Can a walk in the forest boost our immune system?

Walking in nature is usually associated with relaxation, physical activity, and a chance to temporarily step away from the hectic pace of daily life. In recent years, however, scientists have been paying increasing attention to whether spending time in the forest can also have a measurable effect on immune system function. Research in this field examines not only the impact of physical activity and fresh air but also the roles played by stress, the nervous system, and specific substances released by plants.

In Japan, the practice of spending time in the forest is known as *shinrin-yoku*, or “forest bathing.” The term does not imply actual bathing but rather a conscious immersion in the forest environment through walking, observing nature, listening to its sounds, and breathing in the forest air. Accumulated scientific data indicate that such an experience can trigger a range of physiological changes, although the claim that the forest directly “boosts immunity” should be approached with some caution.

The forest, stress, and the immune system

One of the primary mechanisms by which spending time in nature affects the body is through the reduction of psychological and physiological stress. When the body is under prolonged strain, the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system are activated. This is linked to changes in the secretion of cortisol and catecholamines; when levels of these remain chronically elevated, they can adversely affect various components of the immune response.

Numerous studies associate contact with the natural environment with lower stress markers, improved mood, and changes in physiological parameters such as blood pressure and heart rate. Thus, a walk in the forest can indirectly create more favorable conditions for the normal functioning of the immune system.

The act of moving itself should not be overlooked. Walking constitutes moderate physical activity, and regular physical activity is linked to a range of benefits for metabolic, cardiovascular, and immune health. Consequently, some of the benefits of a forest walk likely stem from a combination of movement, stress reduction, and time spent in a natural setting, rather than from any single factor.

Also of interest are so-called phytoncides—volatile organic compounds released by trees and other plants. Part of the forest’s characteristic aroma is due to such compounds, including various terpenes like alpha-pinene and limonene. It is hypothesized that inhaling some of these substances may play a role in the physiological reactions observed during time spent in the forest. However, current data are insufficient to claim that phytoncides alone provide significant protection against infections.

What happens to natural killer cells? One of the most interesting lines of research focuses on so-called natural killer (NK) cells. These are part of the innate immune system and play a crucial role in recognizing and destroying specific infected or altered cells.

Japanese researchers have found in relatively small-scale studies that spending time in a forest environment can lead to increased NK cell activity, as well as changes in certain proteins associated with their function. Interestingly, some of these studies show that the effect persists for some time after the forest visit has ended.

While these results are intriguing, they must be interpreted with caution. Many of the initial studies involved small numbers of participants, and the conditions and duration of time spent in nature varied. Furthermore, an increase in a specific immunological marker does not automatically mean a person will suffer less frequently from colds, the flu, or other infections.

The immune system is an incredibly complex network of cells, signaling molecules, and organs. Consequently, the concept of “boosting immunity” is often an oversimplification. It is more accurate to speak of an influence on specific components of immune function.

How much time should we spend in nature?

There is no established universal “forest dose” that guarantees improved immune function. Some studies examine short walks, while others look at stays lasting several hours or even several consecutive days in nature. This makes it difficult to determine the specific duration required for physiological effects to occur.

However, even a relatively short walk can be beneficial for overall health. In addition to increasing physical activity, it offers an opportunity for mental recovery, a break from prolonged sitting, and a temporary escape from urban noise and daily stressors.

The quality of the experience likely plays a role as well. Rushing through a park with one’s attention entirely focused on a phone is not necessarily equivalent to a leisurely stroll that involves actively taking in the surrounding sounds, scents, and sights. It is precisely this combination of movement, sensory engagement with nature, and psychological relaxation that lies at the heart of the “forest bathing” concept.

There is also another potential factor: exposure to diverse natural environments and microorganisms. Hypotheses suggest that contact with biodiverse natural settings may influence the human microbiome and the regulation of the immune system. While this area of ​​study is particularly interesting, it is still ongoing, and current evidence does not yet allow for definitive conclusions regarding the specific effects of an ordinary walk in the forest.

References:

1. Li, Qing. ‘Effect of Forest Bathing Trips on Human Immune Function’. Environmental Health and Preventive Medicine, vol. 15, no. 1, Jan. 2010, pp. 9–17. PubMed Central, https://doi.org/10.1007/s12199-008-0068-3

2. Wen, Ye, et al. ‘Medical Empirical Research on Forest Bathing (Shinrin-Yoku): A Systematic Review’. Environmental Health and Preventive Medicine, vol. 24, 2019, p. 70. PubMed Central, https://doi.org/10.1186/s12199-019-0822-8

Illustrative photo: pexels-mikhail-nilov-7683244