Exploring the possibility of using CO2 as a proxy for exhaled particles to predict the risk of indoor exposure to pathogens.

Airborne transmission has been confirmed as one of three principal ways of SARS-CoV-2 transmission. To reduce the transmission of SARS-CoV-2 indoors, understanding the distribution of respiratory droplets (or aerosols) present in human breath seems therefore important. To study whether the CO₂ concentration can be used as a proxy for the number of exhaled particles present in an occupied space, the distribution of particles with different diameters (0.3, 0.5, 1.0, 2.5, 5.0 and 10 μm) and CO₂ concentrations were monitored in a classroom setting with six healthy subjects. Additionally, numbers of particles with the same sizes were measured in the breath of the same six healthy subjects separately. Results showed that (1) on the contrary to CO₂, the main source of indoor particles came from outdoor air, and not from occupants; (2) the impacts of ventilation regimes on indoor particle numbers were different to the impacts on CO₂ concentrations; and (3) almost no significant relationship between the number of indoor particles and CO₂ concentration was observed. Based on these results, this study could therefore not conclude that the CO₂ concentration in a classroom can be used as a proxy for the number of exhaled particles by the occupants. [ABSTRACT FROM AUTHOR]