"Why Does Forest Bathing Boost Natural Killer Cell Function?"
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 increases 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.
OK, 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 in 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 that 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 indoor air
(which is dominated by organisms that either live on us or trying 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 6-fold increase
in circulating pinene levels circulating throughout your system.
OK, 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.