Effects of odor types and concentration adjustment modes of olfactory stimulation on driver fatigue

This study employed a 2 × 2 within-subjects design to investigate the effects of olfactory stimulation on driver fatigue. The independent variables were odor type (stimulating: peppermint; mild: lavender) and concentration adjustment mode (dynamic incremental vs. static constant). Based on existing literature identifying 10% as the optimal peppermint concentration, the constant and medium level in the incremental mode were set at 10%. However, definitions of “high” and “low” concentrations vary across studies. Tang and Cai found that high-concentration peppermint effectively reduces fatigue, although they did not disclose specific composition details. Moreover, the concentration levels employed in their study differ markedly from those reported by Tang and Jiang et al. Despite limited details on formulation, Tang and Cai’s findings suggest that high-concentration olfactory stimuli may be effective in mitigating fatigue. From a chemical and biological standpoint, a 30% concentration is a critical threshold, potentially enhancing key reaction pathways or physiological responses. As no prior studies have investigated how olfactory concentration varies with increasing fatigue, existing research only informs the concentration settings used here. Batiha et al. found that a 1% solution enhanced sedative effects through olfactory transmission. Similarly, Komiya et al. reported that inhaling 1% essential oil dilution reduced heart rate (HR), boosted vagus nerve activity (measured by HRV), and improved sleep quality. These studies support the use of low-concentration scents in vehicles, highlighting their physiological and psychological benefits. Aromatherapy at 1% concentration may reduce driver fatigue by enhancing attention without causing discomfort or overstimulation. Accordingly, this study set the low and high concentrations at 1% and 30%, respectively. A pre-experiment with five participants assessed subjective perceptions to both concentrations. Final concentration settings were 10% for the constant condition and 1%, 10%, and 30% for the incremental mode.Before the experiment began, the driving simulator and odor emission device were calibrated to ensure proper functioning and experimental accuracy. The HR monitoring device was also tested to confirm that ECG parameters met the required standards. The experimental procedure is illustrated in Fig. 4.To ensure that participants understood the experiment’s purpose and procedure, they completed a basic information form (age, gender, and health status) before participation. The full protocol was explained prior to the formal experiment to ensure clarity on the requirements. All participants provided informed consent .Before the formal experiment, participants sat quietly for five minutes to collect baseline HR data. Subsequently, they completed the Karolinska Sleepiness Scale (KSS) to record their initial state. Next, they were given five minutes to acclimatize with the experimental environment and procedure.The formal experiment comprised four rounds, each consisting of the following steps:1) Participants completed a 10-minute driving simulation to induce fatigue, after which experimental data were collected. After each initial driving segment, participants completed a scale to record their experiences.2) The experimenter emitted the scent continuously for one minute, followed by a 10-minute drive under olfactory stimulation. Participants again completed the scale afterward.3) Another 1-minute scent emission occurred, and participants drove for another 10 minutes under continued stimulation. The scale was completed again.4) A third 1-minute scent release was followed by the final 10-minute drive. Upon completion, participants filled out the scale one last time, concluding the round under the same experimental condition.After each experimental round, participants received a 1-hour break before the next session. The experiment comprised four rounds, representing four conditions: stimulating and mild scents, each delivered in either constant or incremental concentration modes. Each round involved the same procedure—driving, completing questionnaires, and resting. The order of conditions was randomized for each participant. After all rounds, participants were thanked and invited to a brief post-experiment interview regarding their experience.