Indoor carbon dioxide (CO2) accumulation in university classrooms is associated with fluctuations in students’ cognitive performance. This study investigated the preliminary associations between short-term CO2 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 CO2 in two stages under the observed conditions: an initial sensitive response with relatively stable HR when CO2 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 CO2 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 CO2 and cognitive performance in university classrooms. Specifically, CO2 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 CO2 accumulation. Additionally, the observed CO2-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.
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