Forest bathing and the immune system
Research is available showing that forest bathing and similar experiences have a positive effect on the immune system and have anti-inflammatory, anti-allergic and anti-asthmatic effects (Peterfalvi et al. 2021; Andersen et al 2021; Li 2006 et al). However, when assessing the results, it is important to bear in mind how these research studies have been designed Firstly, many of them are small and not randomised. In addition, they have a limited number of subjects and no control groups. This means that the studies can show positive results, but this does not mean that these can be considered significant, i.e., that chance can be excluded. In order to demonstrate scientifically significant effects, larger randomised studies with a control group are needed, for example, like the study that examined whether aerosols (airborne particles) from waterfalls had an effect on the immune system and stress (Grafetstätter et al. 2017). In this study, the study subjects were divided into three groups; one that stayed close to a waterfall, another group that stayed outdoors and a third group that stayed at home. The outcomes were then compared. Both outdoor groups improved their lung function and other physiological stress-test parameters. In addition, those who were close to the waterfall showed positive mucosal immune response and reduced stress.
Phytoncides – the scents of the forest
In simple terms, phytoncides are a kind of substance secreted by plants, for example as a defence against other plants and organisms. But can the substances also affect humans? Can they affect our immune system? There is research to suggest that they can.
In some studies, subjects were allowed to breathe in phytoncides, for example in the form of scents from conifers. The effects of the immune system were then investigated. In some cases, the number of NK cells (which are activated in response to tumours and infections, amongst other things) was assessed before and after inhalation of phytoncides. The results of the research are contradictory. Several smaller studies have yielded positive results, whilst another higher quality study cannot find any differences after inhalation of phytoncides (Andersen et al. 2021).
A great deal of research has also been devoted to testing the effect of phytonciders on different types of cells and tissues in pure laboratory or animal experiments. This type of study has seen positive effects on the immune system with an increased number of more active NK cells and the elimination of various cancer cells (Jo et al. 2021; Memon et al. 2021: Antonelli et al. 2020; Shin et al 2020; Cho et al 2017; Kang et al 2016). In an attempt to go beyond standardized forms XXX, 15 men stayed in hotels and breathed in essential cypress oil, which was disseminated with the aid of humidifiers (Li et al 2009). The results showed that the phytoncides a-pinene β-pinene were found in the hotel room air and that the number and activity of NK cells increased, and adrenaline and noradrenaline decreased in the study subjects, which is why the authors believe that phytoncides and stress hormones probably increased the activity of NK cells.
However, it is important to bear in mind that there is a big difference between laboratory and animal experiments, where phytoncides are administered artificially, and a situation where a person is in reality in forests or other natural environments. It is not clear whether the amount of phytoncides inhaled, for example, during forest bathing, has as positive an effect as the scents inhaled in the lab experiments. However, it is often assumed that the positive effect of phytoncides on the human immune system is self-evident. In addition, a number of scientific review articles have been written highlighting and consolidating the positive results from the lab experiments and from less reliable studies. It is not easy as a layman to be critical, but we should be anyway. We need larger randomised studies conducted on people ‘on-site’ in nature if we are to obtain a more reliable answer about the importance of the phytoncides to us.
Daylight and sunlight – do they protect against disease?
We are exposed to daylight and sunlight as soon as we poke out noses outside during the summer months. What is the significance of this for our health?
Vitamin D is the vital factor in this regard. When solar radiation hits our skin, vitamin D is formed which is vital for the regulation of calcium in the blood. Calcium is in turn needed to build up the skeleton, so too little sunlight can result in, amongst other things, degeneration of the skeleton and deformed joints. Vitamin D also reduces the risk of developing heart disease and cancer, has a positive impact on diabetes and alleviates colds (Kauffman 2009). Some studies have found that vitamin D reduces the risk of allergies while others have not identified any effect. Vitamin D is particularly important when the skeleton is growing, which is why so-called AD drops are given to small children.
For those who want to know more about the health effects of daylight, there is an excellent summary by Beute and de Kort (2014).
The impact of microbes
When we are in nature, we are exposed to more microbes (organisms so small that we cannot see them) than we are indoors. Is this a good or a bad thing?
There are scientists who believe that it is a problem that we live in an increasingly sterile environment and are not exposed to enough microbes from an early age (Pfefferie et al 2021). This causes allergies to increase. Research by the Swedish doctor and professor of bacteriology, Agnes Wold, has found that the best environment for growing children is a farm with animals. She and her colleagues have studied farm children and urban children and have shown that children who grow up on a farm with animals – and therefore are exposed to more microbes in everyday life – have significantly less allergies (Lundell et al. 2015). By being exposed at a very early age, and probably at moderately low doses, children develop resistance.
