SARS-CoV-2 infectivity in exhaled aerosols can drop by 90 percent in 20 minutes depending on environmental conditions.

Images from scanning electron microscopy show the crystallization of two aerosol droplets at different humidities.Credit: Courtesy of Henry P. Oswin
Images from scanning electron microscopy show the crystallization of two aerosol droplets at different humidities.Credit: Courtesy of Henry P. Oswin

When you exhale, the aerosol droplets you expel receive a powerful shock. In an instant, these particles are ejected from the humid, carbon dioxide-rich environment of the lungs and into the often polar opposite indoor air. This abrupt change can be jarring for any viruses that have decided to tag along, including the novel coronavirus SARS-CoV-2. SARS-CoV-2 infectivity can be reduced by up to 90% within minutes of entering indoor air, according to new research (Proc. Natl. Acad. Sci. U.S.A. 2022, DOI: 10.1073/pnas.2200109119).

The research is the first to look into how environmental factors affect SARS-CoV-2 survival in aerosols shortly after exhalation. The researchers discovered that the relative humidity (RH) of the aerosol’s new environment has a significant impact on the virus’s lifespan.

For example, at low RH (less than 50%), the particles crystallize as water evaporates off the aerosols and the salts within them concentrate, according to Jonathan Reid, lead author of the study and director of the Bristol Aerosol Research Centre at the University of Bristol. This can inactivate 50% of the virus within the particle in seconds.

At higher humidity levels, another—slower—mechanism takes over. “A big driver for the loss of infectivity is actually a very rapid pH rise,” Reid says when RH approaches 90%. As it expels dissolved carbon dioxide and adjusts to lower CO2 levels outside the lungs, the aerosol becomes more alkaline. As a result, in more humid air, infectivity drops by 50% within the first five minutes and drops further to 90% within 20 minutes.

According to Linsey Marr, an environmental engineer at Virginia Tech, the findings are similar to those reported for other aerosolized viruses, such as influenza (Epidemiol. Infect. 1961 DOI: 10.1017/s0022172400039176). “The mechanisms and time scales sound extremely plausible,” she writes in an email. Marr adds that, pending further evidence, these findings could eventually guide practices to slow the spread of the virus. Dryer indoor air, for example, could help limit exposure by making aerosolized viruses less viable.

However, Marr notes that “other studies have shown that the virus survives better at lower RH,” a discrepancy she attributes to differences in methodology as well as timescales considered across the studies. “I’d like to be more confident in the results before recommending low RH,” she says. According to Marr and Reid, dry air can also make people more vulnerable to viral infections by impeding immune response or impairing the lung’s natural defense mechanisms.

Even if the study does not result in immediate updates to current mitigation strategies, Reid points out that it is the first to provide insight into some of the processes that can alter the infectivity of the airborne virus. “It’s another piece of the jigsaw puzzle that will help us understand transmission risk for COVID-19 and other airborne diseases,” he says.

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