225 1. Introduction Traditional horticultural science has focused on plant production. Nowadays, the interaction between the wel- fare of human beings and plants is receiving widespread interest (Relf and Lohr, 2003). People-plant interactions are defined as the wide array of human responses (men- tal, physical, and social) that occur as a result of both active and passive participation with plants (Relf, 1992). One example of a positive response to plants is observed in the postexposure recovery from attention fatigue (Ka- plan and Kaplan, 1989; Tennessen and Cimprich, 1995; Lohr et al., 1996; Herzog et al., 1997; Wells, 2000). In addition, activities that involve direct contact with plants, such as gardening and horticulture, have been used as therapy for different groups of people in various settings to promote health, well-being, and social inclusion (Da- vies, 1998; Sempik et al., 2003). The natural environment, which includes plants, af- fects humans via the five senses and provides stimulation via vision (scenery) and olfaction (aroma of the plant) (Tsunetsugu et al., 2010). There are two areas of ex- perimental methods to clarify the interactions between people and plants. The first and predominant area of study is field experiments, clarifying the effects of total environments. The second area is through indoor experi- ments that test each stimulation on the basis of the five senses; the results of indoor experiments can support the outcome of field experiments. Therefore, we focused on indoor experiments based on the physiological effects of plant odor stimulation. The physiological and psychological effects of essen- tial oils have been acknowledged in folk medicine and aromatherapy for a long time (Tisserand, 1988). Also, essential oils and their components are widely used as constituents of different medical products in the medi- cine industry, as flavoring additives in the food industry, and as cosmetics and fragrances (Cawan, 1999). Oranges are a favorite fruit worldwide. In a previous ambient odor study, exposure to orange essential oil in a dentist’s wait- ing room decreased anxiety and improved mood in fe- male patients (Lehrner et al., 2000). The present paper reports an indoor experiment con- ducted in humans to determine the physiological effects of the odor of orange essential oil by measuring HRV, blood pressure, and pulse rate before and during inha- lation of orange essential oil and comparing the values with those obtained before and during inhalation of fresh air (control). Physiological effects of orange essential oil inhalation in humans B.-J. Park*, K. Ono**, Y. Yoshinami**, Y. Miyazaki*** (1) * Department of Environment & Forest Resources, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea. ** Nissan Motor Co., Ltd. 1-1, Takashima 1-chome, Nishi-ku, Yokohama City, Kanagawa Prefecture 220-8686, Japan. *** Center for Environment, Health and Field Sciences, Chiba University, 6-2-1 Kashiwanoha, Kashi- wa City, Chiba Prefecture 277-0882, Japan. Key words: aroma therapy, blood pressure, horticulture therapy, heart rate variability, people-plant interaction. Abstract: This study was conducted to clarify the physiological and psychological effects of the odor of orange essential oil in humans. Thirteen healthy male university students (mean age 23.0±1.1 years) participated. The study was conducted in an artificial climate chamber with temperature 24°C, relative humidity 50%, and illumination 50 lux. The subjects ran- domly inhaled orange essential oil for 120 s. Fresh air inhalation was used as the control condition. Heart rate variability (HRV), blood pressure, and pulse rate were continuously measured before (resting time) and during inhalation of the experimental odor. In addition, sensory evaluation and subjective odor intensity were evaluated after inhalation. The high frequency component of HRV was significantly higher, systolic and diastolic blood pressure was significantly lower, and the subjective “feeling of comfort” was significantly greater during inhalation of the orange essential oil than during inha- lation of fresh air. These findings indicate that inhalation of orange essential oil effectively induces relaxation in humans. Adv. Hort. Sci., 2014 28(4): 225-230 (1) Corresponding author: ymiyazaki@faculty.chiba-u.jp Received for publication 17 September 2014 Accepted for publication 6 October 2014 226 2. Materials and Methods Subjects Thirteen healthy male university