Given the 7 million years of human history, humans, who have spent most of their time in forests, have evolved to adapt to and feel comfortable in forest environments. (Brunet et al., 2002; Park et al., 2010; Miyazaki, 2018; Song et al., 2020; Rajoo et al., 2020; Kim et al., 2023). Park et al. (2010) reported that compared to urban environments, forest environments suppress the secretion of the stress hormone cortisol, lower blood pressure, and promote parasympathetic nerve activity. Song et al. (2020) also suggested that appreciation of scenery in forest environments increased positive mood (e.g., vitality) and decreased negative mood (e.g., depression, fatigue, and confusion) compared to the appreciation of scenery in urban areas.

Humans perceive forest elements through their five senses (Tsunetsugu et al., 2011). Through their eyes and ears, they see the beautiful scenery of various plant species and hear the sounds of forests, birds and insects. Through their senses of touch and smell, they feel the soil and trees, and smell the scent emanating from trees. The sensory data collected by the sense organs, including sight (vision), taste (gustation), and hearing (audition), is transmitted to the cerebrum through the thalamus of the interbrain (Jones, 2011). Meanwhile, the sensory data gathered by the sense of smell (olfaction) is transmitted directly to the orbitofrontal cortex of the cerebrum without any intermediate processes, and is connected to the hippocampus and the amygdala, which are associated with memory and emotion (Zhou et al., 2021). Because of these characteristics, olfaction has a more direct impact on emotions, memories, and behavior than the other senses (Carmichael et al., 1994; Poo et al., 2022) and is considered a highly utilized sense.

“Essential oil,” an olfactory resource obtained from forests, is financially inexpensive and easy to use spatiotemporally, making it easy to use in daily life. Various studies have been conducted to determine the effects of inhaling tree essential oils on the human body (Miyazaki et al. 1992; Li et al., 2008; Yu et al., 2022; Kim and Song, 2022; Kim et al., 2023). Miyazaki et al. (1992) reported that the inhalation of cypress (Chamaecyparis obtusa; Japanese cypress or hinoki cypress; hereinafter referred to as “cypress”) essential oil reduced blood pressure and improved mood, and Li et al. (2008) found that inhalation of cypress essential oil improved immune function by increasing human natural killer cell activity. Yu et al. (2022) reported that the inhalation of essential oil distilled from wood chips of Taiwania cryptomerioides reduced the heart rate and alleviated negative mood, and Kim and Song (2022) presented that inhalation of Abies holophylla essential oil helps relieve stress by reducing sympathetic nerve activity. Various findings have shown that smelling the scent of forests helps with physiological and psychological stability, but when it comes to measuring physiological responses, research has mainly focused on investigating the activity of the autonomic nervous system.

In a competitive and rapidly changing society, modern people spend more than a third of their day working (OECD, 2022). As studies have reported that continuous and excessive work accelerates the load of brain activity, causing mental exhaustion, headaches, anxiety, depression, decreased concentration, and sleep disorders (Lockley et al., 2004; Afonso et al., 2017; Ogawa et al., 2018), the importance of resting brain activity is increasing.

Recently, with the development of scientific technology, devices that can measure brain activity have been developed and improved. Among them, functional near-infrared spectroscopy (fNIRS) is widely used in relevant application fields (Gunasekara et al., 2022). fNIRS is a non-invasive method for measuring oxidized hemoglobin in the blood using near-infrared light, which has high penetration into biological tissues (Holtzer et al., 2011; Ferrari and Quaresima, 2012). Compared to MRI and PET, which are similar approaches to measure blood flow response, fNIRS has advantages that include high temporal resolution, great accessibility for use in different settings due to low constitutivity of research subjects, and a compact device with good portability (Seok, 2011). For this reason, it has recently been used in research in the field of forest therapy. Previous studies have suggested that when the elements of forests are perceived through the five senses, including through hearing forest sounds (Jo et al., 2019; Song et al., 2023), appreciating forest scenery (Joung et al., 2015; Lee, 2017; Song et al., 2020; Kang et al., 2024), and touching wood (Ikei et al. 2017; Ikei et al. 2018; Ikei et al. 2020), the concentration of oxidized hemoglobin in the prefrontal cortex decreases significantly, thereby calming brain activity. It was also found to decrease negative mood and increase positive mood, improving overall mood. Olfactory stimulation with essential oils is expected to be very useful in daily life because it is financially inexpensive and easy to use spatiotemporally, but there are very few studies that have measured the brain activity that occurs when smelling forest scents. Therefore, this study aimed to analyze and determine the effects of wood scent inhalation on brain activity in a multifaceted manner using multi-channel fNIRS.

