How do aromatic oils from trees boost immunity?

In Japanese, forest bathing – called Shinrin-yoku – refers to walking in a forest or wood, ‘taking in’ the atmosphere using all the senses. People are encouraged to savour aromas, textures, sounds, colours, and shapes of the environment.

Previous research has found the forest environment has many health benefits, including; lowering the stress hormone cortisol, reducing blood pressure and easing those thorny moods. But, something else intriguing happens to the immune system.

When Professor Qing Li, at Nippon Medical School in Tokyo, analysed participants blood results after a forest walk, he found increased numbers of natural killer cells compared to a city walk. What’s more, the percentage of natural killer cells remained high up to a month after the walk.

If you’re wondering, natural killer cells form part of the immune system and are released when body cells are under attack from nasty invaders like viruses. They also have a role in slowing down tumour formation, therefore having anti-cancer properties.

The relaxing forest atmosphere lowered stress hormones helping to boost the immune system, but something else was going on. Li speculated breathing in aromatic oils from the tress could have also influenced the immune system, similar to how aromatherapy works.

Li designed a small study to test his theory to see whether exposure to essential oils outside the forest environment produced the same results. Twelve males aged 37-60 spent three nights in a hotel room where essential oils from the cypress tree were diffused into the room.

After three nights, Li found a higher percentage of natural killer cells and a lower percentage of T-cells, adrenaline and noradrenaline were found in the blood and urine after exposure to the essential oils.

High levels of T-cells in the blood have been linked to mental stress and unhealthy lifestyles. Adrenaline and noradrenaline form part of the sympathetic nervous system and prepare the body for the fight-flight stress response.

The study results indicate that essential oils from trees and lowering stress hormones boost the immune system without actually going to a forest. Helpful for people unable to access woodland or a forest and diffuse essential oils at home.

The study was published in the International Journal of Immunopathology Pharmacology.


Li, Q. Effect of forest bathing trips on human immune function. Environ Health Prev Med 15, 9–17 (2010).

Li Q, Kobayashi M, Wakayama Y, Inagaki H, Katsumata M, Hirata Y, Hirata K, Shimizu T, Kawada T, Park BJ, Ohira T, Kagawa T, Miyazaki Y. Effect of phytoncide from trees on human natural killer cell function. Int J Immunopathol Pharmacol. 2009 Oct-Dec;22(4):951-9. DOI: 10.1177/039463200902200410. PMID: 20074458.

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