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Studies on the effects of “forest bathing,” a traditional practice in Japan of visiting a forest and breathing its air, have found it “can induce…significant increase[s] in the number and activity of natural killer cells” that can last for as long as a month. And, because natural killer cells are one of the ways your body fights cancer (by killing off tumor cells), the findings suggest that forest visits “may have a preventive effect on cancer generation and progression.” Okay, but how? “Why did the forest environment increase…natural killer cell activity?” 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 hanging out in a forest—but not a city—drop adrenaline levels down? Yes; so that checks out, but drip some adrenaline on human blood cells in a petri dish, and there does not appear to be any effect. The stress hormone cortisol, on the other hand, dramatically suppresses natural killer cell activity. So, maybe the forest led to less stress; less cortisol, which released the natural killer cells under its thumb––and you get the big boost?

We know being surrounded by nature can decrease levels of cortisol in our saliva, but what about our bloodstream? A significant drop after a single day trip to the forest. But a week later, the cortisol was normalizing, and the forest 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 floating around from the soil or water, which are absent in our indoor air (which is dominated by organisms that either live on us or try to attack us). So, maybe 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 microbes of the modern synthetic indoor environment that lacks some of the Old Friends, and probably bears little resemblance to the microbes we evolved to live with over millions of years.

Or, maybe it’s the plants themselves. Maybe it’s the aroma of the forest? Trees produce aromatic volatile compounds called phytoncides, like pinene, which you can breathe into your lungs in the forest. But do these compounds actually get into your bloodstream? One hour in the woods, and you get like a six-fold increase in circulating pinene levels circulating throughout your system. Okay, but to fully connect all the dots, the phytoncides like pinene, these tree essential oils, would have to then induce human natural killer cell activity. And…guess what? Phytoncides induce human natural killer cell activity. If you stick natural killer cells in a petri dish with some unsuspecting leukemia cells, your killers can wipe out some of the cancer cells; but add a whiff of cypress, white cedar, eucalyptus, or pine, and the cancer cells don’t stand a chance.

A combination of wood aromas improved the recovery of mice put through the wringer. But this is the study I was looking for. 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 just vaporizing some essential oil from one of the trees into a hotel room overnight. And it worked! A significant boost in natural killer cell activity; though it just boosted their activity, rather than their number, and being in the actual forest can do both. So, maybe 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. The researchers speculate these compounds may be playing some role in the fact that more heavily forested regions in Japan appeared to have lower death rates from breast cancer and prostate cancer. Being out in nature has been found to be an “important coping strategy among cancer patients.” It turns out this could potentially be helping more than just with the coping, thanks to the fragrance of trees.

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