With the rise of COVID-19, the world learnt how being in close quarters with another person or people can increase the risk of infection with the virus. New research led by the University of Bristol in the UK has provided an answer to the question of how and why airborne respiratory viruses linger longer in enclosed spaces: carbon dioxide.

“We knew SARS-CoV-2, like other viruses, spreads through the air we breathe,” said Allen Haddrell, lead and co-corresponding author of the study and senior research associate in Aerosol Science at the University’s School of Chemistry. “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.”

Carbon dioxide (CO2) is a good indicator of ventilation in an indoor space, with the number of people in a room affecting CO2 concentration. As both CO2 and respiratory viruses are present in exhaled air, it makes sense that CO2 concentration is used as a proxy indicator of viral transmission risk.

Here, we need to delve a little deeper into the science of breathing. The high pH (alkalinity) of exhaled breath results from the respiratory secretions from which it originates. For example, saliva and lung fluid contain elevated levels of bicarbonate, an alkaline. The pH of droplets expelled in the breath changes as bicarbonate evaporates into gaseous CO2 but is affected by things like relative humidity, droplet size, and ambient CO2 concentration. As pH is thought to be a driver of an airborne virus’ infectivity, the researchers explore whether ambient CO₂ concentration affected the stability of airborne viruses (aerostability) and, therefore, their risk of transmission.

In the setting of the COVID-19 pandemic, CO2 monitors were used to estimate building ventilation. Normal outdoor air contains around 400 ppm of CO2; in typical, well-ventilated indoor spaces, the concentration is between 400 and 1,000 ppm. In poorly ventilated, occupied spaces, the CO2 concentration can exceed 2,000 ppm and rise above 5,000 ppm in more crowded environments.

By varying the CO₂ concentration in the air between 400 parts per million (ppm) and 6,500 ppm, the researchers confirmed a correlation between CO₂ concentration and the length of time airborne viruses remained infectious. When compared to typical atmospheric CO₂ of around 500 ppm, a moderate increase in CO₂ from 400 ppm to 800 ppm – still within range for a well-ventilated room – resulted in a significant increase in viral aerostability for all SARS-CoV-2 variants (Delta, Beta, Omicron) after two minutes. No difference in infectivity was seen between 800 ppm and 6,500 ppm.

An elevated CO₂ concentration profoundly affected the infectivity of SARS-CoV-2 over time. Compared to normal air, when CO₂ concentrations were 3,000 ppm, similar to that of a crowded room, around 10 times as much virus remained infectious after 40 minutes.

“This relationship shed important light on why super spreader events may occur under certain conditions,” Haddrell 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.”

Thankfully, the researchers’ recommendation for reducing infectivity is a simple one.

“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,” Haddrell said.

Given the global focus on reducing atmospheric CO2, which climate scientists predict will rise to over 550 ppm in the coming decades, the researchers say their findings have broader implications.

“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. “Data from our study suggests that rising levels of CO2 in the atmosphere may coincide with an increase in the transmissibility of other respiratory viruses by extending how long they remain infectious in the air.”

The study was published in the journal Nature Communications.

Source: University of Bristol