Phytoncides and immunity from forest to facility: A systematic review and meta-analysis

Background

Forest bathing is a traditional Japanese custom that involves immersing oneself in forest settings for extended periods. It is recognized for its positive impacts on psychological and physiological well-being. Phytoncides play a key role in the benefits of forest bathing and have begun to be investigated for their immunotherapeutic potential. It is important to investigate their immunomodulating effects within both forest and clinical settings.

Purpose

We conducted a systematic review and meta-analysis to investigate the effects of phytoncides on immune functioning.

Materials and methods

A PICO-SD framework was used to screen studies from databases, including PubMed, Web of Science, Scopus, Cochrane, and Trent University’s Omni portal. Selection criteria involved studies of adults aged 18+ exposed to phytoncides, comparing those exposed with control groups. The outcomes of eligible studies focused on immunological measures, excluding survey and qualitative research. Risk of bias was assessed using the ROBINS-I tool for non-randomized controlled trials and the ROB-2 for randomized controlled trials. Six studies (79 participants) were included in this meta-analysis. The meta-analysis included standardized mean difference effect sizes (Cohen’s d) with a random effects model using the Hartung-Knapp adjustment and 95 % confidence intervals for continuous data.

Results

This review found favourable immunological outcomes of phytoncide treatment, including increases in NK cells, T-cells, and cytotoxic effector molecules. Meta-analysis indicated a significant increase in NK cell activation (Effect Size: 2.50; 95 % CI [1.94–3.05]; p < 0.05; I² = 50.47 %).

Conclusions

The evolving landscape of phytoncide research calls for randomized controlled trials using specific phytoncides to establish the efficacy and safety of phytoncides in diverse healthcare settings.