Scholars recently noted that humans release a wide variety of gases from the skin, whose components may act as a barometer for indicating the state of the body.
For example, persons with diabetes characteristically release acetone, whereas those with liver diseases typically emit ammonia. Moreover, the study confirmed that the human skin releases specific components due to changes in the body unrelated to illnesses, such as aging and constipation.
Alternatively, studies reported body odor in samples collected mainly from the armpit, which is caused by various emotional states. Majority of the studies examined the reactions of dogs and humans who smell human odors of different states [13–15]. Reports of human odors estimated by statistical methods do not indicate how such odor affects humans . Therefore, this study hypothesized that changes in psychological state chemically affect skin gas and its components affect human psychology. This study was conducted with the aim of identifying the main components of skin gas released during certain emotional states and examining their psychological effects. In the middle of various psychological changes, the study opted to induce mild tension in a manner that is easy to control and with reference to the Trier social stress test method [17, 18].
Collecting body odor from humans can be conducted using various methods, such as armpits using cotton balls [13–15], clothing [2, 19], and the back and neck . However, such methods contain a mixture of bacterial metabolites of sebum and/or apocrine gland secretions in addition to the odor that comes directly from the body. Thus, the study examined the specific relationship between changes in skin-gas components and the body. For this reason, samples were collected from the hand because it contains less sebaceous glands and no apocrine glands .
During the interviews with instigated tension, physiological indicators before, during, and after the interview were measured to confirm the subject's condition. The results indicate that the LF/HF values during the interviews were significantly higher than those before and after the interviews, which indicate sympathetic dominance. Cortisol levels of saliva during the interviews were also significantly higher than those collected at baseline at the same time on another day. In particular, when the effects of the circadian rhythm  on cortisol was canceled and compared, the finding demonstrated that the stress level was high. The results suggested that the subjects were psychologically tense during the tension-inducing interviews. Additionally, sensory assessment was performed to confirm changes in odor components as a result of the altered psychological states. Consequently, a common characteristic odor that smells similar to sulfur-containing compounds was recognized in all samples of skin gas collected during the interviews despite their short duration (20 min). Notably, the intensity of this characteristic odor was positively correlated with the rate of increase in sympathetic activity during the interviews. Moreover, a positive correlation was found between heart-rate variability and odor intensity. On another test, this phenomenon was also observed in men or non-Japanese individuals while they were feeling tensed. These findings suggest that with greater emotional tension, characteristic odor is emitted more strongly from humans, and this phenomenon may be an instinctive physiological reaction among humans. Conversely, the characteristic odor was unrecognized in the results of the sensory evaluation of skin-gas samples collected during a bike exercise, which featured an elevated heart rate (> 140bpm for 20 min) (Fig. 5). This result suggests that a mere increase in heart rate and accompanying increase in thermal sweating was not the direct cause of the characteristic odor released by the skin.
The characteristic odor was clearly recognized through sensory evaluation. However, determining the target compounds was extremely difficult due to the very low concentrations. Thus, the study searched for effective sample preparation methods and analytical conditions and finally succeeded in identifying the target compounds as DMTS and AM. The successful identification of the two sulfur-like compounds rendered possible the artificial recreation of the model tension-stress odor. Moreover, the study examined the psychological effects of subjects directly inhaling and smelling the odor. The second edition of the short version of the Profile of Mood States (POMS 2) was used to evaluate psychological status. POMS 2 is a widely used self-report questionnaire for the rapid assessment of mood states, including transient, fluctuating feelings, and enduring affect states. The higher the T scores for anger–hostility (AH), confusion–bewilderment (CB), depression–dejection (DD), fatigue–inertia (FI), and tension–anxiety (TA), the more negative the mood. Conversely, positive mood states, such as vigor–activity (VA) and friendliness (F), indicate that the higher the T score, the better the mood . POMS is also used in various research fields to measure mood disturbance and/or fatigue [24, 25]. The results showed that sniffing the model's tension-stress odor increased negative emotions such as tension–anxiety (TA), fatigue–inertia (FI), and confusion-confusion (CB). This finding suggests that the odor emanating from emotionally stressed people may exert a psychological impact on people within their immediate vicinity. If odors released from people are a profound means of communication, then this finding may have a great impact on various fields. Thus, the elucidation of the mechanism and significance of this characteristic odor development is a subject for future research. Moreover, the current finding that emotional changes may influence body odor produced from areas apart from the apocrine glands, which is specific to axillary odor, may indicate new concepts and potential applications in the field of biogas research. Thus, the study aims to proceed with the development of a practical fragrance material as it endeavors to elucidate the odor generation mechanism. The study intends to eliminate the discomfort that may be cause by a tension-induced odor by formulating a scent that harmonizes with this odor and removes its negative psychological effects instead of covering it with a strong odor.
This research is not only an etiquette issue in interpersonal relationships but is also applicable to the personal management of mental health and smooth communication in social interaction in the case of a serious COVID-19 pandemic. Conversely, in the rescue of victims of natural and man-made disasters, consideration is given to the use of human body odor as a clue for those who are still alive [25–27]. The perspective of tension- and stress-induced human odor in the field of rescue can be extremely helpful in detecting humans in crisis. Such applications hold great potential. Therefore, further research is required.