## 4 Discussion  
The COS, HbO, and HbT measured at different atmospheric concentrations increased in the order of C1 to C4, whereas the HbR decreased. The increase or decrease in each condition was greater in the high CO\({}_{2}\) concentration (C3) than in the high O\({}_{2}\) concentration (C2), with the greatest change observed under combined high O\({}_{2}\) and CO\({}_{2}\) concentrations (C4). Therefore, both O\({}_{2}\) and CO\({}_{2}\) influence changes in COS, HbO, and HbT, with CO\({}_{2}\) causing greater changes than O\({}_{2}\). This is consistent with previous studies reporting that chemoreceptors are more sensitive to CO\({}_{2}\) stimulation than to O\({}_{2}\)[30]. Under C2, the increase in COS, HbO, and HbT was attributed to the high O\({}_{2}\) concentration in the inhaled air, leading to a high partial pressure of O\({}_{2}\) in the alveoli [31], which facilitated O\({}_{2}\) transfer from the alveoli to the bloodstream, ultimately delivering blood with a high HbO concentration to the brain [32]. Conversely, in C3, the alveoli were maintained at a high partial pressure of CO\({}_{2}\) due to the high CO\({}_{2}\) concentration in the inhaled air [33], which likely restricted CO\({}_{2}\) exchange from venous blood to the alveoli and limited the ability of Hb to bind with O\({}_{2}\) due to the inability to dissociate from carbaminohemoglobin [4]. When blood lacking adequate gas exchange moves into the arterial circulation, chemoreceptors detect low O\({}_{2}\) and high CO\({}_{2}\) concentrations, triggering physiological responses, such as increased HR and blood flow velocity, ultimately enhancing O\({}_{2}\) supply [34]. Additionally, C4 demonstrated the most significant increase compared to C2 and C3, suggesting that the physiological responses elicited by O\({}_{2}\) and CO\({}_{2}\) acted synergistically. In contrast, HbR exhibited a decreasing trend, likely due to the supply of blood with a high HbO concentration in C2 and the increased blood flow in C3, which accelerated HbO delivery, resulting in faster HbR displacement. The greatest change in C4 appears to stem from the simultaneous increases in O\({}_{2}\) and CO\({}_{2}\) concentrations. COS and HbO on the left side did not differ significantly between C1 and C2. However, these measurements differed significantly on the right side. This suggests that left-brain function was activated, leading to increased O\({}_{2}\) demand, even under normal air concentrations, as all participants in this study were right-handed. This finding aligns with research linking left- and right-brain activation to handedness [35]. Even at normal atmospheric concentrations, relatively more O\({}_{2}\) is likely supplied to the left brain. Consequently, while COS and HbO increased, owing to the high O\({}_{2}\) concentration in C2, the difference was smaller than that observed in the right brain. Furthermore, the HbR differences were significant only in the left brain under all conditions, indicating that the left brain was activated in response to changes in the atmospheric concentration, resulting in increased O\({}_{2}\) consumption. C3 and C4, which had high CO\({}_{2}\) concentrations, may have been interpreted as crisis situations, prompting physiological responses to mitigate their effects [36; 37]. HbT, which represents the sum of HbO (Hb bound to O\({}_{2}\)) and HbR (after O\({}_{2}\) has been delivered to tissues), is indicative of blood flow supplied through vessels. Like HbO, HbT