Phytoncide: the word is often linked to Shinrin yoku or forest bath and its positive effects on humans, especially on the immune system. What are phytoncides? How do they work? What does the research say about them?
What are phytoncides?
The term "phytoncides" was first used by the Russian biologist Boris Petrovich Tokin in the 1920s. The word comes from two Greek words: "phyto" meaning "plants" and "cide" meaning "to kill". Then phytoncides are molecules that plants produce that can kill. They can be seen as the plants' protection system against attacks from fungi, mold, bacteria, insects and even mammals. However, the word can be confusing because it is used for molecules that are excreted in the air or stay in plants, and sometimes also for molecules found in essential oils. When we talk about phytoncides in forest baths, we often refer to the molecules that end up in the forest air and that we can breathe in. These belong to a larger group of substances called VOC (Volatile Organic Compound). There are hundreds of different VOCs that involve many metabolic processes and are produced by flowers, roots and vegetative parts of plants. In an attempt to summarize this in a simple way, one can say this: not all VOCs are phytoncides and not all phytoncides are VOCs.
The VOCs at high levels in the forest atmosphere belong to the chemical family of isoprenoids which include terpenes such as limonene and pinene. This applies to both coniferous and deciduous forests.
What functions do they have for plants?
Some VOCs are produced by plants all the time, others are produced in response to a stimulus or stress. For example, when a herbivorous animal begins to graze on the tree's leaves, then a substance can be produced that becomes toxic to the animal. The tree makes itself inedible. More, VOC will be excreted in the air and be recognized by other trees of the same species as a signal to begin producing the substance and protect themselves preventively against herbivores. This occurs, for example, between a type of acacia (Senegalia caffra) and the antelope kudu. The antelope only has a short time to eat the acacia leaves before they become toxic. This ensures a balance to avoid the antelope eating all the leaves at once. Another example is plants that secrete VOCs during infestation by herbivorous mites. In this case, the VOC will attract predators of the mites to the plants that have produced them. This "strategy" is used by Lima beans against a small voracious mite, Tetranychus urticae. You could say that the plants "call" for help against the attack.
VOC / phytoncides thus functions as a means of protection and communication between individuals of the same species, but also between species.
And what about us, humans?
When we are in the forest, we breathe in the air of the forest. Inhalation is the main route of intake of VOC / phytoncides in our body and blood. Research studies have shown that the effects of phytoncides on humans during forest baths are linked to our immune system and a type of white cell: NK cells (Natural Killer).
NK cells are lymphocytes that belong to our non-specific immune system and that have the ability to "kill" cells that are damaged such as cells infected by a virus or tumor cells. NK cells are very important, they are the first line of our defense against pathogens (viruses, bacteria) and there are many studies that focus on their potential functions in the fight against cancer.
Dr. Qing Li was one of the first to be interested in the effects of forest baths on the immune system. He led a series of studies, both in laboratory with cultured cells and in the forest on humans, to measure the levels and activity of NK cells. The results showed that NK cells "respond" to phytoncides and that their number and activity increase when they are exposed to substances from hinoki cypress oil that contains alpha- and beta-pinene. These increases could also be measured in people who stayed in a room where these substances were diffused into the air and after a stay in the forest. In this case, the effects continued even up to one week after returning in a urban environment. It is reasonable to suggest that these effects may be due in part to exposure to phytoncides in the forest atmosphere.
Other studies have shown similar results and recently an article was published showing that alpha-pinene improves the anti-cancer activity of NK cells and describes the metabolic pathway that appears to be involved in this activation. The author concludes the article by discussing phytoncides as a new approach in immunotherapy and a complement in cancer treatment.
Does that mean that forest bathing will improve my immune system?
As you walk in the forest, you will inhale molecules that all plants secrete into the air. Some of them, we now know, have positive effects on you and can "boost" your immune system. Unfortunately, this does not mean that you will never get sick even if you are often in the woods, but if you ask me if it can help: YES absolutely! I do believe in it.
Do you need a forest bath to get these effects? No, it's not necessary. What you need is to breathe in the forest. However, forest baths can help you calm down, breathe more deeply and then perhaps promote these effects. Here I admit that I am speculating. No such studies exist as far as I know.
When it comes to VOC / phytoncides, there is still a lot research to do. It is known that VOCs in the air are different depending on tree species found in the forest and that the levels vary during the day, with season, weather, altitude above sea level, etc. The amount of parameters makes it very difficult to be able to compare different studies. But research is ongoing and I am sure it will show more and more how important trees, plants and the forest as a whole are for our health. Many of us feel this intuitively, while others seem to have forgotten it.
Phytoncides: a message from the forest
There is one more thing that I would like to share and that I find very exciting with VOC / phytoncides, NK cells and the immune system. I wrote at the beginning of the article that these molecules can be a type of communication tool between species, do you remember acacia trees and antelopes? Think of it this way: VOC / phytoncides are actually molecules that plants produce and that we, humans, can feel and react to. In other words, plants send a signal, a chemical message, that our body has the ability to capture, understand and respond to. Is this not a definition of communication? I think so.
So no matter what you think or believe: plants "talk" to you and you "understand" them. Your body does it. Speaking of connection with nature! You can understand trees! And it's not science fiction, it's nothing you need to learn, which is cultural. It is natural in you. And this is just the beginning…
The question now is, can you communicate the other way with trees and the forest?
Maybe a forest bath can help you here…
￼￼￼￼Antonelli M., Donelli D., Barbieri G., Valussi M., Maggini V. and Firenzuoli F. Forest Volatile Organic Compounds and Their Effects on Human Health: A State-of-the-Art Review. Int. J. Environ. Res. Public Health. 2020. 17: 6506.
van den Boom CE, van Beek TA, Dicke M. Attraction of Phytoseiulus persimilis (Acari: Phytoseiidae) towards volatiles from various Tetranychus urticae-infested plant species. Bull Entomol Res. 2002. 92(6): 539.
Li Q, Nakadai A, Matsushima H, Miyazaki Y, Krensky AM, Kawada T, Morimoto K. Phytoncides (wood essential oils) induce human natural killer cell activity. Immunopharmacol Immunotoxicol. 2006. 28(2): 319.
Li Q. Effect of forest bathing trips on human immune function. Environ Health Prev Med. 2010. 15(1): 9.
Jo H., Cha B., Kim H., Brito S., Mun Kwak B., Tae Kim S., Bin BH and Lee MG. α-Pinene Enhances the Anticancer Activity of Natural Killer Cells via ERK/AKT Pathway. Int. J. Mol. Sci. 2021. 22: 656.