There are now known positive immune boosting effects gained from being in a forest. The trees themselves release a class of chemical constituents referred to as PHYTONCIDES.
This chemical class has been shown to increase natural killer NK cells in humans for days even weeks after exposure in the wild. NK cell activities include killing tumor cells and virus infected cells through the release of perforin and granzymes via granulated exocytosis pathways.
There are significant effects on the autonomic system such as lowering blood pressure suggesting the inhalation of certain forest phytoncides could be regulators of sympathetic nervous dysfunction.
Inhalation of hinoki essential oil stimulated a pleasant mood status. The essential oil of hinoki contains alpha and beta pinene that are found in forest air. These chemical constituents are also present in the distilled form of hinoki essential oil, as well as various forms of cadinene, limonene and alpha terpineol.
Chen, C.J., Kumar, K.S., Chen, Y.T., Tsao, N.W., Chien, S.C., Chang, S.T., Chu, F.H. and Wang, S.Y., 2015. Effect of Hinoki and Meniki essential oils on human autonomic nervous system activity and mood states. Natural product communications, 10(7), p.1934578X1501000742. Li, Q., 2010. Effect of forest bathing trips on human immune function. Environmental health and preventive medicine, 15(1), pp.9-17. Li, Q., Nakadai, A., Matsushima, H., Miyazaki, Y., Krensky, A.M., Kawada, T. and Morimoto, K., 2006. Phytoncides (wood essential oils) induce human natural killer cell activity. Immunopharmacology and immunotoxicology, 28(2), pp.319-333. Putra, R.R., Veridianti, D.D., Nathalia, E., Brilliant, D., Rosellinny, G., Suarez, C.G. and Sumarpo, A., 2018. Immunostimulant Effect from Phytoncide of Forest Bathing to Prevent the Development of Cancer. Advanced Science Letters, 24(9), pp.6653-6659.