.S. (2020). "Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies." Journal of Exposure Science & Environmental Epidemiology, 30(2), 253-261. https://doi.org/10.1038/s41370-019-0175-9 (12 chamber studies, 196 results; median single-plant clean air delivery rate 0.023 m³/h.)
  22. 10.Donovan, G.H., Butry, D.T., Michael, Y.L., Prestemon, J.P., et al. (2013). "The relationship between trees and human health: evidence from the spread of the emerald ash borer." American Journal of Preventive Medicine, 44(2), 139-145. https://doi.org/10.1016/j.amepre.2012.09.066 (1,296 counties in 15 states, 1990-2007; 6.8 additional lower-respiratory and 16.7 additional cardiovascular deaths per year per 100,000 adults; falsification test on accidental deaths p = 0.22; the 100-million-tree figure is attributed in the paper to a personal communication from D. McCullough, Michigan State University.) (Deaths modeled 1990-2007; 6,113 additional lower-respiratory and 15,080 additional cardiovascular deaths between 2002 and 2007; extra cardiovascular deaths rose from 9.68 per 100,000 in the first year of infestation to 38.59 in the sixth; the borer kills all 22 North American ash species, and by 2012 had killed roughly 100 million US trees.)
  23. 12.Donovan, G.H., Prestemon, J.P., Gatziolis, D., Michael, Y.L., et al. (2022). "The association between tree planting and mortality: a natural experiment and cost-benefit analysis." Environment International, 170, 107609. https://doi.org/10.1016/j.envint.2022.107609 . Donovan's quotation is from the EurekAlert! press release of 17 November 2022, which also gives the total of 49,246 trees planted by Friends of Trees between 1990 and 2019. (The release's conversion of the rate coefficients into "20%" and "6%" reductions is a misreading and is not used here.) (Trees planted in the preceding 15 years; models adjusted for year, race, education, income and age. Donovan: "We observed the effect both in green and less green neighbourhoods, which suggests that street tree planting benefits both.")
  24. 18.Branas, C.C., South, E., Kondo, M.C., Hohl, B.C., et al. (2018). "Citywide cluster randomized trial to restore blighted vacant land and its effects on violence, crime, and fear." PNAS, 115(12), 2946-2951. https://doi.org/10.1073/pnas.1718503115 (38-month study period; 445 residents sampled; the "roughly the size of Switzerland" comparison is the authors'.) (Around treated lots in neighborhoods below the poverty line: gun violence -29.1%, burglary -21.9%; residents' use of outdoor space for relaxing and socializing +75.7%.)
  25. 43.Ideno, Y., Hayashi, K., Abe, Y., Ueda, K., et al. (2017). "Blood pressure-lowering effect of Shinrin-yoku (forest bathing): a systematic review and meta-analysis." BMC Complementary and Alternative Medicine, 17(1), 409. https://doi.org/10.1186/s12906-017-1912-z (20 trials, 732 participants; systolic MD -3.15 mmHg, 95% CI -4.12 to -2.18; only two trials lasted longer than one day.)
  26. 44.CDC (2024). Preventing Tick Bites, last reviewed 28 August 2024. https://www.cdc.gov/ticks/prevention/index.html

This topic carries 44 notes in the research manuscript; the 26 above are those behind the material on this page. Full source register.