Forest bathing, described with an increasingly clear scientific background since 1982, encompasses the concept of spending time in wooded areas and 'bathing' all the senses in the experience of being in the forest. Researchers describe a tremendous number of benefits, including lower stress hormones (such as cortisol), lower anxiety and better ability to withstand stress, and in general a significantly increased sense of well-being. These are to a large extent attributed to phytoncides, which include the class compounds called terpenes. You should read about the many benefits of Forest Bathing.
As written by Bum-Jin Park, et. al., in The Physiological Effects Of Shinrin-Yoku (Taking In The Forest Atmosphere Or Forest Bathing): Evidence From Field Experiments In 24 Forests Across Japan 1 , "From the perspective of physiological anthropology, human beings have lived in the natural environment for most of the 5 million years of their existence. Therefore, their physiological functions are most suited to natural settings. This is the reason why the natural environment can enhance relaxation”. He continues "While subjects viewed forest landscapes or walked around forest environments, their pulse rate, and cortisol concentration decreased.”
The Olfactory System
While the beneficial effects of forest bathing can be attributed to more than just the sense of smell, there is no denying that the olfactory system is largely involved. Alpha-pinene is a commonly occurring terpene found in a number of plants, including pine forests. "Collectively, these results indicate that a weak smell of alpha-pinene induces a relaxed physiological state 2 ..." Also "blood pressure started to decrease after the 20s from the commencement of inhalation [of limonene ]." 3
When further analyzing whether these effects occur largely from the olfactory system (the sense of smell), it is interesting to note that Yuko Tsunetsugu, goes on to state in Trends In Research Related To "shinrin-yoku" (Taking In The Forest Atmosphere Or Forest Bathing) In Japan 4 that:
"The results of various studies on other scents, such as lavender, lemon, valerian, and others, showed considerable agreement with respect to the fact that the effectiveness of odors in... lowering blood pressure, and suppressing renal sympathetic nerve activity disappeared in anosmic mice or rats, thus indicating the mediation of the olfactory system in the whole process."and "it is likely that phytoncides exert their effects via the olfactory processing pathway, and not via the blood-borne route.”
One conclusion that can be drawn from these various statements is that the tremendously beneficial physiological effects of being in a forest, more specifically forest bathing, including lowering of blood pressure, pulse rate, and the stress hormone cortisol, are significantly due to merely smelling the wide variety of terpenes and other phytoncides which are emitted by plants in the forest.
This makes sense as follows: Just as there is sodium chloride (salt) in animals' cells, most probably in relation to our early evolution from the oceans, our later evolution in the forests and savannas would lead to an environment in which the secondary metabolites of plants, such as terpenes and terpenoids, were very prevalent for millions of years.
Animals evolved, and developed their homeostatic mechanisms while breathing in these terpenes. It is well known that many terpenes are anxiolytic, that is they lower stress levels.
Now that we have largely migrated into cities, it may be the lack of terpenes in our daily breathing these days that leads to an increase in stress. It is the replacement of these naturally occurring terpenes through aromatherapy that brings us to the natural state of lowered stress.