Review ArticleA systematic review of evidence of additional health benefits from forest exposure
Introduction
The forest environment is increasingly recognized as the enhancement of salutary or health promoting factors, and exposure to forest environments can lead to disease prevention (e.g., enhanced cardiovascular and immune function) or improve well-being (e.g., happiness) (Park et al., 2007, Park et al., 2010, Tsunetsugu et al., 2007, Bowler et al., 2010; Lee et al., 2011, Barton et al., 2012; Lee et al., 2017, Song et al., 2016). Shinrin-Yoku or “forest bathing”, which appeared in Japan, is an important part of preventive health care and treatment in Japanese medicine, and has spread all over the world (Li et al., 2007, Li et al., 2008, Lee et al., 2012, Miyazaki et al., 2014). Previous studies investigating forest exposure have found numerous synergistic benefits to reduce blood pressure levels (Suda et al., 2001, Park et al., 2007, Li et al., 2011, Ideno et al., 2017), decrease sympathetic parameters, increase parasympathetic parameters (Tsunetsugu et al., 2007, Park et al., 2010), reduce levels of stress (Antonelli et al., 2019), improve self-esteem (Barton et al., 2012) and is thus an important restorative practice widely-used in rendering physical and psychological health benefits. In addition, Attention Restoration Theory (Kaplan and Kaplan, 1989), Stress Recovery Theory (Ulrich et al., 1991) and Biophilia Hypothesis (Kellert and Wilson, 1993) support the benefits of forests to human well-being. Nowadays, the healing power of forest environment has been used to promote public health and solve the problem of health inequality (Lanki et al., 2017). Therefore, forest exposure would play a vitally important role in preventive medicine in the future, especially for stress and stress-related diseases (Song et al., 2016).
Forest environment contains a large number of healing factors, which play an important role in preventing and treating health benefits through sensory input (e.g., visual, olfactory, auditory, or tactile) (Ulrich, 1983, Kaplan and Kaplan, 1989, Kaplan, 1995). Firstly, visual information from natural realm (e.g., green foliage and vegetation, blue sky and water) can generate a powerful response in human body (Akers et al., 2012). Previous studies have found that exposure to green and blue can make people feel less depressed, anxious and nervous and more calm (Kaya and Epps, 2004, Mehta and Zhu, 2009, Clarke and Costall, 2008, Hoshi et al., 2011). Briki and Majed (2019) revealed that compared with other colors, exposure to green would lower heart rate when walking, because green can promote energy conservation. Secondly, a tree-derived phytoncide had positive affected on physiological and psychological parameters. It has been reported that cedrol (cedar wood oil) (Dayawansa et al., 2003), Taiwan Hinoki wood oil (Miyazaki and Motohashi, 1996), and Hinoki cypress leaf oil (Ikei et al., 2015) inhalation had health benefits, namely by reducing blood pressure. In addition, Tsunetsugu et al. (2010) reported that phytoncides produced by forest trees and other plants could significantly reduce stress, anxiety and depression. Thirdly, the sounds of nature containing numerous rhythms are sufficient to elicit therapeutic effects (Pijanowski et al., 2011, Benfield et al., 2018, Zhao et al., 2018). Study has showed that the sounds of running water and the rustling of wind could reduce systolic blood pressure and improve brain activity (Yamada, 2003). In addition, the touch and feel of natural substances such as tree-bark and leaves are beneficial to the mind in terms of relaxation. For example, Koga and Iwasaki (2013) reported feeling a measurable sense of “calm” when touching natural plant material. Several studies on tactile stimulation with different kinds of wood have also been conducted to demonstrate the effects of physiological relaxation (Morikawa et al., 1998, Sakuragawa et al., 2008, Ikei et al., 2018, Ikei et al., 2017a, Ikei et al., 2017b). Visual cues, sounds, smells and tactile experiences are considered as a connection with the environment, and they are related to the positive feelings of one’s own environment (Franco et al., 2017). Besides, feelings of connectedness can enhance the psychological benefits of human (Nisbet et al., 2011, Lawton et al., 2017). Research findings have also suggested that people who have a feeling of connectedness to nature not only report pro-environmental behaviour (Barton and Pretty, 2010), but also have happiness and positive effects (Gosling and Williams, 2010, Mayer and Frantz, 2004, Mayer et al., 2009).
