Forest bathing is based on a Japanese practice called shinrin-yoku developed in 1982 by the Japanese Ministry of Agriculture, Forestry and Fisheries. Shinrin-yoku which translates to forest well or forest bathing in English is defined as making contact with and taking in the atmosphere of the forest. This is something you can do on your own, but the practice of forest therapy uses trained guides who set the pace and help you mindfully experience the forest using all of your senses.
A few references:
     Environmental health and preventive medicine, 15(1), 18–26.
a separate study on trees' phytoncide and natural killer cell function:
https://pubmed.ncbi.nlm.nih.gov/20074458/
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