Psychological and physiological effects of a green wall on occupants: A cross-over study in virtual reality

Social change due to COVID-19 has had negative impacts on the mental health of people who live indoors. One of the possible solutions to these problems is to introduce biophilic design. Among the elements of this design, green walls are often favored due to their spatial efficiency. This study investigated the effectiveness of green walls in relieving stress indoors. First, the stress level of the occupants of an indoor space that has green walls was measured. Then their psychological and physiological responses to the green walls were analyzed. To this end, using virtual reality, an experimental condition was set up in which a green wall was installed, and the stress level of the subjects was analyzed based on their state-trait anxiety (STAI), heart rate variability, electrodermal activity (EDA), and electroencephalogram (EEG). Twenty-seven subjects were recruited and the changes in their psychological and physiological responses when their environment changed from a non-green wall condition to a small to large green wall condition were measured. The results showed statistically significant differences in STAI, EDA, the relative alpha power in the parietal and occipital lobes, mental stress, and mental fatigue. Ultimately, the small green wall had a more significant relaxing effect. On the other hand, in the indoor space with a large green wall, the stress level of the residents increased. Thus, when installing a green wall in an indoor space, a small green wall is proposed.