Evidence shows that exposure to forests (i.e., time spent in, and engagement with, forest-rich environments) can lead to positive mental and physical health outcomes (Tsunetsugu et al., 2007, Tsunetsugu et al., 2010, Lee et al., 2009, Lee et al., 2011, Lee et al., 2014, Park et al., 2008, Park et al., 2009, Park et al., 2010). A linear hypothesis is the basis of most literature, in which the increase of forest exposure is considered to lead to the improvement of health and wellbeing. However, this relationship may be determined by many factors, such as duration in forest, the method of engagement with forest, and the type of forest setting. For example, in a study of 1538 Brisbane respondents, it was reported that visits to outdoor green spaces of 30 min or more during the course of a week could reduce the population prevalence of these illnesses by up to 7% and 9% respectively (Shanahan et al., 2016). According to a meta-analysis report by Barton and Pretty (2010), dose responses for duration showed that the benefits of short-term participation in green exercise were greater and then gradually decreased, but there were still positive returns. Experimental studies have also shown that viewing or walking in forest has positive physiological and psychological effects, however the responses were quantitatively different (Song et al., 2016, Kobayashi et al., 2021).
Although many studies have shown that forest exposure has positive impact on physiological and psychological indicators, little is known about how long you stayed in the forest, or what method should be the best. The purpose of this research was thus to assess the best regime of dose(s) of acute exposure to forest required and the methods of interaction with forest to improve mental and physical health. In this paper, we conducted a meta-analysis to reveal the general responses of physiological and psychological effects to forest experiments. This study is aimed to: (1) identify the general responses of physiological and psychological effects following forest exposure; (2) explore the responses of physiological and psychological effects to different types of interaction with forest environment (viewing, walking, and other), for different time spent in forest settings (<0.5 h, 0.5–2 h, and > 2 h).
Section snippets
Literature survey
An extensive literature survey was conducted through Web of Science, MEDLINE, and PubMed. The keywords used for the literature search were related to: “Shinrin-Yoku” or “forest bathing” or “forest therapy” or “forest walking” or “forest viewing” or “natural therapy”. All the literature were retrieved by three authors for more careful examination. We used the citation traceability method and Google Scholar to search for the supplemented literature. The results ranged from the beginning to March
Description of studies
40 studies were included in the meta-analyses of this review. Mean age-range of the included participants was 21.0–66.4 years of age. Thirty-one studies reported the male/female ratio. No attempt was made to elucidate age- or gender related correlations. The countries with the highest number of reports were Japan (n = 27), China (n = 5), Korea (n = 5). Forests were mostly composed of broadleaf deciduous and/or evergreen coniferous trees. Details of individual papers is shown in Supplementary
Physiological and psychological responses to forest exposure
Recent reviews including one review on blood pressure-lowering effect of forest bathing (Ideno et al., 2017), one on levels of cortisol (Antonelli et al., 2019), three on health and well-being benefits (Hansen et al., 2017, Oh et al., 2017, Park et al., 2010), have outlined the evidence that forest exposure can improve health and mood. Some limitations of those reviews are that most previous studies did not have enough sample size, or they were narrative rather than systematic. A recent
Conclusion
The quantitative synthesis indicated that forest exposure elicited both physiological and psychological relaxation. Our meta-analysis results suggested that SBP, lnHF, ln(LF/HF), tension-anxiety would be significantly affected by duration and HR, lnHF would be significantly affected by methods of intervention, which help us understand how people’s physical and psychological parameters will change under the different duration and methods of intervention of forest exposure. Of the reviewed
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Shasha Lu conceived and designed the paper, contributed analysis tools, authored or reviewed drafts of the paper, approved the final draft.