Higher levels of CO2 increase lifespan of airborne SARS-CoV-2

Research led by the University of Bristol has revealed that carbon dioxide (CO₂) is a major factor in prolonging the life of SARS-CoV-2 variants present in tiny droplets circulating in the atmosphere. It shows that keeping CO₂ levels in check helps to reduce virus survival, and therefore the risk of infection.

During the COVID-19 pandemic, carbon dioxide monitors were used to help estimate ventilation in buildings, as both CO₂ and the virus are present in exhaled breath. But the new research has uncovered how CO₂ itself actually makes the virus survive longer in the air. The researchers also found that different SARS-CoV-2 variants had different aerostabilities, with the latest Omicron variant having an extended lifespan.

The researchers made these discoveries using bioaerosol technology they dubbed CELEBS (Controlled Electrodynamic Levitation and Extraction of Bioaerosols onto a Substrate), which allows the survival of different SARS-CoV-2 variants to be measured in laboratory-generated airborne particles that mimic exhaled aerosol. By varying the concentration of CO₂ in the air between 400 ppm (the level in normal outdoor air) and 6500 ppm, the team confirmed a correlation between increases in CO₂ concentrations and the length of time airborne viruses remain infectious in air, compounding the risk of transmission.

Increasing the CO₂ concentration to just 800 ppm — a level considered ‘well ventilated’ — resulted in an increase in viral aerostability. 40 minutes after increasing the CO₂ concentration to that of a crowded room (3000 ppm), around 10 times as much virus remained infectious when compared to clean air. These results were published in the journal Nature Communications.

Image shows droplets containing the COVID-19 virus being held in the air by electric fields. Image credit: Allen Haddrell.

“We knew SARS-CoV-2, like other viruses, spreads through the air we breathe,” said Bristol’s Dr Allen Haddrell, lead author on the study. “But this study represents a huge breakthrough in our understanding of exactly how and why that happens and, crucially, what can be done to stop it.

“The high pH of exhaled droplets containing the SARS-CoV-2 virus is likely a major driver of the loss of infectiousness. 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.

“That’s why opening a window is an effective mitigation strategy — because it both physically removes the virus from the room but also makes the aerosol droplets themselves more toxic to the virus.”

Between now and the end of the century, recent climate science research has projected the concentration of CO₂ in the atmosphere is expected to reach more than 700 ppm. The study findings therefore have implications not only in our understanding of the transmission of respiratory viruses, but also how changes in our environment may exacerbate the likelihood of future pandemics, Haddrell noted.

“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.”

Top image shows a top-down view of the suspended viral droplets, spanning around 2 cm, in the CELEBS bioaerosol device. Image credit: Allen Haddrell.