Can a stroll in the forest ward off cancer?

Trees in Olympic National Park, Washington. Photo by Jeff Fox.

You’ll be surprised by the answer. Find out more in this essay from Wilderness Magazine. To receive the annual magazine and quarterly newsletters from The Wilderness Society, become a member today!

Breathing Trees

By Jennifer Ackerman

There is a particular forest odor that haunts me. In it I smell home, as if my Russian and German ancestors had passed along in my olfactory genes their roots in dark woods. A whiff of this volatile cocktail travels directly to that corner in my brain that regulates my sense of well-being. I find it calming, comforting, even — I would venture — healing. When I catch hint of it, I inhale deeply, drawing in what I guess is a mix of red cedar and pine mingled with the musty litter of maple and oak leaves. But I suspect this is a crude gleaning on the surface of a deeper olfactory mystery, a rich and subtle chemical text I can discern but can’t fully describe. When it comes to smell, words fail.

I first experienced this olfactory response when I was a child sent away to camp at too tender an age. What consoled me during bouts of piercing homesickness was not the companionship of my fellow campers but wandering the rough path among the mossy hummocks and decaying stumps surrounding our camp site and breathing in that sweet dank odor.

Now, when life dishes up difficulties, I seek this smell where I can, most often in the scented breath of the Appalachians. I once thought of trying to reproduce it in my yard, model my own aromatic grove. But I knew that key ingredients would be missing: time (to create that fragrance of decay) and space (how many trees does it take to generate forest air?)

Why this woody bouquet should make me feel good has always puzzled me. Hence it was with some delight that I stumbled on a Japanese study published last year revealing the salubrious effects of shinrin-yoku, or Taking in the Atmosphere of the Forest. The study compared the physiological effects of walking in the city with strolling in one of Japan’s few remaining stands of old-growth broadleaf forest. The researchers found that the forest stroll had beneficial effects on blood pressure, heart rate, and immune function. Their findings belong to a growing body of evidence that ‘forest bathing’ can affect everything from blood glucose levels to the activity of immune cells. It may even help protect against cancer.

Many of us know from experience that being in the wild relieves stress and mental fatigue. When I walk in the woods, I surrender my vigilant urban eyes in favor of ‘involuntary attention,’ the effortless noticing of pleasant sensory stimuli, the thrum of a woodpecker, the shifting lozenges of light playing on the forest floor.

That the human body and mind should find a walk in the woods restorative is hardly surprising. After all, our tribe evolved in the forests and fields of Africa. Our ears evolved to detect the rustle of leaves that betrays a predator or prey; our eyes, to tease apart the subtle hues that signal ripe fruit and tender leaves; and our noses, to recognize not only members of our own species but the wood and floral signposts of our homes.

I can easily see how tracing the slip of water over bright stones in a forest brook might calm the hypertensive heart. Or how glimpsing the marbled salamander skittering over a froth of moss could banish anxiety. I can imagine that listening for the liquid trill of a woodthrush or the singular call of a whippoorwill might obliterate an incipient cancer cell.

But this is soft speculation.

In their experiments, the Japanese scientists uncovered hard evidence that walking in the forest decreases the blood glucose levels of diabetic patients, and that people who view forest scenery for 20 minutes have a 13 percent lower blood concentration of the stress hormone cortisol than people viewing urban settings. They have revealed that forest walking compared with city walking boosts the activity of natural killer cells, immune cells that fight cancer—an effect that may last for as long as 30 days. And in 2008, they reported that people living in areas with a higher percentage of forest cover had lower mortality rates for cancers of the lung, breast, uterus, prostate, kidney, and colon, compared with people living in areas with lighter forest cover, even after factoring in socioeconomic status.

The healthy effects of forests may be related to the cooler temperatures beneath a canopy or the softer intensity of light, say some scientists, or possibly to the higher concentrations of oxygen. Perhaps the pleasant aroma of plants makes us want to breathe deeply through the nose, and that deep breathing produces physiological benefits, lowering blood pressure and heart rate and boosting circulation.

Or maybe it’s something in the smells themselves. When researchers analyzed air samples in the forests of the Sierra Nevada Mountains, they found a blend of 120 different volatile organic compounds. (Many of these chemicals play a vital role in forest ecosystems, repelling herbivores or attracting insects for pollination.) When we inhale forest air, some of these molecules ride the currents into our lungs and there pass into our bloodstream to affect our cells. This may explain why researchers have found that the smell of Japanese cedar lowers blood pressure or that two woody compounds, alpha-pinene and borneol, reduce fatigue. The Japanese scientists who studied the effects of shinrin-yoku in that old-growth forest attribute some of the boosted activity of natural killer cells to inhaling phytoncides, essential wood oils emitted by plants.

I love this notion that plant molecules speak to human cells, revealing as it does that deep common kinship of living things long known by poets. As George Herbert wrote, “Herbs gladly cure our flesh, because that they/Find their acquaintance there.”

The researchers who parsed the Sierra forest air samples could identify only 70 of the 120 compounds they found. We are as ignorant of this unseen component of forest ecology as we are of the myriad intricate interactions between genes and the environment that shape our bodies, or of the range of interlocking links in ecological webs that make predicting the effects of habitat destruction such a tricky business. When I think that we are losing forests at rate of 7 million hectares a year, and with them, who knows how many pinenes and phytoncides, known and unknown, with potential to ease our stress and heal our bodies, I feel a black wave of despair.

Still, the new research gives hope.

The Japanese have a word, aware, that describes the feelings that arise from the poignant beauty of an ephemeral thing. The word refers not to the loss of the thing itself, but to the human feelings evoked by its passing. I’d like to invent a word to describe the human body’s beneficial response to the secret elements of forest air. Maybe bringing the concept under the net of language would help us embrace the idea that in saving forests, we may quite literally be saving ourselves.

Jennifer Ackerman of Charlottesville, Virginia, writes about science and nature for National Geographic and many other publications. Her most recent book is Sex Sleep Eat Drink Dream: A Day in the Life of Your Body.

photo: Trees in Olympic National Park, Washington. Photo by Jeff Fox.