Keeping CO₂ levels low reduces infectious airborne viral loads, new research suggests. While the study focused on the pathogen behind COVID-19, it has clear implications for reducing the risk of transmitting viruses in spaces where ventilation is limited.

"Opening a window may be more powerful than originally thought," says 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.

Image shows an aerosol of droplets containing the COVID-19 virus being held aloft by electric fields. (Allen Haddrell/University of Bristol)

Interestingly, different strains of SARS-CoV-2 had different patterns of stability in the air. After only 5 minutes viable viral particle concentrations were 1.7 times higher for Omicron (BA.2) than for Delta. This suggests there may be a lot of variability between viral particle types.

So while more research is required to confirm the relationships between CO₂ and other types of viruses, the researchers suspect this could explain why many respiratory viruses have seasonality. During colder weather people are likely to spend more time indoors experiencing greater exposure to air with higher levels of CO₂.

The amount of CO₂ in our outdoor air is also increasing thanks to global warming. Recent projections predict concentrations could exceed 700 ppm by the end of the century.

"[This study] also highlights the importance of our global net zero goals because the research indicates even slightly raised levels of CO₂ , which are increasing in the atmosphere with the onset of climate change, can significantly improve the rate of virus survival and the risk of it spreading," Haddrell adds.

"These findings can serve as a scientific basis for the design of mitigation strategies that could save lives in any future pandemic," concludes University of Bristol physical chemist Jonathan Reid.

This research was published in Nature Communications.