students were re- cruited for the study (mean age 23±1.1 years), which was conducted according to the guidelines of the Institutional Review Committee of the Center for Environment, Health and Field Sciences, Chiba University, Chiba, Japan. Be- fore beginning the experiment, the subjects provided written informed consent after a detailed description of the aim and experimental procedures were provided to them. Experimental stimuli and odor delivery system Orange essential oil (Ogawa & Co., Ltd., Japan) was used as an experimental stimulus. Fresh air without any odor was used as a control. The odor stimuli were con- trolled by an odor delivery system that comprised four parts: 1) a polypropylene odor bag, 2) a container, 3) an air pump with a gauge, and 4) a funnel with a plastic tube. The polypropylene odor bag was filled with 24 L of diluted orange essential oil. The odor of orange essential oil flowed from the plastic odor bag through an air pump set at 2.5 L/min and was delivered through a Teflon tube to a funnel located 15 cm from the subject’s nostrils. After inhalation of the odor stimuli, the subjects were asked to evaluate the subjective odor intensity on a 6-point scale, from an “insensible level” (0) to an “un- bearable level” (6). The level of subjective intensity was controlled at an “easily sensible level.” Experimental design The study was conducted in an artificial climate chamber with temperature 24°C, relative humidity 50%, and illumination 50 lux. The time schedule for the ex- periment is shown in figure 1. After the attachment of sensors for physiological measurement, the subjects were asked to close their eyes and rest. Blood pressure and pulse rates were monitored in real time. After 30 s in a stable state, the subjects were exposed to odor simula- tion for 120 s. Sensory evaluation was conducted after physiological measurements. The subjects were asked to evaluate their subjective feelings and subjective intensity. Measurement HRV, blood pressure, and pulse rates were measured as physiological indices. These indices are frequently employed to estimate changes in autonomic nervous activity (Tsunetsugu et al., 2010). The time interval be- tween two consecutive R waves (R-R interval) on an electrocardiogram was measured using a portable elec- trocardiograph (Activtracer AC-301A, GMS, Japan) with three disk electrodes that were attached to the patient’s chest; this data was then analyzed using the maximum entropy method (Memcalc/win; GMS, Japan). Two major spectral components of HRV were calculated: low fre- quency (LF: 0.04-0.15Hz) and high frequency (HF: 0.15- 0.4 Hz) components (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996). HF is considered to reflect parasympathetic nervous activity (Cacioppo et al., 1994), which increases under relaxation conditions, while LF/ (LF+HF) is considered to reflect sympathetic nervous activity (Weise and Heydenreich, 1989), which increases under stressful conditions. Blood pressure and pulse rates were measured on the left middle finger (Finometer pro, FMS Ltd. Co., Neth- erland). This method is noninvasive, and data is available per second. Sensory evaluation was conducted after odor inhala- tion. The subjects were asked to evaluate and rate their “feeling of comfort,” “feeling of being soothed,” and “feeling of naturalness” on a 13-point scale. Statistical analysis A paired t-test was used for comparison of HRV, blood pressure, and pulse rate measurements during inhala- tion of orange essential oil with those during inhalation of fresh air. The Wilcoxon signed-rank test was used to analyze the subjective feelings scores. Statistical analysis of physiological data was processed with EXCEL 2003 (Microsoft Inc., Japan). Each measured value is repre- sented as mean ± SD. A P-value of <0.05 was considered statistically significant. 