High Levels of Carbon Dioxide Increase Virus Survival and Transmission

Researchers at the University of Bristol have shown that CO₂ is a major factor in prolonging the life of SARS-CoV-2 variants present in tiny droplets circulating in the atmosphere.

The findings suggest that keeping CO₂ levels low helps to reduce virus survival, and therefore the risk of infection. The study authors highlight the importance that global net zero goals could play in controlling the transmissibility of other respiratory viruses. The findings are published in Nature Communications.

Maintaining low CO₂ levels mitigates transmission

SARS-CoV-2, like many other viruses, spreads through the air we breathe. 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. These findings now suggest that opening a window may be more effective at combatting transmission than originally thought by not only physically removing the virus but also causing the virus to become inactivated much faster.

Previous studies have shown that an increase in the pH of respiratory aerosols following generation is a significant factor in reducing SARS-CoV-2 infectivity. In this study, the researchers set out to further understand the underlying physicochemical properties of respiratory aerosols that influence viral infectivity.

To perform the research, the scientists developed a unique bioaerosol technology called CELEBS – Controlled Electrodynamic Levitation and Extraction of Bioaerosols onto a Substrate. This technology enabled 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 the level in normal outdoor air (400 parts per million (ppm)) and 6,500 ppm, the team confirmed a correlation between increases in CO₂ concentrations and the length of time airborne viruses remain infectious in the air.

Results showed that a moderate increase in CO₂ concentration to just 800 ppm increased viral aerostability. After 40 minutes, when compared to clean air, around 10 times as much virus remained infectious when the air had a CO₂ concentration like that of a crowded room (3,000 ppm).

“These observations confirm the critical importance of ventilation and maintaining low CO₂ concentrations in indoor environments for mitigating disease transmission,” the researchers said.

High pH drives the loss of viral infectivity in aerosols

Discussing the link between CO₂ levels and infection risk, Dr. Allen Haddrell, senior research associate in aerosol science at the University of Bristol, said: “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.”

Investigating the effects of CO₂ on different variants of SARS-CoV-2, the researchers found that the various strains had different aerostabilities, with the latest Omicron variant having an extended lifespan. The scientists suggest this is likely a product of the differing pH sensitivities for different variants, with the more pH-sensitive variants more sensitive to changes in CO₂ concentrations.

“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,” said Haddrell.

Increasing atmospheric CO₂ levels could improve virus survival

Recent climate studies suggest atmospheric CO₂ is on pace to surpass 550 ppm in the next 30–80 years. “These findings therefore have broader implications not only in our understanding of the transmission of respiratory viruses, but how changes in our environment may exacerbate the likelihood of future pandemics,” said Haddrell.

This data emphasizes the importance of global net zero goals showing that even slightly raised levels of CO₂ can significantly improve the rate of virus survival and the risk of it spreading. The researchers concluded, “The correlation of increased CO₂ concentration with viral aerostability need to be better understood when considering the consequences of increases in ambient CO₂ levels in our atmosphere.”

Reference: Haddrell A, Oswin H, Otero-Fernandez M, et al. Ambient carbon dioxide concentration correlates with SARS-CoV-2 aerostability and infection risk. Nat Commun. 2024;15(1):3487. doi: 10.1038/s41467-024-47777-5

This article is a rework of a press release issued by the University of Bristol. Material has been edited for length and content.