As you note, forest therapy originated in Japan, where researchers have been studying its physiological effects for many years. It appears that forest therapy does have measurable health benefits; for example, it can lower levels of salivary cortisol, the hormone that rises when we're under stress. One Japanese study showed that gazing at forest scenery for as little as 20 minutes reduced salivary cortisol levels by 13.4 percent. Forest therapy can also lower blood pressure and heart rate and trigger a dramatic increase in the activity of natural killer (NK) cells (produced by the immune system to ward off infection and fight cancer). Spending three days in the forest has been shown to increase NK activity by 50 percent, a beneficial effect that can last up to one month.
Most recently, researchers from UK's University of East Anglia analyzed 143 studies of forest therapy including data on some 290 million participants from 20 different countries. Not only was forest bathing associated with lower levels of cortisol, lower blood pressure and heart rate, it also lowered blood cholesterol and reduced rates of diabetes, coronary heart disease, stroke, asthma and death from heart disease. In addition, it was associated with decreased risk of preterm birth and lower all-cause mortality. Some studies suggested that forest therapy helped people sleep better and improved outcomes in those with cancer and neurological conditions. Finally, people exposed to forest therapy were found to be more likely to report that their overall health was good.
One mechanism that underlies these benefits may be inhalation of phytoncides, aromatic oils with antibacterial properties released by trees. Some studies suggest that these compounds increase the activity of natural killer cells.
I understand that in the U.S., guided forest therapy walks typically last two to four hours but cover very little distance - no more than a mile, according to the Association of Nature and Forest Therapy Guides and Programs. You may sit and focus on natural sights and sounds much more than you walk. The idea is to slow down and experience nature in a deeper way than you have before.
On this site and in my book, Spontaneous Happiness, I've written about the importance of connecting with nature. We are creatures of nature and cannot enjoy optimum physical or emotional well-being if we have too little contact with it. Biologist E.O. Wilson coined the term biophilia, meaning "love of life or living systems," to describe this innate human need. I believe it to be as real as our needs for love, food, sex, and community.
Your health will benefit from letting nature into your awareness as often as you can, any way you can. Watch the ever-changing shapes of clouds, admire trees, listen to the wind and look at the moon, at birds, at mountains. When you do, remember that you are part of nature, connected through it to something larger than yourself.
Andrew Weil, M.D.
Sources:
Qing Li et al, "Effect of phytoncides from trees on human natural killer cell function." International Journal of Immunopathological Pharmacology, October-December 2009.
Gregory M. Bratman, et al, "Nature experience reduces rumination and subgenual prefrontal cortex activation." Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1510459112
Caoimbe Twohig-Bennett and Andy Jones et al, "The health benefits of the great outdoors: A systematic review and meta-analysis of greenspace exposure and health outcomes." Environmental Research October 2018, doi.org/10.1016/j.envres.2018.06.030
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