Don’t Hold Your Breath: Indoor CO₂ Exposure and Impaired Decision Making

Elevated indoor levels of carbon dioxide (CO₂) have been associated with impaired work/school performance, a variety of health symptoms, and poor air quality. But CO₂ has been assumed to be simply a marker of poor ventilation rather than a detrimental exposure unto itself. Now researchers document evidence of adverse effects on adult decision-making performance associated with exposure to commonly encountered indoor levels of CO₂, even at fixed high ventilation rates [EHP 120(12):1671–1677; Satish et al.].

Typical outdoor CO₂ concentrations range from approximately 380 ppm to a high of about 500 ppm. Average indoor concentrations tend to be higher because occupants exhale CO₂. Some studies suggest CO₂ levels in crowded indoor spaces such as conference rooms, classrooms, and aircraft cabins can reach several thousand ppm. The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends a maximum indoor CO₂ level of 1,000 ppm as a marker of adequate ventilation.

In the current study, researchers exposed 22 university students to CO₂ concentrations of 600, 1,000, and 2,500 ppm in a controlled environmental chamber simulating an office environment. Participants were exposed to each concentration for 2.5 hours; all exposures occurred over the course of one day, but their sequence varied among participants. Ventilation rate, temperature, relative humidity, and other key factors were unchanged across exposures.

During each session, the participants engaged in a variety of activities, including computer-based questionnaires on perception of air quality and health symptoms. In the last 1.5 hours of each exposure, after a brief training session, they completed computer-based tests designed to analyze their decision-making performance.

The investigators observed a moderate decrease in performance for 6 of 9 decision-making measures at CO₂ concentrations of 1,000 ppm and a more substantial decrease for 7 of 9 measures at 2,500 ppm. The authors note that the findings need to be confirmed but suggest, in a surprising turnabout, that CO₂ should be considered an indoor pollutant, not just a proxy for other toxic pollutants. The findings also support the enforcement of current ventilation standards in buildings, and argue against reducing ventilation for the sake of energy savings.