Doctors are prescribing trees
In four countries a walk among trees is now something a physician can write down. The mechanism is a chemical the trees make for themselves.
Doctors in the United Kingdom, Japan, Canada and South Korea can prescribe time in nature. Not as a metaphor. As a written recommendation, in some cases with a park pass attached.
The Japanese version has a name, shinrin-yoku, usually translated as forest bathing, and it has been studied since the 1980s. The others are newer. The interesting question is what the trees are doing that a treadmill is not.
Phytoncides
Trees release volatile compounds called phytoncides. They make them for their own reasons: to defend against insects and rot. When you walk among trees you breathe them in.
Breathing them appears to lower the body’s main stress hormones.
The practical dose in the studies is modest: a walk of an hour or two, or in the daily version, ten minutes near trees.
The Japanese studies that started this line of work measured the same thing from the other direction: lower cortisol, lower blood pressure and lower pulse after time in forest compared with time in a city, in the same people.
The effect is modest, repeatable and free.
Why this is a sustainability story
Because the prescription only works if the trees are there.
A city that loses canopy loses an intervention that four national health systems have decided is worth writing down. The street that gets paved for parking is not just hotter. It is one fewer place a person can go to lower a hormone.
That reframes an urban tree from decoration to infrastructure. We already fund infrastructure that lowers stress hormones. It is called a hospital, and it is expensive. Trees are the cheap version, and they run without staff.
What the research does not say
It does not say a walk in the park treats depression, or replaces medication, or does anything at all for a condition you have not had diagnosed. The studies are mostly small, mostly short, and measure stress markers rather than outcomes years later.
It also does not require a forest. Ten minutes can be a street with trees on it. The point is regular, not remote.
The prescription, in full
Go outside where there are trees. Stay ten minutes. Do not bring the inbox. Repeat tomorrow.
That is the whole thing, and a doctor in Tokyo, Toronto, Seoul or Sheffield can now sign it.