Phytoncides: What Forest Air Actually Does, and What It Doesn't

Phytoncides are the volatile organic compounds that trees and other plants release into the air, mostly to defend themselves against insects, fungi and bacteria. In a forest the largest group of them are monoterpenes, and the most abundant single one is usually alpha-pinene, the sharp resinous note you smell in a spruce stand on a warm afternoon. The popular claim is that breathing these molecules boosts your immune system. The pooled research says something narrower, and more interesting.

The first systematic review and meta-analysis of the immunological effects of phytoncides, published in 2024, found a significant increase in natural killer cell activation after phytoncide exposure, with an effect size of 2.50 and a 95 percent confidence interval of 1.94 to 3.05. The same authors graded the certainty of that evidence as low. Six studies with 79 participants between them entered the pooled analysis, five of those six came from one Japanese research group, and the authors rated the evidence down specifically because most studies could not separate the effect of the molecules from the effect of the walk people took to reach them (Fleming et al., 2024).

I guide forest therapy in Slovenia and have facilitated people in nature since 2019, and phytoncides are the single thing people most often bring to a session already believing. It is a lovely idea: the forest as a medicine you inhale. This article sets out what the compounds are, how much of them is actually in the air you breathe, what the immune research found and did not find, how big the anxiety effect really is, and what I think an honest guide should say about all of it. For the wider picture I keep a separate summary of what the research says about forest therapy.

What is a phytoncide?

A phytoncide is a plant-made airborne compound that kills or slows the growth of micro-organisms. The Russian biochemist Boris Tokin coined the word in 1928, joining the Greek for plant to the Latin for killing, after noticing that crushed plant tissue could destroy bacteria at a distance. Trees are not producing these compounds for us. They are chemical defence, and some of them double as signals to other trees and to the insects that prey on the ones doing the damage.

The phytoncides that matter in forest research are mostly monoterpenes: alpha-pinene and beta-pinene, limonene, 1,8-cineole, camphene, sabinene. They are the same family of molecules that make up much of a conifer essential oil, which is why a warm spruce forest smells the way it does. Slovenia is 58 percent forest, third in the European Union behind Sweden and Finland, and spruce and fir are among its most common trees (Zavod za gozdove Slovenije). Whatever monoterpenes do, this country has no shortage of the trees that make them.

How much of it is actually in the air?

Monoterpene concentrations under a forest canopy are low, variable, and strongly patterned by season and time of day. A Catalan monitoring study measured breathable air beneath a Mediterranean holm oak canopy continuously from June to November 2019 and recorded total monoterpene concentrations between 0.02 and 0.13 parts per billion by volume across the middle half of readings, with peaks reaching 1.24 ppbv. Concentrations were clearly higher in summer, and within any given day they peaked during daylight hours.

Alpha-pinene made up close to half of all monoterpenes measured in the Italian forest therapy study described below, across mountain, hill, pinewood and urban park sites. Urban parks trended lower than forest sites in that dataset, but the difference did not reach statistical significance, partly because there were few urban park sessions and the variability was high (Donelli et al., 2023). That is worth holding on to: the gap between a city park and a real forest is probably real, and it has not been demonstrated cleanly.

The practical reading is simple. A still, warm, sunlit hour in a conifer-rich stand carries more of these molecules than a cold windy morning in a bare beech wood. That is a reason to pick a July afternoon in the spruce above Pokljuka if the chemistry is what you are after, and it is not a reason to stay home between November and March. The evidence that a forest helps in the cold rests on quite different measurements, which I go through in the article on winter forest bathing.

Do phytoncides boost the immune system?

The honest answer is that a small body of low-certainty evidence consistently points that way, and no study has yet shown it cleanly. The 2024 review screened 468 records and ended with eight studies published over sixteen years. Seven of the eight measured how strongly natural killer cells were activated, and all seven reported a significant increase. Six studies with 79 participants had data in a form that could be pooled, giving the effect size of 2.50 quoted above, with moderate heterogeneity.

What did not change is as informative as what did. Every one of the five studies that counted white blood cells found no significant difference after the intervention, and four of them found no change in lymphocytes. Three studies found a significant decrease in the percentage of T-cells. So this is not a general immune boost in the way the phrase is usually meant. It is a shift in one specific cell population and its cytotoxic machinery, the granzymes and perforin that natural killer cells use to destroy infected and tumour cells.

