When you exhale, the aerosol droplets you expel experience a big shock. In an instant, these particles are thrust from the humid, carbon dioxide-rich environment of the lungs into indoor air that’s often the polar opposite. This abrupt transition can be a rude awakening to any viruses that have decided to hitch a ride, including the novel coronavirus, SARS-CoV-2. According to new research, SARS-CoV-2 infectivity can decrease by up to 90% within minutes of hitting indoor air (Proc. Natl. Acad. Sci. U.S.A. 2022, DOI: 10.1073/pnas.2200109119).
The study is the first to investigate how environmental conditions can influence SARS-CoV-2’s survival in aerosols shortly after exhalation. The scientists determined that the virus’s lifespan is greatly affected by the relative humidity (RH) of the aerosol’s new environment.
For example, at low RH (below 50%), the particles undergo a phase change: They crystallize as water evaporates off the aerosols and the salts within them concentrate, explains Jonathan Reid, lead author of the study and director of the Bristol Aerosol Research Centre at the University of Bristol. Within seconds, this can inactivate 50% of the virus within the paritcle.
At higher humidities, another—slower—mechanism dominates. When RH approaches 90%, “a big driver for the loss of infectivity is actually a very rapid pH rise,” Reid says. The aerosol becomes more alkaline as it expels dissolved carbon dioxide while equilibrating to the lower CO2 levels outside the lungs. As a result, in more humid air, infectivity drops by 50% within the first five minutes, further decreasing to 90% within 20 minutes.
The findings are similar to what has been reported for other aerosolized viruses, such as influenza (Epidemiol. Infect. 1961 DOI: 10.1017/s0022172400039176), says Linsey Marr, an environmental engineer at Virginia Tech. “The mechanisms and time scales sound very plausible,” she writes via email. Marr adds that, pending further evidence, these findings could eventually guide practices that mitigate the virus’s spread. For example, drier indoor air could help limit exposure by making the aerosolized viruses less viable.
But Marr points out that “other studies have shown that the virus survives better at lower RH,” a discrepancy she attributes to differences in both methodology and the timescales considered across the studies. “I would want to be more sure about the results before recommending low RH,” she says. Dry air can also make people more vulnerable to viral infections, Marr and Reid explain, by either impeding immune response or hindering the lung’s natural defense mechanisms.
However, even if the study doesn’t yield any immediate updates to current mitigation strategies, Reid points out that the research is the first to provide insight into some of the processes that can alter the airborne virus’s infectivity. “It’s another piece of the jigsaw puzzle to help us understand transmission risk,” he says, both for COVID-19 and other airborne diseases.