## 4 Discussion
Furthermore, fNIRS can penetrate deeper into tissue layers, capturing hemodynamic signals from cortical structures in the brain. In contrast, earlier methods are limited to detecting blood flow near the skin surface. These advancements make fNIRS a superior tool for brain research and clinical applications requiring precise and localized analysis. However, this study has several limitations. First, the inclusion criteria for this study specified healthy adults aged 20 years or older. However, the participants were overwhelmingly dominated by individuals in their twenties, and as a result, the study exclusively targeted adults in their 20s. Thus, the results lack insight into other age groups. Second, the present study included only healthy individuals, leaving a gap in the understanding of its applicability to patients with respiratory or circulatory conditions. Third, various atmospheric concentration conditions were not compared and analyzed. Thus, further investigation is warranted to identify the optimal atmospheric concentration for maintaining brain function and cerebral blood flow across different environments and individuals. Additionally, direct information on blood vessels, such as blood pressure, blood diameter, and blood stiffness, was lacking. Future studies should include diverse age groups and patients with diseases to confirm the physiological changes resulting from variations in atmospheric concentrations. Moreover, direct measurements of blood vessel diameter and stiffness using ultrasonography or magnetic resonance imaging may further elucidate the relationship between O\({}_{2}\) saturation and Hb concentration. In conclusion, the human body is significantly influenced by continuous changes in atmospheric concentrations, particularly increased CO\({}_{2}\) concentrations, which trigger physiological responses to protect the O\({}_{2}\)-dependent brain. This knowledge has been applied in various fields, including cognitive function enhancement, disease treatment, and recovery by utilizing appropriate O\({}_{2}\) and CO\({}_{2}\) concentrations. However, research on changes in cerebral blood flow and their effects on brain function relative to the quantitative gas concentration remains limited. This study used fNIRS to compare and analyze the physiological changes in the brain associated with variations in atmospheric concentrations. At high O\({}_{2}\) and CO\({}_{2}\) concentrations, COS, HbO, and HbT increased, whereas HbR decreased. Notably, high CO\({}_{2}\) concentrations resulted in significant differences compared to standard atmospheric concentrations, and the simultaneous application of high levels of both O\({}_{2}\) and CO confirmed that enhanced O\({}_{2}\) due to the increased blood O\({}_{2}\) saturation and increased blood flow due to the high CO\({}_{2}\) worked in tandem, resulting in the most substantial changes in COS and Hb concentrations. Thus, combining appropriate gas concentrations is a simple yet effective method of enhancing cerebral blood flow, potentially facilitating optimal brain function, promoting cerebrovascular health, and preventing disease. Further research in diverse age groups and patients with specific conditions is essential to validate the positive effects of changes in atmospheric concentrations observed in the present study. [PERSON]-S.P., [PERSON]-J.K., [PERSON]-H.K., [PERSON]-K.K. and [PERSON]-B.L. were involved in conceptualization, data curation, formal analysis, investigation, methodology, resources, software, validation, and visualization. [PERSON]-S.P. and [PERSON]-K.K. were involved in writing the original draft. [PERSON]-K.K. and [PERSON]-B.L. were involved in funding acquisition, project administration, supervision, writing, reviewing, and editing as corresponding authors. All authors have read and agreed to the published version of the manuscript. This research was supported by grants from the National Research Foundation of Korea (NRF), funded by the Korean government (MSIT) (No.