A Japanese immunologist named Qing Li spent 20 years proving the chemicals trees release into the air walk into your bloodstream, hunt down your stress hormones, and arm your immune system in ways no therapist or pharmaceutical can match.

Li found the activity of a specific type of immune cell called the natural killer cell, the one your body uses to hunt down cancer cells and virus-infected cells before they can spread, jumped around 50 percent during a forest trip.

The mechanism turned out to be a class of airborne molecules called phytoncides. Trees produce these compounds to defend themselves against insects, bacteria, and fungi. When you walk through a forest, you are inhaling those molecules into your lungs and absorbing them through your skin, and once inside your body they appear to directly stimulate immune cell production and activity.

In addition, he found the cortisol levels of the people who walked in the forest were lower than the cortisol levels of the people who walked in the city by a significant margin. Their heart rates and blood pressure were also lower.

According to Li’s data, roughly 50 percent of the health benefit of a forest walk comes from the air chemistry. The other half comes from what the forest is doing to your nervous system.

While nature is not a substitute for therapy or medication when needed, it is a complementary intervention. Li's research has proven that the human body has a physiological response to being among trees that operates on the same biological systems modern medicine is trying to reach with drugs and clinical protocols, and that response is fast, measurable, and free.

The average person now spends more than 90 percent of their life indoors. This makes their cortisol stay elevated and their natural killer cells stay sluggish. Their parasympathetic nervous system, tuned by millions of years of life under a canopy of trees, is having to run inside a box made of drywall and screens.

Your body has not forgotten what it is supposed to do in a forest. It is waiting for you to walk into one.

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Qing Li is a clinical professor at Nippon Medical School in Tokyo and the president of the Japanese Society of Forest Medicine. The Japanese government has been funding his research since 2004, and the body of work he has produced is the reason forest bathing is now an officially prescribed clinical therapy in Japan and Korea.

Qing Li's research is available at