3. Results The subjective odor intensity ratings revealed the orange essential oil to be at the “easily sensible level”, Fig. 1 - Time schedule for the experiment conducted to investigate the physiological indices of the olfactory effects of orange essential oil odor. 227 which was significantly different from control (fresh air) inhalation. Figure 2 reports results of sensory evaluation of subjective feelings during orange essential oil inhala- tion. With regard to the “feeling of comfort,” the subjects rated orange essential oil as “moderately comfortable,” which was significantly different from control inhalation (Fig. 2 A). With regard to the “feeling of being soothed” (Fig. 2 B) and “feeling of naturalness” (Fig. 2 C), no sig- nificant differences were observed between the ratings for orange essential oil and those for fresh air. Figure 3 shows the HF component of HRV recorded every 30 s during the 120 s of essential oil inhalation; this was significantly enhanced compared with that dur- ing control inhalation (Fig. 4). The relative systolic blood pressure, measured every 30 s during exposure, tended to be lower with orange essen- Fig. 2 - Changes in subjective feelings after inhalation of an orange essential oil or fresh air as a control. (A) feeling of comfort; (B) feeling of being soothed; (C) feeling of naturalness. n= 13, mean ± standard deviation. **P < 0.01 as determined by the Wilcoxon signed-rank test. Fig. 3 - Changes in the high frequency component of heart rate vari- ability during 120 s of inhaling an orange essential oil or fresh air as a control. n= 13, mean ± standard deviation, determined by the paired t-test. Fig. 4 - Changes in the average high frequency component of heart rate variability during 120 s of inhalation of an orange essential oil or fresh air as a control. n= 13, mean ± standard deviation *P < 0.05 as determined by the paired t-test. 228 tial oil inhalation than during control inhalation (Fig. 5). The average relative systolic blood pressure (Fig. 6), rela- tive diastolic blood pressure (Fig. 7), and average relative diastolic blood pressure showed similar results (Fig. 8). 4. Discussion and Conclusions In this study, subjects exposed to the odor of orange essential oil for 120 s showed a significantly greater “feel- ing of comfort” compared with those who were exposed to fresh air without any odor. The results regarding psycho- logical states indicated that inhalation of orange essential oil has beneficial effects. Inhalation of essential oil is be- lieved to produce reliable and predictable effects on psy- chological state (Sanderson and Ruddle, 1992), and previ- ous studies have investigated this possibility. The effects of orange essential oil have been demonstrated through re- lief from anxiety and tension and improvements in mood (Lehrner et al., 2000, 2005). Subjective relaxation effects have also been found for lavender essential oil (Diego et al., 1998; Motomura et al., 2001; Moss et al., 2003; Bur- Fig. 5 - Changes in the relative systolic blood pressure during 120 s of inhalation of an orange essential oil or fresh air as a control. n= 11, mean ± standard deviation, determined by the paired t-test. Fig. 6 - Changes in the relative average systolic blood pressure during 120 s of inhalation of an orange essential oil or fresh air as a control. n= 11, mean ± standard deviation. *P < 0.05 as determined by the paired t-test. Fig. 7 - Changes in the relative diastolic blood pressure during 120 s of inhalation of an orange essential oil or fresh air as a control. n= 11, mean ± standard deviation, determined by the paired t-test. Fig. 8 - Changes in the relative average diastolic blood pressure during 120 s of inhalation of an orange essential oil or fresh air as a control. n= 11, mean ± standard deviation. *P < 0.05 as determined by the paired t-test, 229 nett et al., 2004) and peppermint essential oil (Ilmberger et al., 2001; Raudenbush et al., 2001, 2002). The data obtained in this study suggest that the inhala- tion of orange essential oil has positive effects on auto- nomic nervous system activity. The test subjects showed a significant enhancement of parasympathetic nervous ac- tivity, which has a relationship with relaxation. The results from the present study are consistent with those of a previ- ous study that investigated the HF component of HRV in healthy, young, adult university students (Park et al., 2007; Tsunetsugu et al., 2007; Park et al., 2008; Lee et al., 2009; Park et al., 2009, 2010; Lee et al., 2011). During inhalation of orange essential oil for 120 s, both systolic and diastolic blood pressure were significantly lower than those during control inhalation, indicating that the inhalation of orange essential oil has a significant re- laxing effect on the human body compared with the inha- lation of fresh air. The results of this study demonstrate that when sub- jects were exposed to an odor simulating the natural en- vironment, they experienced the environment via the five senses, including olfaction. 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