Exploring the effects of short-term indoor CO ₂ exposure on heart rate and cognition in university students: A pilot study

Indoor carbon dioxide (CO 2 ) accumulation in university classrooms is associated with fluctuations in students’ cognitive performance. This study investigated the preliminary associations between short-term CO 2 exposure, heart rate (HR), and cognitive accuracy (ACC), leveraging a small-N intensive longitudinal design with 54 synchronized observation sets over 6 days. Results indicated that HR responded to CO 2 in two stages under the observed conditions: an initial sensitive response with relatively stable HR when CO 2 was below the 1000 ppm reference level, followed by a gradual adaptive decline as concentrations increased. Standardized mediation analysis confirmed a global indirect effect (β = −0.106, p < 0.05). Notably, segmented analysis revealed that this physiological-cognitive coupling was primarily driven by the intensified impact of HR on cognitive accuracy in Phase II (β = 0.389, p = 0.022), whereas the mechanism remained exploratory in Phase I. These preliminary findings suggest that the relationship between CO 2 and cognition may be mediated by autonomic regulation (reflected by HR). Under the observed classroom conditions, 1000 ppm may serve as a guideline-aligned environmental reference associated with physiological–cognitive shifts. Practical applications This pilot study suggests that heart rate (HR) mediates the association between indoor CO 2 and cognitive performance in university classrooms. Specifically, CO 2 levels exceeding 1000 ppm were associated with distinct physiological changes and reduced cognitive accuracy, highlighting this value as a critical reference for ventilation control. Practically, these results support implementing occupancy-sensitive ventilation strategies to limit CO 2 accumulation. Additionally, the observed CO 2 -HR coupling indicates that aggregate HR trends derived from wearable devices could serve as non-invasive, supplementary indicators of indoor environmental conditions. These exploratory findings inform future human-centric approaches to indoor environmental quality (IEQ) management in educational settings.