Other researchers believe that the lack of exposure to natural microbes has an even more negative effect to the immune system than exposure to air pollution (Pfefferie et al 2021; Weiland et al 1999). When children from East and West Germany were examined after the fall of the Berlin Wall, it was found that children from the West, where air pollution was lower, had more allergies than those from East, where air pollution was higher.
/Text: Ann Dolling
Andersen, Corazon SS, Stigsdotter UK (2021) Nature exposure and its effects on immune system functioning: a systematic review. International Journal of Environmental Research and Public Health 18:1416. https://doi.org/10.3390/ijerph18041416
Antonelli M, Donelli D, Barbieri G, Valussi M, Maggini V, Firenzuoli F (2020) Forest volatile organic compounds and their effects on human health: A state-of-the-art review. International Journal of Environmental Research and Public Health 17:6506. https://doi.org/10.3390/ijerph17186506
Beute F, de Kort YA (2013) Salutogenic effects of the environment: review of health protective effects of nature and daylight. Appl Psychol Health Well Being 6:67-95. https://doi.org/10.1111/aphw.12019
Cho KS, Lim Y, Lee K, Lee J, Lee JH, Lee I (2017) Terpenes from forests and human health. Toxicol Res 33:97-106. https://doi.org/10.5487/TR.2017.33.2.097
Grafetstätter C, Gaisberger M, Prossegger J, Ritter M, Kolarž P, Pichler C, Thalhamer J, Hartl A (2017) Does waterfall aerosol influence mucosal immunity and chronic stress? A randomized controlled clinical trial. J Physiol Anthropol. 13:36. https://doi.org/10.1186/s40101-016-0117-3
Jo H, Cha B, Kim H, Brito S, Kwak BM, Kim ST, Bin B-H, Lee M-G (2021) Alfa-Pinene enhances the anticancer activity of natural killer cells via ERK/AKT pathway. Int. J. Mol. Sci. 22:656. https://doi.org/10.3390/ijms22020656
Kang S, Lee JS, Lee HC, Petriello MC, Kim BY, Do JT, Lim DS, Lee HG, Han SG. (2016) Phytoncide extracted from pinecone decreases LPS-induced inflammatory responses in bovine mammary epithelial cells. J. Microbiol. Biotechnol. 26:579-587. https://doi.org/10.4014/jmb.1510.10070
Kauffman JM (2009) Benefits of vitamin d supplementation. Journal of American Physicians and Surgeons 14 38-45. https://www.grc.com/health/pdf/benefits_of_vitamin_d_supplementation.pdf
Li Q, Nakadai A, Matsushima H, Miyazaki Y, Krensky AM, Kawada T, Morimoto K (2006) Phytoncides (wood essential oils) induce human natural killer cell activity. Immunopharmacology and Immunotoxicology 28:319-333. https://doi.org/10.1080/08923970600809439
Li Q, Kobayashi M, Wakayama Y, et al. (2009) Effect of phytoncide from trees on human natural killer cell function. International Journal of Immunopathology and Pharmacology 951-959. https://doi.org/10.1177/039463200902200410
Lundell AC, Hesselmar B, Nordström I, Adlerberth I, Wold AE, Rudin A (2015) Higher B-cell activating factor levels at birth are positively associated with maternal dairy farm exposure and negatively related to allergy development. Journal of Allergy and Clinical Immunology 136:1074-1082. https://doi.org/10.1016/j.jaci.2015.03.022
Memon A, Kim BY, Kim S, Pyao Y, Lee YG, Kang SC, Lee WK (2021) Anti-inflammatory effect of phytoncide in an animal model of gastrointestinal inflammation. Molecules 26:1895. https://doi.org/10.3390/molecules26071895
Peterfalvi A, Meggyes M, Makszin L, Farkas N, Miko E, Miseta A, Szereday L (2021) Forest bathing always makes sense: Blood pressure-lowering and immune system-balancing effects in late spring and winter in Central Europe. Int. J. Environ. Res. Public Health 18:2067. https://doi.org/10.3390/ijerph18042067
Pfefferle PI, Keber CU, Cohen RM and Garn H (2021) The hygiene hypothesis – learning from but not living in the past. Front. Immunol. 12:635935. https://doi.org/10.3389/fimmu.2021.635935
Shin M, Liu QF, Choi B, Shin C, Lee B, Yuan C, Song YJ, Yun HS, Lee IS, Koo BS, Cho KS (2020) Neuroprotective effects of limonene (+) against Aβ42-induced neurotoxicity in a drosophila model of alzheimer's disease. Biol Pharm Bull.1:43:409-417. https://doi.org/10.1248/bpb.b19-00495
Weiland SK, von Mutius E, Hirsch T, Duhme H, Fritzsch C, Werner B, Hüsing A, Stender M, Renz H, Leupold W, Keil U (1999) Prevalence of respiratory and atopic disorders among children in the East and West of Germany five years after unification. Eur Respir J. 14:862-70. https://doi.org/10.1034/j.1399-3003.1999.14d23.x