Effects of Plant Essential Oils on Physiological Changes

1. Introduction

1.2. Study content and composition

The scents extracted from plants are recognized for their diverse effects, including stabilizing emotions, inducing excitement, and effectively regulating mood. Additionally, they can aid in improving concentration and alleviating various neurological disorders caused by stress (Choi, 2021). This study aimed to explore the efficacy of essential oils extracted from plants through theoretical review. Additionally, it sought to verify whether the scent effects of pine, a key component in forest therapy, could be replicated in urban environments without direct forest exposure for therapy.

The experiment was conducted twice, on 4th and 7th Dec 2023, targeting 31 male and female students from G University in Gangwon Province, South Korea. The procedure was as follows:

• 1) Stress index examination was conducted using the Modified Form of the Stress Response Inventory (SRI-MF) questionnaire;
• 2) measurements related to the autonomic nervous system, brainwaves, and heart rate variability (HRV) were taken using the OMNIFIT Mindcare device;
• 3) one drop of essential oil, randomly chosen from pine (Pinus densiflora), cypress (Cupressus sempervirens), lavender (Lavandula species), and chamomile (Matricaria recutita L.), known for their stress- relieving effects, was applied to a mask (to prevent errors in the application of the plant's essential oil). Participants wore the mask with the oil and closed their eyes while assuming a relaxed posture, inhaling the scent for 1 min;
• 4) after removing the mask, participants took a 5-min rest; and
• 5) measurements related to the autonomic nervous system, brainwaves, and HRV were taken again.

Research parameters, such as measurements related to the autonomic nervous system, brainwaves, and HRV, were obtained using the OMNIFIT Mindcare device, as shown in Fig. 1. The collected data were analyzed using the statistical software SPSS ver24.0. This experiment enabled a practical verification of whether effects similar to forest therapy could be achieved using aromatherapy.

Fig. 1.

The OMNIFIT Mindcare device (A) and the measurement set-up (B).

2. Research Subjects and Methods

3. Research Results and Considerations

To investigate the effects of plant essential oils, the following sequential steps were undertaken:

• 1) Measurements of autonomic nervous system activity, brainwaves (electroencephalogram (EEG)), and HRV were conducted before inhalation;
• 2) participants randomly selected and inhaled the aroma of one of the four essential oils (pine, cypress, lavender, or chamomile) for 1 min and then took a 5-min rest in a comfortable position, as shown in Fig. 1;
• 3) EEG and HRV measurements were taken again to assess the effects of inhaling the essential oils; and
• 4) finally the data obtained was analyzed using statistical tools.

The experiment aimed to investigate the effects of essential oils derived from woody and herbaceous plants on physiological stress. Participants were instructed to inhale 1–2 drops of the essential oil (derived from either woody or herbaceous plants) applied to a mask, and the impact on physiological stress was examined.

To verify the homogeneity in the relationship between woody and herbaceous plants, gender, and stress index, a cross-analysis was conducted. The results, as presented in Table 1, indicated no significant differences between woody and herbaceous plants, confirming the pre-existing homogeneity between the two plant types in terms of gender and stress index.

Table 1.

Homogeneity in the relationship between woody and herbaceous plants in terms of gender and stress index

To assess the homogeneity of HRV, theta waves, and alpha waves between the woody and herbaceous plant groups, homogeneity tests were conducted. Due to the small sample sizes in each group, both parametric (independent t-test) and non-parametric (Mann-Whitney U test) tests were performed. The results (Table 2) indicated no statistically significant differences in any of the variables (HRV, theta waves, and alpha waves) between the woody and herbaceous plant groups. Therefore, it can be concluded that there was homogeneity between the woody and herbaceous plant groups in terms of HRV, theta waves, and alpha waves before the experiment.

Table 2.

