Field Investigations on Subjective Perception, Physiological Responses, and Cognitive Performance under Increasing CO2 Concentration in an Underground Confined Space

Abstract

The objective of this research was to explore the impacts of heightened CO₂ concentrations on human health and wellness in an underground confined space. A total of 180 participants were subjected to CO₂ concentrations ranging from 1000 to 10000 ppm within a confined underground environment. The study assessed not only subjective perceptions and physiological responses but also cognitive performance, integrating novel measures such as emotion, skin conductance (SC), and heart rate variability (HRV). The findings demonstrated a notable variation in thermal sensation votes (TSV) and perceived air quality acceptability with the change in CO₂ concentration. A significant increase in total mood disturbance (TMD) of 1.5 units was observed at a CO₂ concentration of 8500 ppm, compared to 1000 ppm. Cognitive performance remained consistent for concentrations below 8500 ppm; however, a substantial alteration was noted at 10000 ppm. In terms of task difficulty, numerical calculations were perceived to require 0.74 units more effort than letter searches. As CO₂ concentration exceeded 7500 ppm, significant variances were noted in physiological parameters such as diastolic blood pressure (DBP), heart rate (HR), LF/HF, MF/HF ratios, PNN 50, and frequency domains of HRV (LF, MF, and HF) in comparison to the parameters at 1000 ppm. At 8500 ppm, the LF and HF parameters were found to be 780 and 452.3 units, respectively, higher than at 7000 ppm. These findings suggest that high humidity, low temperature, and elevated CO₂ concentrations collectively contribute to the significant human stress responses. This study is of interest as there are limited reported researches on the air quality in underground confined space.