How Is Natural Killer Cell Function Boosted by Forest Bathing? 

Can the aroma of wood essential oils replicate the immune-boosting effects of walking in a forest?

Studies on the effects of “forest bathing,” or shinrin-yoku in Japanese, “a traditional practice characterized by visiting a forest and breathing its air,” have found it “can induce a significant increase in the number and activity of natural killer (NK) cells” that can last as long as a month. And because natural killer cells are one of the ways our body fights cancer by killing off tumor cells, the “findings suggest that visiting forest parks may have a preventive effect on cancer generation and progression.” How? “Why did the forest environment increase human NK activity? What kind of factors in the forest environment activated human NK cells?” What is it about the forest environment?

One thought is that the boost may be related to a reduction in stress. If you measure the amount of adrenaline flowing through people’s systems, did spending time in a forest—but not in a city—lower adrenaline levels? Yes, as you can see at 0:58 in my video Why Does Forest Bathing Boost Natural Killer Cell Function?. 

However, if you drip some adrenaline on human blood cells in a petri dish, there does not appear to be any effect on NK cells. The stress hormone cortisol, on the other hand, dramatically suppresses natural killer cell activity, as shown below and at 1:09 in my video. So, might being in the forest lead to less stress and less cortisol, which releases the natural killer cells under its thumb, giving us a boost?

We know being surrounded by nature can decrease levels of cortisol in our saliva, but what about our bloodstream? Researchers found a significant drop in the stress hormone after a single-day trip to the forest. A week later, cortisol was normalizing, as you can see here and at 1:30 in my video, but the forest’s effects sometimes appeared to last an entire month. Anything else that could cause a longer-term immune system change?

Maybe we’ve been missing some of our “Old Friends.” If you sample outdoor air, you can pick up an abundance of microorganisms from the soil and water that float around—but are absent from indoor air. (Indoor air is dominated by organisms that either live on us or try to attack us.) So, on a day-to-day basis, in terms of keeping our immune system on ready alert, it “might not be sufficient to encounter only the biased microbiota of the modern synthetic indoor environment that lacks some of the Old Friends and probably bears little resemblance to the microbiota we encountered throughout our evolutionary history,” the microbes we evolved to live with over millions of years.

Or perhaps it’s the plants themselves. Maybe it’s the aroma of the forest. Trees produce aromatic volatile compounds called phytoncides, like pinene, which we can breathe into our lungs when we’re in the forest. Do these compounds actually get into our bloodstream? Indeed, after spending one hour in the woods, we get about a sixfold increase in pinene levels circulating throughout our system. To fully connect all the dots, phytoncides like pinene—those essential oils from wood—would then have to induce human natural killer cell activity. And guess what? The study title gives it away: “Phytoncides (Wood Essential Oils) Induce Human Natural Killer Cell Activity.” If you put natural killer cells in a petri dish with leukemia cells, our NK cells can wipe out some of the cancer cells. But, if you add a whiff of cypress, white cedar, eucalyptus, or pine, the cancer cells don’t stand a chance, as shown below and at 3:14 in my video. 

A combination of wood aromas improved the recovery of mice put through the wringer, but this is the study I was looking for: “Effect of Phytoncide from Trees on Human Natural Killer Cell Function.” If we want to know if the magic ingredient is the fragrance of the forest, then let’s see if we can get that same boost in natural killer cell activity by just vaporizing some essential oil from one of the trees into a hotel room overnight. It worked! There was a significant boost in natural killer cell activity, as seen here, and a 3:40 in my video.

Only NK activity was boosted, though, rather than the number of the natural killer cells, and being in an actual forest can elevate both. So, perhaps it’s a combination of the tree fragrance and the lower cortisol levels working together.

Ironically, these phytoncide compounds are part of the tree’s own immune system, which we may be able to commandeer. As you can see at 4:05 in my video, researchers speculate that these compounds may be playing a role in the fact that more heavily forested regions in Japan appear to have lower death rates from breast cancer and prostate cancer. Indeed, being in nature has been found to be an “important coping strategy among cancer patients,” but it may help us cope better, thanks to the fragrance of trees. 

Spending time in a forest significantly increases NK cell activity. In my previous blog, I discussed Boosting Anticancer Immunity with Forest Bathing.

You may recall that I’ve touched on this topic before. See Are There Health Benefits of Spending Time in Nature?.

For more on aromatherapy, check out the related posts below.