The research shows it: wandering mindfully in forests positively effects the physiology of the wanderer. But why is this? What's in the forest that's absent in the city?
Is there something floating around in forest air? Maybe some kind of molecule?
They are volatile, odiferous compounds released by plants and trees, protecting them from microbial and fungal infections.
One study of a pine forest concluded that for every 2.4 acres of forest, eleven pounds of phytoncides were released in a 24 hour period. LINK
Forest air is thickly imbued with these compounds. When I walk in a forest I am immmersed in a sea of phytoncides - I draw them into my body.
What happens to me when I breathe them in?
My immune system get's a jumpstart. I am able to fight off infection and disease more effectively.
Japanese researchers isolated NK cells (white blood cells specializing in eliminating cancer and virus cells) and immunity proteins (perforin, GRN and GRA/B). Then they introduced the phytoncides Limonene and Pinene (emitted by conifers). They recorded significant increases in NK cell activity and amounts of intercellular perforin, GRN and GRA/B. LINK
In another study, Cedrol, a phytoncide emitted by cedar trees was administered to subjects. After inhaling Cedrol, their heart rates, blood pressure and respiratory rates all decreased. LINK
Another study found phytoncide inhalation significantly reduced concentrations of urinary adrenaline and noradrenaline, indicating that phytoncides decrease activation of the sympathetic (fight or flight) nervous system. LINK
The forest air is imbued with phytoncides - when I walk under trees I draw them in with every breath. They surround, infuse, relax, and calm me.
So it's not just that forests are nice to look at, listen to, smell, and feel.