Nature's Medicine: Can Forests Calm Inflammatory Conditions?

The prescription for what ails us may, at least in part, be written not in the pharmacy but in the forest.

Every weekend, Louise and I visit Forest Park here in Portland, the largest wild area within a city in the world, with 5,200 acres and over 80 miles of trails. There are some pretty steep climbs and we try to get seriously out of breath.

Not only is it good exercise and a nice opportunity to get away from our electronics and just talk with each other, but communing with nature has well-known positive emotional and psychic effects that help regulate mood and even improve physical health.

On last Sunday’s walk, Louise wondered out loud if regularly walking in nature could also help reduce inflammatory conditions like asthma. It reminded me that back in the early 1990s, I met a woman named Sarah at a conference in Portland who told me something I found both fascinating and counterintuitive.

Sarah had suffered from severe asthma her entire adult life, requiring multiple medications and occasional emergency room visits. “Then I moved to a small cabin near Mount Hood,” she said, “and within three months, I was using my rescue inhaler maybe once a month instead of daily.” When I asked her what changed, she simply said, “I walk in the forest every morning. That's the only thing different in my life.”

I thought about Sarah recently when my conversation with Louise led to my reading about the “hygiene hypothesis,” first proposed by epidemiologist David Strachan in 1989. His radical suggestion was that our increasingly sanitized modern environments might actually be contributing to the alarming rise in allergies, asthma, and autoimmune disorders we’ve seen over the past several decades. It struck me as deeply ironic — our quest for cleanliness potentially undermining our health.

The hygiene hypothesis has since evolved into what scientists now more accurately call the “Old Friends'“ hypothesis. The problem isn't simply that we’re too clean; it's that we’ve lost regular contact with specific microorganisms that humans co-evolved with throughout our evolutionary history. These “old friends” — found in soil, plants, animals, and unpolluted natural environments — appear critical in training our immune systems to distinguish between harmless environmental particles and genuine threats.

Think about it: for 99% of human existence, we lived intimately connected to natural environments, constantly exposed to a rich diversity of microbes. Only in the past few generations have we created virtually sterile indoor environments, doused everything in antimicrobial chemicals, and separated ourselves from the microbial world that shaped our immune system's development. Our bodies simply haven’t adapted to this dramatic change.

The consequences are becoming increasingly clear. A landmark 2015 study published in the New England Journal of Medicine compared the immune profiles of Amish children (who grow up on traditional farms with daily exposure to livestock and soil) to genetically similar Hutterite children (who live on industrialized farms with limited direct exposure to animals and soil). Despite their genetic similarities, Amish children had dramatically lower rates of asthma and allergies, a difference the researchers attributed directly to their microbial exposure.

But what about those of us who didn’t grow up on farms or spend our childhoods playing in the dirt? If you’re like Sarah, already suffering from asthma or other inflammatory conditions, is it too late to recalibrate your immune system?

Emerging research suggests it may not be. A fascinating body of evidence comes from studies on “forest bathing” (Shinrin-Yoku), a Japanese practice of immersing oneself (with your clothes on!) in forest environments. Dr. Qing Li, an immunologist at Nippon Medical School in Tokyo, has conducted extensive research showing that when people spend time in forests, they experience measurable changes in their immune function.

In one study, Li took a group of middle-aged Tokyo businessmen into the forest for three days. Blood tests revealed significant increases in natural killer (NK) cells, the immune cells that play a vital role in fighting infections and cancerous cells. More remarkably, these participants showed decreased levels of pro-inflammatory cytokines, the very signaling molecules that drive excessive inflammatory responses in conditions like asthma. Most surprisingly, these beneficial immune changes persisted for more than 30 days after their forest visit.

It makes perfect sense, after all. Our immune systems didn’t evolve in a vacuum; they developed in constant dialogue with the microbial world around us. When we restore some of that dialogue through forest exposure, we’re essentially reminding the immune system how it was designed to function.

This makes intuitive sense when you consider our evolutionary history. Our ancestors didn’t suffer from high rates of allergies and asthma despite (or perhaps because of) their constant exposure to diverse microbial environments. The human immune system appears to need this microbial diversity to develop properly and maintain balance.

A groundbreaking 2020 study published in Science Advances provided even more direct evidence. Researchers in Finland modified urban daycare playgrounds, replacing sterile gravel with forest floor materials: rich soil, native plants, and the diverse microbiota they contain. After just 28 days, children playing on these naturalized playgrounds showed increased diversity in both their skin and gut microbiomes. More importantly, they exhibited increases in regulatory T cells, those specialized immune cells that help prevent allergic reactions and dampen excessive immune responses.

While this particular study focused on children, its implications extend to adults as well. Our microbiomes remain somewhat plastic throughout life, capable of responding to environmental changes. When we walk through biodiverse forests, we expose ourselves to a rich tapestry of microorganisms that can influence our own microbial communities and, by extension, our immune function.

There’s another fascinating aspect to forest exposure that has particular relevance for asthma sufferers: phytoncides. These are natural oils released by trees and plants as part of their defense system against bacteria and insects. When humans inhale these aromatic compounds during forest walks, they trigger beneficial physiological responses.

Research shows that phytoncide exposure reduce levels of stress hormones, decreases sympathetic nervous system activity (associated with the “fight-or-flight” response), and enhance parasympathetic nervous system function (associated with “rest-and-digest” activities). For asthma sufferers, whose symptoms can be triggered or worsened by stress, these effects could provide additional relief beyond the direct immune-modulating benefits.

Laboratory studies have gone further, demonstrating that certain phytoncides have direct anti-inflammatory properties. Alpha-pinene, for example, a compound abundant in pine forests, has been shown to reduce inflammatory markers in lung tissue and may help soothe irritated airways, potentially reducing the hypersensitivity characteristic of asthma.

Not all green spaces offer equal benefits, however. A 2019 study published in Environmental Health Perspectives found that adults living near areas with greater biodiversity had lower rates of asthma exacerbations and hospital visits compared to those living near less diverse green spaces. This suggests that the microbial diversity of the environment plays a crucial role; old-growth forests with varied plant species likely offer greater immune benefits than manicured urban parks with limited biodiversity.

It turns out there’s a growing body of research suggesting that regular forest exposure may help moderate inflammatory conditions. The combination of stress reduction, exposure to diverse microbiota, and inhalation of beneficial plant compounds appears to create an environment conducive to immune balancing.

None of this means we should abandon modern medicine or hygiene practices. Prescription medications save lives, and basic hygiene prevents the spread of dangerous pathogens. Forest walks won’t “cure” asthma or completely reverse the effects of growing up in highly sanitized environments. But they may offer a meaningful complement to conventional treatments; a way of addressing one of the potential root causes of our modern epidemic of inflammatory conditions.

If you suffer from asthma, allergies, or other inflammatory conditions, consider adding regular walks in biodiverse natural environments to your self-care routine. Seek out older, established forests with diverse plant species rather than recently planted or highly manicured parks. Go regularly: benefits appear to accumulate with consistent exposure. And when you're there, engage fully: touch plants (safely), breathe deeply, and fully engage your senses.

The prescription for what ails us may, at least in part, be written not in the pharmacy but in the forest. Our immune systems evolved in constant dialogue with the natural world. Perhaps it’s time we rejoined that conversation.