If we can bathe in sunshine or in glory or in misery, why not in a forest? I didn’t invent the expression: it’s from the Japanese (shinrin-yoku) who believe time spent in nature boosts health. Most people agree that gulping fresh air on a forest walk or on a mountain or beside the sea is beneficial, although there is little scientific evidence to support the belief. Perhaps that disconnect shouldn’t surprise us in an era when beliefs often trump facts in national debates and people still turn to quackery and naturopathy to cure their ills. But the idea seduces me. Breathing pine-scented air and inhaling the “sighs” of vegetation along a trail is more than engaging; it resonates with a belief in “biophilia,” that despite our retreat to urban living we have not lost an inborn sympathy with mother nature and need to spend time with her.
Our remote ancestors regarded the outdoors otherwise. They crouched in caves and huts to shelter from the elements and large predators, and the Brothers Grimm played on primitive fears by inventing menaces prowling in the Black Forest. We may be alienated from nature by modern living, but it is now regarded more as a kind mother than a cruel step-father, even while it still harbors dangers. Forests are called lungs of the world, and even if we never visited the tropics we know they throb with biodiversity we need to survive. We are told that bathing in the bosom of the plant kingdom, where our ancestors spent their days, restores health and well-being, but it doesn’t need to be a forest. It can be a garden, woodlot or urban park will do, and immersion should start in childhood (Last Child in the Woods by Richard Louv).
We now live mostly indoors which is a novelty in human history, but no English word describes it. The closest I can find is the Japanese hikikomori for people whose lives are spent indoors mostly around movies and the Internet, although this word often refers to recluses rather than the general habit I have in mind.
After musing about these words I wondered if my belief in the healthy outdoors was mere wishful thinking, based on quasi-science. Hocus pocus. Of course, walking is an exercise proven to promote health, but that wasn’t the evidence I was seeking because an ardent hikikomori can take exercise in a gym with barely any exposure to outdoors, and even then to a polluted street. I wanted more evidence of forest bathing to justify what my gut told me, although I knew that a negative result would not diminish my pleasure. I found a trio of factors: Bugs, Ions and Oils. I remember them by the acronym BIO, although I found that lends far more biological authority than warranted by the data. This is what I learned.
Bugs. We are more likely to be caught indoors by a bug like staph, strep, tubercle bacteria or by one of the host of viruses and allergenic molds, and other hazards exist there too, including carbon monoxide, radon, toxic vapors and smoke from fires and tobacco. Since Ms. Nightingale started throwing hospital windows open we accept the virtues of fresh air, but outdoor air isn’t empty ether even deep in the countryside where it still swarms with microbes and spores, exposing us to 2,000 types a day according to one estimate. Most of them are benign, but are they good like the country folk who offer a friendly handshake and refreshment to hikers? I read that inhaling bugs in country air can boost our microbiome, the vast community of commensals in our bodies that is now touted as a touchstone of health. I wish it were true, but the claim stretches credibility as much as homeopathic medicine. The evidence is as microscopic as the microbes.
Ions. I also read that woodland air and sea air zap microbes in the way that air ionizers and ozone generators are claimed to cleanse homes, offices and hospitals, but it’s a false parallel. These devices are not necessarily safe because ozone can be hazardous to breathing, and so fresh air is safer because the concentrations are far lower, and below the threshold for killing bugs. Besides, the old story about the smell of ozone at the beach is a myth, and a meta-analysis in BMC Psychiatry (2013) failed to trawl from multiple studies any hint of association between charged ion concentration and mood, sleep or anxiety.
I was losing hope in BIO when I reviewed the last factor in the trio.
Oils. Air is wonderfully scented by terpenes and other odoriferous oils in groves of pines, firs or cypress. These oils are called phytoncides because they are plant vapors with antimicrobial activity. They serve plants by inhibiting molds that would otherwise cause rot, and they make fruit and foliage less appetizing to herbivores. Garlic and tea tree oil are familiar domestic examples of phytoncides. Unfortunately for the hypothesis, the concentrations of phytoncides likely to be encountered on forest walks are far too low to eliminate bacteria and spores, unless you spray the air with a bottle of Pine-Sol.
This quick survey of the literature confirmed the doubts I began with, so I cast around for another explanation before abandoning the notion of forest bathing.
I wondered if the sight of green vegetation was the key. Green is close to blue in the spectrum, beloved by artists and interior decorators for pacifying our emotions, and just might account for some of the benefits of light therapy for the winter blues (Seasonally Affective Disorder). If green is naturopathic, does it follow that people who can’t see the color (deuteranopia) have poorer health? I found evidence for this wild hypothesis in old clinical studies where patients lacking the retinal pigment for green were more likely to have bipolar disorder. But this connection with mental health was weak, and it can be explained not only by the inability to see green fields and luxuriant foliage but more likely by another gene that is closely linked to the pigment gene on the same X-chromosome. Another dead end!
Perhaps there is no physical basis for forest bathing, leaving the explanation all in the mind. Maybe the uplifting feelings we get from gazing at nature affect the endocrine and autonomic nervous systems to reduce stress and anxiety, and the beauty of nature distracts us from worries carried from home and work. Dr. Roger S. Ullrich published a study over thirty years ago that lends some support for a psychosomatic explanation. He monitored two groups of surgical patients who were comparable in every respect except one: they were either recovering in rooms with windows overlooking a blank wall or had a view of trees and other vegetation. Those who could see a more natural landscape were released earlier from hospital, needed less analgesia and had fewer post-operative complications. The windows were closed and the indoor environment was controlled so the difference outcomes were not explained by outdoor air. His research, which has influenced hospital design in many countries, began inauspiciously when he was bed-ridden as a teenager and often looked out at the lone pine tree outside his window.
Anne Frank was also confined to home as a young person, but for a very different reason. She often gazed out her window at the large chestnut tree in her street in Amsterdam. She couldn’t go for a forest walk but the tree connected her to nature and raised her spirits.
Perhaps forest bathing is no more, and no less, than a soulful experience, and we don’t need bugs, ions and molecules to account for it. It’s not a new explanation, and John Muir wrote about it long ago: “Nature’s peace will flow into you as sunshine flows into trees. The winds will blow their own freshness into you, and the storms their energy, while cares will drop off like autumn leaves.”
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