The results of analysis of the HbO concentration in each channel are shown in Table 2. In channels 7 and 13, the HbO concentration decreased when fir essential oil was inhaled, and increased when room air was inhaled, indicating a significant difference between stimuli (p < .05). In channels 6 and 12, HbO concentration increased for both fir essential oil and room air inhalation, but the increase in HbO concentration was less for fir essential oil inhalation than for room air inhalation (p < .05).

The results of an analysis of the mean HbO concentration of all channels (CH 1-15) are shown in Fig. 4. The change in HbO concentration for each section is shown on the left, and the mean HbO concentration during inhalation is shown on the right. The decrease in mean HbO concentration was significantly greater with fir essential oil inhalation than with room air inhalation (Fig. 4; p < .05).

An analysis of the mean HbO concentration for the left hemisphere channels (CH 9-15) showed that fir essential oil inhalation resulted in a significantly greater decrease in HbO concentration than room air inhalation (Fig. 5; p < .05). However, no significant differences were found when analyzing the mean concentration for the right hemisphere channels (CH 1-7).

The results of this study were consistent with those of previous studies, in that the HbO concentration in the prefrontal cortex decreased when natural substances such as cypress, rose and orange essential oils were inhaled (Igarashi et al., 2014; Ikei et al., 2015a; Ikei et al., 2015b). Ikei et al. (2015a) reported that the inhalation of cypress essential oil significantly decreased the HbO concentration in the prefrontal cortex, while Igarashi et al. (2014) presented that inhalation of the scent of rose and orange essential oils decreased the HbO concentration in the prefrontal cortex.

In a competitive and rapidly changing society, the demand to visit forests is increasing, as the relaxing effects of forest environments are being discovered (Park et al., 2008; Tyrväinen et al., 2014; Lanki et al., 2017; DeBloom et al., 2017). However, in such a society, where modern people spend more than a third of their day working (OECD Statistics, 2022), it is difficult to actually visit forests in person. Under these circumstances, aromatherapy using fir essential oil, which can be used anytime, anywhere, and is inexpensive, seems to help modern people effectively relax their brain activity.

Meanwhile, previous studies (Igarashi et al., 2014; Ikei et al., 2015a; Ikei et al., 2015b), which were consistent with this study in that the concentration of HbO in the prefrontal cortex decreased when natural substances were inhaled, showed partially different results. First, the locations of the channels where HbO concentration changed when the scent was inhaled were different. In previous studies that examined brain activity when natural substances were inhaled (Igarashi et al., 2014; Ikei et al., 2015a), HbO concentration decreased mainly in the right prefrontal cortex, while in this study, HbO concentration decreased in the left prefrontal cortex. Some previous studies that identified the relationship between olfactory stimulation and brain activity reported that the difference in brain activity from olfactory stimulation was even greater in the left hemisphere than in the right hemisphere (Boyle et al., 2016; Hucke et al., 2018). Although these findings are consistent with the results of this study in that fir essential oil inhalation resulted in more significant changes in the left hemisphere than in the right hemisphere, various factors, including the preference for the scent of natural substances, the components of essential oils, the concentration of scent, and the duration of scent inhalation seem to have had an effect. Therefore, it seems necessary to examine brain activity in response to different essential oils and from various perspectives in the future. Second, by analyzing the HbO concentration for each channel, the pattern of change in HbO concentration was found to be different, with the concentration even increasing in some channels. This is a novel finding that could not be confirmed with single-channel fNIRS. In this regard, there are not many studies that have examined changes in brain activity by applying multi-channel fNIRS to scent inhalation experiments, making direct comparisons difficult. In the future, comparisons by channel through accumulating sufficient research data will be necessary. This study is expected to provide important baseline data for determining changes in brain activity based on multi-channel fNIRS.

This study was conducted to determine the effect of forest scent inhalation on brain activity. Multi-channel fNIRS was used as an indicator to measure brain activity. By analyzing each of the 15 channels, right hemisphere channels (CH 1-7), left hemisphere channels (CH 9-15), and all channels (CH 1-15), it was found that inhalation of fir essential oil resulted in a significantly greater reduction in HbO concentration than inhalation of room air.

However, because the participants in this study were limited to college students in their 20s, it is difficult to generalize to all people. It seems that verification through the accumulation of data targeting different age groups will be necessary in the future. Moreover, in 2020 when the experiment was conducted, it was difficult to recruit participants due to the COVID-19 pandemic, so a large amount of data could not be secured. It seems that experiments with a larger number of participants will be needed in the future, and thus continuous data collection will be necessary.