Aerosols in indoor air can vary in acidity, and the acidity determines how long viruses remain infectious in the air, research finds.

Viruses such as SARS-CoV-2, influenza, and others travel from person to person by “hitchhiking” on aerosols. An infected person expels the finely dispersed particles when they cough, sneeze, or just exhale. Someone else can then inhale them.

It’s not clear how long viruses in aerosols remain infectious. Some studies suggest that the humidity and temperature of the air may play a role in virus persistence. A factor that has been underestimated so far is the exhaled aerosols’ chemical composition, in particular their acidity and their interactions with indoor air. Many viruses, such as influenza A virus, are acid-sensitive; exhaled aerosol particles can absorb volatile acids and other airborne substances, among them acetic acid, nitric acid, or ammonia, from the indoor air, which in turn affects the acidity (pH) levels of the particles.

The new study from ETH Zurich, EPFL, and the University of Zurich shows how the pH of aerosol particles changes in the seconds and hours after exhalation under different environmental conditions. It also shows how this affects the viruses contained in the particles. The study appears in the journal Environmental Science & Technology.

The team tested the sensitivity of influenza A and coronaviruses to different acidic conditions in artificially generated lung fluid and in nasal or lung mucus, which the scientists had previously harvested from specially grown mucus cell cultures.

Researchers from the Atmospheric Chemistry Group at ETH Zurich, led by Thomas Peter and Ulrich Krieger, investigated the behavior of the mucus aerosols using an electrodynamic particle trap. With this apparatus researchers can “hold” individual suspended particles for days or weeks and study them without contact to surfaces, for example to see how changes in humidity affect them.

Read the full article about preventing the spread of viruses by Peter Rüegg at Futurity.