Only one of the eight studies was a randomised controlled trial. It vaporised phytoncides indoors for gynaecological cancer survivors, was assessed as low risk of bias, and found a non-significant increase in natural killer cell count alongside significant changes in specific cell receptors. The review also notes that it had no pre-registered protocol of its own, and that publication bias could not be assessed because fewer than ten studies existed to assess it with.

The part most forest bathing articles leave out

The review's own authors name three things that could explain the immune findings without any molecule doing anything. Walking by itself increases circulating natural killer cells. Group sessions bring social contact, and positive social connection has its own documented effect on antiviral immunity. Simply looking at natural scenery, indoors, shifts autonomic activity toward the parasympathetic side, and that shift has been linked to reduced inflammation. Nearly every study in the review sent people for a walk in a forest, usually in a group, for three days. Any of those ingredients could be doing the work.

There is also a direct test of the inhalation story, and it came back negative. A field experiment in a Mediterranean holm oak forest measured four monoterpenes in participants' blood before and after a two-hour forest exposure, with an urban group as control, and found no significant change in blood monoterpene concentrations in either group. The one clear relationship was that people who started with less in their blood absorbed more (Bach et al., 2021). Two hours of forest air, measured directly, did not move the numbers.

None of this means phytoncides do nothing. It means the causal chain that gets repeated as fact, from tree to air to bloodstream to immune cell, has a measured gap in the middle of it. A two-hour session is also much shorter than the three-day forest trips that produced most of the immune findings, which is worth saying plainly to anyone booking a single walk on the strength of a headline about cancer-fighting cells.

How big is the effect on anxiety?

The best evidence linking forest air chemistry to how people feel found a reduction of about 1.3 points on an anxiety scale that runs from 20 to 80. Italian researchers collected data from 505 participants across 39 structured forest therapy sessions, measured the monoterpene concentration in the air at each site, and administered the state anxiety questionnaire before and after every session. Participants exposed to above-average total monoterpene concentrations showed an estimated reduction of 1.28 points, with a confidence interval running from 2.51 down to 0.06. For alpha-pinene specifically the estimate was 1.31 points, and in the top quartile of exposure the estimates grew to 1.61 and 1.68 points (Donelli et al., 2023).

That scale runs from 20 to 80, so the effect is small and it is worth saying so before anyone else does. Three other monoterpenes tested the same way, sabinene, o-cymene and camphene, produced no statistically significant effect at all. The design was observational rather than randomised, 150 of the 655 people recruited were dropped for incomplete questionnaires, and two thirds of the final sample were women. The authors state directly that their results should not be read as causation.

Set that against the pooled picture and the tension is obvious. A 2026 systematic review and meta-analysis of eleven forest bathing studies found consistent improvements in self-reported tension and anxiety, a mean heart rate drop of four beats per minute, no clinically meaningful change in salivary cortisol, and no clear effect on the same state anxiety inventory the Italian team used (Andrade et al., 2026). One well-powered observational study says the chemistry matters a little; the pooled trials cannot confirm it. Promising and unsettled is the accurate description. I unpack the physiology side of this in nature and the nervous system.

What this means for how I guide

I do not sell the molecule. A forest therapy session with me runs two to three hours over a very short distance, built from sensory invitations and circle sharing and closing with a tea ceremony, and none of that depends on what the air assays at. If I told a group that the spruce around them was raising their natural killer cell count, I would be stating as fact something that six small studies suggest and cannot demonstrate. The practice is strong enough without it.

What the chemistry does change is where and when I choose to guide, because those decisions are free. Conifer-rich stands, sheltered spots where the air sits still, the hours after midday in summer, the twenty minutes after rain when the whole forest smells different: I would pick all of those anyway, for the smell and the light, and if the monoterpenes are contributing something then nothing has been lost. When a company asks me what their team will get out of a half day in the forest, I answer with what the research supports: lower heart rate, better mood, a genuine break from directed attention. Not immunity.

Forest therapy may support relaxation and wellbeing, and it does not replace medical or psychological care. If you want to test the air of a Slovenian forest on yourself rather than read about it, the sessions I run are the place to start, and what forest therapy actually is explains the practice from the beginning.