Homogeneity in the physiological stress indicators between the woody and herbaceous plant groups before treatment

Table 3 presents the results of the independent t-test and Mann-Whitney U test conducted to examine the pre–post differences between the woody and herbaceous plant groups in terms of physiological stress indicators, including HRV, theta waves, and alpha waves. Post-treatment measurements showed increases in HRV, total power (TP), high frequency (HF), and low frequency (LF); however, the differences were not statistically significant.

Table 3.

Differences in physiological stress indicators between the woody and herbaceous plant groups before and afterr treatment

Table 4 presents the results of the independent t-test and Mann-Whitney U test conducted to examine the differences between the woody and herbaceous plant groups in terms of physiological stress indicators, including HRV, theta waves, and alpha waves, after inhaling the essential oil. Post-treatment measurements showed increases in HRV, TP, HF, LF, and theta waves when using essential oil derived from woody plants; however, the differences were not statistically significant.

Table 4.

Differences in the physiological stress indicators between the woody and herbaceous plant groups after treatment

Table 5 shows the results of the paired t-test and Wilcoxon test conducted to examine the differences in physiological stress indicators, including HRV, theta waves, and alpha waves, before and after treatment in all participants. The paired t-test revealed statistically significant differences in TP, HF, LF, and theta waves, while the Wilcoxon test showed statistically significant differences only in the theta and alpha waves.

Table 5.

Differences in the physiological stress indicators before and after treatment in all participants

Table 6 presents the results of the paired t-test and Wilcoxon test conducted to examine the differences in physiological stress indicators, including HRV, theta waves, and alpha waves, before and after treatment based on the oil type. For pine oil, both paired t-test and Wilcoxon test revealed statistically significant differences in TP, HF, LF, HRV, theta waves, and alpha waves. For cypress oil, both tests revealed statistically significant differences only in TP. Lavender oil did not exhibit statistically significant differences in any of the variables tested. Regarding chamomile oil, the paired t-test revealed statistically significant differences in TP and HF, while the Wilcoxon test showed significant differences in TP, HF, and autonomic nervous system balance.

Table 6.

Differences in physiological stress indicators before and after treatment based on oil type

4. Conclusion

This study investigated the relationship between inhaling plant-derived essential oils and physiological stress responses among university students exposed to stress. Lavender and chamomile essential oils, renowned for their stress-alleviating properties, alongside pine and cypress oils, associated with forest therapy effects, were employed to induce potential changes in stress levels. By measuring the stress stimuli-induced changes in HRV, θ waves, and α waves, the homogeneity of the relationship between woody and herbaceous plants in terms of gender and stress index was verified through a cross-analysis. The results confirmed the homogeneity between woody and herbaceous plants.

Unlike previous studies that simply verified changes in physiological responses following inhalation of plant essential oils, this study classified essential oils into woody and herbaceous types and aimed to verify whether effects comparable to forest therapy could be attained in urban settings. Paired t-tests revealed statistically significant differences in TP, HF, LF, and theta waves, while Wilcoxon tests showed significant differences only in theta and alpha waves. To determine which essential oil is most effective for alleviating physiological stress, pine, cypress, lavender, and chamomile oils were individually tested using paired t-tests and Wilcoxon tests. Pine oil showed significant differences in TP, HF, LF, HRV, theta waves, and alpha waves. Conversely, cypress oil showed significant differences only in TP. Lavender oil did not show significant differences in any of the studied variables. Furthermore, chamomile oil revealed significant differences in TP and HF in paired t-tests, as well as in TP, HF, and autonomic balance in Wilcoxon tests. These findings suggest that, among the tested essential oils, pine oil is most effective in providing forest therapy-like effects regardless of location constraints. Moreover, the results of the present study are consistent with those of previous studies by Lee(2014) and Lim(2020), further supporting the effectiveness of inhalation therapy.

As this study marks the initial endeavor to confirm physiological responses to stress, future studies should increase the duration and frequency of inhalation to bolster the reliability of physiological responses. Additionally, further research is warranted to compare the therapeutic effects obtained in natural forest environments with those obtained by inhaling essential oils in urban areas, without visiting forests, to verify the differences in physiological responses.

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