###### Abstract  
Biological volatile organic compounds (BVOCs) are emitted abundantly from the flora. Among BVOCs, monoterpenes (MTs) have been used for aromatherapy (e.g., forest bathing) to treat human physiological and psychological conditions. However, relatively little is known about the therapeutic effects of MTs at low part per billion by volume (ppbv) levels. The effects of artificial MTs on human subjects were assessed by both olfactory and therapeutic parameters (brain alpha waves and stress index). Gaseous standards of three monoterpenes (i.e., \(\alpha\)-pinene, \(\beta\)-pinene, and \(d\)-limonene) prepared at low ppbv levels were used individually and as mixtures. Fifty-nine healthy and non-asthmatics volunteer university students were selected for the test. All human subjects inhaled low ppbv levels of monoterpene in the testing room. Brain alpha waves and stress index were investigated during the inhalation time. Questionnaires were also used after testing. It was found that the detection threshold of MTs was close to 5 ppbv. When the MT levels increased from 0 to 20 ppbv, the mean values of brain alpha waves derived from all participants increased from 9.8 to 15.1. In contrast, the stress index values declined from 46.2 to 34.7. The overall results suggest that MTs have great potential to positively affect the relaxation state of subjects in a manner similar to forest bathing in terms of short-term effect. They can thus be applied as potential therapeutic media for mental health care. monoterpene; psychology; physiology; brain wave; forest bathing 2019 9 4507 000 0000-0001  
compounds (BVOCs) consisting of two isoprene units. They are the second most abundant BVOCs emitted from plants [5]. The emission and composition of MTs have been widely investigated (Table 1) [6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. Of these compounds, \(\alpha\)-pinene, \(\beta\)-pinene, and \(d\)-limonene are predominant. Many studies have explored the effects of MTs on human health with respect to their composition and concentration level [16; 17; 18; 19; 20; 21; 3]. It was also documented that several MTs may exert adverse health effects on humans [21]. However, these adverse effects could only occur when MTs concentrations exceeded the threshold level of human sensory irritation or when they reacted with ozone to produce harmful by-products. Forest environments are known to affect various human senses, including vision, olfaction, and auditory and tactile sensations [22]. To date, much effort has been applied to assess the effect of such environmental conditions on human wellbeing. Some of those studies focused on the complex mixtures of BVOCs released from materials such as wood chips or wood-extracted oil [23; 24; 25; 26; 27; 28]. However, it is not easy to determine the relative significance of a particular component among many different BVOC components. The effects of forest bathing (e.g., Shinrin-Yoku) on human psychology have been well documented [22]. However, the MT levels in a forest can vary widely from pptv to ppbv, due to the combined effects of many factors such as reactivity, temperature, wind speed, and emission rate of MTs from the flora. Moreover, the effects of forest bathing, such as those of phytoncides, are complex, since they include all elements associated with senses (i.e., vision, olfaction, audition and tactile sensations) [22]. Therefore, it is not easy for a human to consistently recognize the odor of MTs. Also, the presence of MTs may not have an effect on human psychology and physiology. Therefore, the results of these previous studies could be biased if they are used to assess the effects of forest bathing. Thus, we attempted to investigate the effect of MTs at fixed concentrations and exposure times on young human subjects in the laboratory to remove the visual and auditory effects present in a forest. It was found that a relaxed physiological state was observed at relatively low concentrations of \(\alpha\)-pinene (e.g., 0.3-3.0 part per million by volume-ppmv) [28].