How air acidity affects how long airborne viruses hang around

Viruses such as SARS-CoV-2 and influenza are mainly passed between individuals via aerosols, tiny particles that an infected person expels when coughing, sneezing, or exhaling. (c) Copyright 2021, Dpa (www.dpa.de). Alle Rechte Vorbehalten

A study by Swiss researchers shows that the acidity of air determines how long airborne viruses remain infectious. This has major implications for virus transmission and strategies to contain it, the authors say.

Viruses such as SARS-CoV-2 and influenza are mainly passed between individuals via aerosols, tiny particles that an infected person expels when coughing, sneezing, or exhaling.

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. The chemical composition of exhaled aerosols, in particular its acidity and its interactions with the indoor air, is also thought to have an influence.

New researchExternal link by scientists at the federal technology institutes in Zurich and Lausanne, ETH Zurich and EPFL, and the University of Zurich shows that the more acidic the air, the faster viruses are inactivated.

Their study published the journal Environmental Science & TechnologyExternal link shows for the first time how the pH of aerosol particles changes in the seconds and hours after exhalation under different environmental conditions. It also reveals how this impacts the viruses in the particles.

The authors say exhaled aerosols acidify very rapidly, faster than some might expect. The speed of acidification depends on the concentration of acid molecules in the ambient air and the size of the aerosol particles.

The team examined tiny droplets – a few micrometres across – of nasal mucus and of lung fluid synthesised specifically for the study. In typical indoor air, it took these droplets only about 100 seconds to reach a pH of 4, which is roughly equivalent to the acidity of orange juice. The pH value is a measure of acidity: a neutral solution has a pH of 7; the pH of acidic solutions is less than 7.

The acidification of aerosols is largely due to nitric acid from the outside air, which enters indoor spaces either through open windows or via ventilation systems.

SARS-CoV-2 and influenza were found to have different acid sensitivities. SARS-CoV-2 is so acid-resistant that at first the experts didn’t believe their measurements. It took a pH of below 2, i.e. very acidic conditions such as those in undiluted lemon juice, to inactivate the coronavirus.

The Swiss scientists were surprised to find that 99% of influenza A viruses are inactivated in aerosols after roughly three minutes. Since aerosol pH hardly ever falls below 3.5 in typical indoor spaces, it takes days for 99% of coronaviruses to be inactivated.

“With this knowledge, we could act much more quickly against infectious viruses in the air,” Tamar Kohn, co-author of the studyExternal link , told the Keystone-SDA news agency.

The study shows that in well-ventilated rooms, inactivation of influenza A viruses in aerosols works efficiently, and the threat of SARS-CoV-2 can also be reduced. In poorly ventilated rooms, however, the risk that aerosols contain active viruses is 100 times greater than in rooms with a strong supply of fresh air.

The team recommends indoor rooms be ventilated frequently and well, so that the virus-laden indoor air and substances such as ammonia from emissions of people and indoor activities are carried outside, while acidic components of the outside air can enter the rooms in sufficient quantities.

The study also found that normal air conditioning systems with air filters can lead to a reduction in volatile acids. “Acid removal is likely even more pronounced in museums, libraries or hospitals with activated carbon filters. In such public buildings, the relative risk of influenza transmission can increase significantly compared to buildings supplied with unfiltered outside air,” the team wrote.

In such situations they suggest adding small amounts of volatile acids such as nitric acid to filtered air and removing basic substances such as ammonia in an attempt to accelerate the aerosols’ acidification.

According to the study, a concentration of nitric acid at levels around 50 ppb (parts per billion of air, which is 1/40th of the 8-hour legal limit in the workplace) could reduce the risk of Covid-19 infection thousandfold.

However, such measures are highly controversial, as it is not clear what consequences such volatile acids may have. Additional studies are needed to assess the risks to people and structures, the scientists conclude.