Boosting Anticancer Natural Killer Cell Function with Trees?

Visiting a forest can induce a significant increase in both the number and activity of natural killer cells, one of the ways our body fights off cancer. Can the aroma of wood essential oils replicate the immune-boosting effects of walking in a forest? Forest bathing is a term coined in Japan in the 1980s. Since then, we have learned it has benefits beyond just the exercise. In this video Dr. Greger looks at some of those benefits, and whether or not you need to actually visit a forest to experience them.

Friday Favorites: Boosting Anticancer Natural Killer Cell Function with Forest Bathing (

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Intro: Forest bathing is a term coined in Japan in the 1980s. Since then, we have learned it has benefits beyond just the exercise. In this video I look at some of those benefits, and whether or not you need to actually visit a forest to experience them.

Previously, I showed how exposure to nature can have self-reported psychological benefits, but there was a dearth of data on changes in objective measurements. So, I was excited to see this paper on the effects on levels of the stress hormone cortisol in the saliva of those partaking in “forest bathing”––which just means visiting a forest and surrounding yourself by trees.

The level of cortisol in your saliva is considered an indicator of your stress level, and after walking in a forest, compared to walking in a city, or even after just hanging out in a forest compared to a city, people’s salivary cortisol levels were significantly lower. But wait a second, the same effect was found before they went to the forest. Huh? Forest bathing was associated with significantly lower salivary cortisol both before and after, compared with visiting an urban area. Therefore, it appears that just the thought of going to spend the day in the forest relieved stress. So, when comparing the effects of forest bathing versus urban visiting, the anticipation placebo effect may play a more important role in influencing stress levels than the actual experience of being in the forest. So, I was ready to dismiss it as just another nebulous psychological effect until I read this. Studies on the effects of forest bathing on the immune function showed that visiting a forest can induce a significant increase in the number and activity of natural killer cells––one of the ways our body fights off cancer. That got my attention.

It all started with this study. Twelve men were taken on a long weekend to walk in some forests, and almost all of the subjects (11 out of 12) showed higher natural killer cell activity after the trip––and not just a little; about a 50 percent increase compared to before the trip. Now, just exercise can affect immune function, but they weren’t walking any extra; they were just walking in a forest instead. Yeah, but they also were taken on a trip somewhere, introducing other variables. So, how about randomizing them to go on some city trip versus the forest trip? And if there is some special forest effect, how long does the effect last? Do you have to, like, walk in the forest every day? Before jumping into all that, how about we first see if it works in women too?

Same kind of setup, and same results: a significant boost in natural killer cell activity walking around in the woods. And this time, they went back a week later to retest them, and they were still up––though after a month, they came back down. But hey, once a week should do it. But it was a multiple-day trip. Who has time to hang out in forests all weekend, every weekend? How about just a little day trip? The title gives it all away. Boom! Same thing! The same big jump measured the day after the trip, compared to before, and with the same staying power. Natural killer cell activity still boosted a week later. This suggests that if people visit a suburban forest park once a week on a day trip, they may be able to maintain the increased anti-cancer immunity.

Okay, but I’m still not convinced. How can you attribute the benefit to the forest itself, when all you have is before and after data? To make the case that nature had anything to do with it, you’d need a control group that took the same kind of trip but went to somewhere else instead. And…here we go. It turns out visiting a forest, but not a city, increases human natural killer activity. Here’s the forest data, just like before, but nothing on a trip to go walking in a city. By the end of the forest trip, 80 percent of the subjects experienced a boost, compared to only 1 in 10 of the city walkers.

And, both trips were matched for physical activity, and alcohol, and sleep—other things that can affect immune function. And so, here we go. Confirmation of boosted immunity––but only on the forest trip, “indicating that forest bathing does indeed enhance [natural killer cell] activity.” Moreover, they found that the increased activity lasted up to 30 days after the trip. Check it out. Still up a week later, and even a bit up a month later. “This suggests that if people visit a forest once a month, they may be able to maintain increased [natural killer cell] activity.”

Okay, so, now that we know that it’s a real effect, the next question is, why? What is it about forests that gives us the boost? And (you can imagine Big Pharma thinking), can you make it into a pill? We’ll find out, next.

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.