Phytoncides are antimicrobial chemicals produced and emitted by trees, flowers, vegetables, and fruit to defend against predators such as bacteria, fungi, insects, and some animals (Lee et al. 2009). The phytoncides, monoterpenes and sesquiterpenes produced and secreted by plants protect the plants from being eaten by the predators (Lee et al. 2009). The aroma that is produced by the plants is key to the plants survival as an offensive odor, however, paradoxically the aroma has shown to enhance human immunity by the activation of natural killer (NK) cell production.
Natural killer cells are cytotoxic lymphocytes, a type of white blood cell whose role is to kill by way of cell apoptosis any invading pathogens such as a virus, bacteria, fungi and parasites. Natural killer cells are a vital component of the human innate immune system (Vivier, Tomasello, Barston, Walzer, & Ugolini, 2008). Phytoncides also increase intracellular anti-cancer proteins in certain lymphocytes in both males and females (Lee et al., 2009) leading to a pronounced submission of cancer cell anastasis (regrowth defined as “rising to life”).
Researchers Lee, et al. (2009) studied the effect of the Japanese experience of Shinrin-yoku, “forest bathing” and the effects of phytoncides on the immune system. Using an experimental design, Lee and his team found elevated NK cells and a higher degree of activation among the NK cells of the study population who spent time walking among the forest as compared to the control group of urban walkers (Lee et al. 2009). The increased NK activity maintained its action for more than a week post Shinrin-yoku experience in both males and female participants. Concurrent data also demonstrated an effect on the stress hormones adrenaline and noradrenaline production by way of urine sampling. The participants who experienced Shinrin-yoku had lower levels of adrenaline and noradrenaline in their urine post forest bathing than the urban walkers who had no interaction with a green-nature environment.
The outcome of the study suggested that people who routinely spend time in nature, walking through forests and other areas of natural vegetation may have increased levels of NK cells, which will create a stronger immunological response to pathogens (Lee et al. 2009).