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... "Opening a window may be more powerful than originally thought," says[1] University of Bristol chemist Allen Haddrell, "especially in crowded and poorly ventilated rooms, as fresh air will have a lower concentration of CO2, causing the virus to become inactivated much faster."

By measuring SARS-CoV-2 capacity to remain infectious while aerosolized in droplets under different environmental conditions, Haddrell and colleagues discovered the virus's stability is directly impacted by CO2 levels in the air. They used a new technique called Controlled Electrodynamic Levitation and Extraction of Bioaerosol onto a Substrate (CELEBS), which measures the impact of temperature, relative humidity, and different gas concentrations on suspended virus particles.

Atmospheric CO2 concentrations are currently around 400 parts per million (ppm). Crowd enough people in a closed room, however, concentrations can soar to around 3,000 ppm. The team found the number of viral particles that can remain infectious under these elevated concentrations can be 10 times higher than what would be found in outdoor air.

"The high pH of exhaled droplets containing the SARS-CoV-2 virus is likely a major driver of the loss of infectiousness," explains Haddrell. "CO2 behaves as an acid when it interacts with droplets. This causes the pH of the droplets to become less alkaline, resulting in the virus within them being inactivated at a slower rate."

What's more, highly crowded environments in poorly ventilated spaces can exceed 5,000 ppm of CO2.

"This relationship sheds important light on why super spreader events may occur under certain conditions," notes Haddrell. ...