28/06/2026
For more than 20 years, Japanese immunologist Qing Li studied what happens to the human body when you spend time in forests.
His research found that when people spend time among trees, the activity of their natural killer cells, a type of immune cell that helps the body fight cancer and virus infected cells, can increase by around 50%.
Part of the reason appears to be chemicals called phytoncides. Trees release these natural compounds into the air to protect themselves from insects, bacteria, and fungi. When you're walking through a forest, you're breathing those compounds in, and research suggests they can help boost immune function.
Li also found that people who walked in forests had significantly lower cortisol levels than people who walked in cities. Their blood pressure and heart rates were lower too, showing a clear reduction in stress.
According to his research, about half of the benefits come from the forest's natural air chemistry. The other half comes from the calming effect nature has on the nervous system.
Nature isn't a replacement for therapy or medicine when those are needed, but it can be a powerful complement. The human body has a built in response to being around trees, one that affects many of the same biological systems modern medicine is trying to support.
The average person now spends over 90% of their time indoors. As a result, many people live with chronically elevated stress levels while spending less time in the environments humans evolved in for thousands of generations.
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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.
