‘Forest bathing’ or shinrin yoku—spending time in a forest or other green space to reap the health benefits—has become an increasingly popular activity in recent years, especially in some countries, like Japan, which take it pretty seriously. And with good reason—the practice isn’t some new-age form of woo-woo healing; it’s an increasingly well-evidenced health habit that’s garnered a lot of popular and scientific interest in the last few years.
A new meta-analysis in the journal Environmental Research finds that people who spend more time in green spaces have significantly reduced risks for a number of chronic illnesses. There are probably several mechanisms behind the connection, but one of the more fascinating ones likely has something to do with phytochemicals that trees emit, and humans breathe in.
The researchers, from the University of East Anglia, looked back at data from a slew of earlier studies—103 observational studies and 40 interventional studies. In the latter, different types of interventions were carried out, like assigning people to engage in shinrin yoku (forest bathing) or the equivalent in an urban space. Others looked at post-operative recovery time of people who could see greenery out their hospital window compared to those who could only see a wall; another compared spending time near greenery vs. water.
All told, the research tracked a whopping 290 million participants, from 20 different countries. Participants were 50% of which were in Europe, but many studies (24) were from Japan, where forest bathing is popular. The team correlated the amount of time people spent in green spaces with 100 health outcomes over time, and found a number of relationships.
Spending more time in green spaces was linked to reduced levels of the stress hormone cortisol, lower heart rate, reduced risk of coronary heart disease, lower blood pressure, lower cholesterol, reduced risk of type II diabetes, reduced all-cause mortality and death from heart disease. For women who were pregnant, more time spent in greenery was linked to reduced risk of “small size for gestational age” in the baby as well as preterm birth. Among all the participants, on average, there was an increased likelihood of self-reporting one’s health as “good.”
Some studies also suggested a possible link between green spaces and cancer outcomes, neurological outcomes, sleep duration, and certain biomarkers. The authors do note that some of the studies included weren’t of the highest quality, and further research is certainly warranted.
A number of plausible mechanisms exist, including the obvious—green spaces promote physical activity, social interaction, exposure to sunlight, and reduced pollution, which all boost health in various ways. The less obvious explanations include the “old friends” hypothesis, which suggests that the increased exposure to microorganisms can bolster the immune system, which in turn leads to reduced risk of chronic disease and early death.
But there’s also another possibility—chemicals emitted by the trees themselves, which may affect our immune systems in various ways. “Much of the literature on forest bathing suggests that phytoncides (volatile organic compounds with antibacterial properties) released by trees may explain the salutogenic properties of shinrin yoku [forest bathing],” the authors write. Some studies have also shown that the compounds may increase the activity of the immune system’s natural killer cells themselves.
According to the paper, people have been aware of the connection between greenery and health since the 19th century, which may be part of why city parks and green spaces were developed so thoroughly in that period. The authors suggest that “green prescriptions,” especially for those who lack everyday access to green spaces, may be a valuable addition to medicine, as they could have measurable health benefits over the years. And, as those who spend regular time in nature know, the mental health benefits of forest bathing may be just as strong.