## 4 Discussion  
In terms of MT detection threshold, our results are similar to those of previous studies. [PERSON] et al. (2007) reported that the detection threshold of limonene was 2 to 4 ppbv [37]. In contrast, the P100 threshold of \(\alpha\)-pinene was about 15 ppbv (90 mg/m\({}^{3}\)) [38]. Furthermore, a larger fraction of female subjects was able to detect the odor of 5 ppbv MTs compared to male subjects (Table 5). This indicates that the female olfactory response is more subtle than the male response. It was suggested that women have more neurons and glial cells than men [36]. Moreover, the odor perception of women is more effective than that men [39].  
The concentration of MTs in our experiments was adjusted on the basis of their natural levels in forests. Moreover, the concentrations of MTs can vary depending on many factors such as season, climate, and forest composition [22]. Therefore, the concentration of MTs must be chosen on the basis of their detection limits when studying MTs effects on human psychology. The results of this study showed that all participants could clearly sense the MTs at a concentration of 7 ppbv, indicating that MTs might not affect human psychology when they are present under the detectable smell level (2 ppbv). In a study, it was concluded that the strong odor of \(\alpha\)-pinene (\(\sim\)16.7 ppmv) was uncomfortable for the subjects [28]. However, this concentration level was much higher (e.g., in ppmv level) than that  
Figure 4: Variations of the mean values of alpha waves and stress index of 30 participants with respect to the concentration of MTs, as obtained from Exp 2.  
in ambient level (e.g., in ppbv level). In this study, the objective was exposure to the low (ppbv) levels of MTs. Thus, it is not easy to directly compare those results to ours.  
It was found that the inhalation of low ppbv levels of MTs was helpful to improve the psychology of all subjects in this study. These results are consistent with those reported previously regarding the inhalation of mixed MTs [23; 24; 25; 26; 28]. Participants who smelled Japanese cedar wood reported enhanced comfort and a relaxation after 40-60 s [26]. The smell of Hiba oil also mitigated depression and anxiety [25], while enhancing contingent negative variation [24]. Moreover, after inhaling Taiwan cypress, the performance of the participants was improved with respect to certain tasks [27]. Besides, their maximum blood pressure also decreased [27]. It was also reported that the inhalation of 0.3-3 ppmv of \(\alpha\)-pinene and 0.3 ppmv of d-limonene from air greatly improved the comfort of subjects [28]. In addition, Cedrol was shown to decrease heart rate and systolic blood pressure [23].  
Accordingly, the inhalation of single MTs revealed a good effect on human psychology. However, high concentrations of MTs could bring about adverse effects on psychology. In addition, mixtures of MTs also helped improve the subjects' comfort at low ppmv levels of MTs. On the basis of this study, it is suggested that the exposure to ppbv levels of artificial MTs should help improve human psychology and physiology. In the combination with an image display device (such as projector, television, and visual-reality device), the use of artificial monoterenes would generate similar positive effects as a forest bathing, without visiting a real forest. It would offer diverse therapeutic opportunities for those who cannot access the forest environment with ease.