In 1875, scientists proposed that carbon dioxide (CO2) concentrations . The more CO2 present, this theory went, the more stagnant the air. Now, nearly a century and a half later, this idea has resurfaced during the COVID-19 pandemic. Mounting evidence suggests that transmission occurs via aerosols that — without proper ventilation — can linger in the air for minutes to hours.
This then raises the question: as we begin to reopen schools and businesses, could we measure CO2 to determine how safe an indoor space is?
In a paper published in the journal scientists from the University of Colorado Boulder, Zhe Peng and Jose Jimenez, studied this question. It turns out, linking ventilation risks to chemistry is more complicated than just ensuring CO2 levels don’t cross a pre-established threshold. Using no more than a couple of low-cost CO2 monitors and a mathematical model, Peng and Jimenez determined that risk depends on a number of factors, such as the number of occupants in a room, the percentage of occupants wearing masks, and the likelihood they’re participating in activities known to more easily spread the virus, such as talking or singing. So there is no single "safe" CO2 range for COVID-19 transmission.
But even though there are a lot of variables that need to be taken into account, this doesn't mean that measuring CO2 isn’t helpful. Public health officials can easily estimate the likelihood of transmission for a number of shared indoor spaces, like classrooms and grocery stores, using the , which is based on the mathematical formulas Peng and Jimenez share in their paper. And monitoring CO2 levels can still indicate someone’s relative risk of infection: all else equal, if better ventilation halves CO2 levels, then the transmission risks go down too, they say.
Their results support that better ventilation can reduce the amount of COVID-filled aerosols in the air. As Jimenez pointed out , "Wherever you are sharing air, the lower the CO2, the lower risk of infection."