Effect of Wood Scent Inhalation on Brain Activity: Focusing on A Multi-Channel Functional Near-Infrared Spectroscopy (fNIRS)

Background and objective: While numerous studies have demonstrated the positive effects of the scent of forests on humans, there has been a lack of research that investigates its effects on brain activity. This study was conducted to investigate the impact of inhaling fir essential oil on brain activity.Methods: Twenty-six university students in their twenties (mean age: 21.5 ± 1.4 years) participated in this study. Participants sat in a chair with their eyes closed and relaxed for one minute. The scent of Abies holophylla (needle fir; hereinafter referred to as "fir") essential oil was dispersed into the air through a digital diffuser for approximately one minute, and the participants then inhaled for two minutes. A control experiment was conducted using the same method without any essential oil (room air). Functional Near-Infrared Spectroscopy (fNIRS) was used as an indicator to measure brain activity, and the oxy-hemoglobin (HbO) concentrations in the fifteen channels located in the prefrontal cortex were measured. The change in the HbO concentration was analyzed: mean concentration during one minute of scent inhalation-mean concentration during two minutes of scents inhalation.Results: By analyzing each of the 15 channels, the decrease in HbO concentration was found to be significantly higher for fir essential oil inhalation compared to room air inhalation. Furthermore, an analysis of the mean HbO concentration across all channels (CH 1-15), right hemisphere channels (CH 1-7), and left hemisphere channels (CH 9-15) showed a significantly higher decrease with fir essential oil inhalation than with room air inhalation. This effect was observed in all channels and left hemisphere channels.Conclusion: It was found that inhaling fir essential oil aids in relaxation brain activity.