Research finds 'exhaled aerosols acidify very rapidly'

A study conducted by Swiss universities revealed the acidity of aerosols in indoor air plays a crucial role in determining the duration of viral infectivity.

The research, led by scientists from ETH Zurich, EPFL and the University of Zurich, sheds light on the impact of aerosol acidity on the transmission of viruses such as SARS-CoV-2 and influenza, according to a news release. The findings emphasize the importance of effective ventilation and air filtration strategies in reducing the risk of viral infections.

"According to the researchers, the exhaled aerosols acidify very rapidly, faster than some might expect," the release reported. "How fast they do this depends on the concentration of acid molecules in the ambient air and the size of the aerosol particles."

Viruses, such as SARS-CoV-2 and influenza, primarily spread through aerosols, which are small suspended particles containing liquid that infected individuals release when coughing, sneezing or even breathing, the release said. Given the importance of airborne transmission, it is widely recognized that appropriate ventilation and air filtration are crucial in reducing aerosol concentrations and thereby minimizing the risk of infection in indoor spaces like homes, offices and public transport.

While the impact of humidity and temperature on the persistence of viruses in aerosols has been researched, the chemical composition of exhaled aerosols and their acidity levels have been largely underestimated, according to the release. Acidic conditions have a significant impact on the survival of viruses in aerosols, and understanding this interaction could have significant implications for public health measures. 

The researchers investigated the changes in pH (a measure of acidity) of aerosol particles in the seconds and hours following exhalation under different environmental conditions, the release reported. They also examined how these changes in acidity affected the viability of viruses present in the particles. 

The team's experiments revealed exhaled aerosols rapidly acidify, a process influenced by both the concentration of acid molecules in the ambient air and the size of the aerosol particles, the release said. When exposed to typical indoor air, the researchers found tiny droplets of nasal mucus and lung fluid took only about 100 seconds to reach a pH of 4, similar to the acidity of orange juice.

One main contributor to aerosol acidification is nitric acid, which enters indoor areas as a result of open windows or ventilation systems that draw in outside air, according to the release. Nitric acid is formed through the chemical transformation of nitrogen oxides released into the environment by combustion processes and exhaust gases. 

AS a consequence, cities and metropolitan areas have a continuous supply of nitrogen oxides, leading to nitric acid adhering to surfaces and being absorbed by exhaled aerosol particles, thereby increasing their acidity, the release said. 

The study also showed the differential sensitivity of viruses to acidity, according to the release. While SARS-CoV-2 was found to be highly resistant to acid conditions, requiring a pH below 2 (similar to undiluted lemon juice) for inactivation, influenza A viruses were inactivated after just one minute in acidic conditions of pH 4. In typical indoor environments, freshly exhaled mucus particles reached this pH level in less than two minutes. 

The researchers were surprised to discover around 99% of influenza A viruses would be destroyed in aerosols after roughly three minutes. The findings suggest well-ventilated rooms with quality air filtration can effectively decrease the chance of influenza A virus transmission through aerosols, the release said. However, in poorly ventilated spaces, the likelihood of active viruses present in aerosols is 100 times greater than in well-ventilated rooms. 

Consequently, the researchers recommend frequent and adequate ventilation to expel virus-laden indoor air and introduce fresh air, balancing the acidification of aerosols and preventing virus survival, the release reported. While the study raises the possibility of adding volatile acids like nitric acid to filtered air to accelerate aerosol acidification, the long-term consequences and potential risks of such measures require further investigation. 

Museums, libraries and hospitals, which employ thorough air filtration systems to protect works of art and books, might face challenges with the addition of acids that could damage materials, according to the release

However, the removal of ammonia, a compound emitted by people that stabilizes viruses and raises pH, remains a less controversial strategy, the release said. This interdisciplinary collaboration between the researchers highlights the significance of grasping aerosol acidity and its effect on virus infectivity. 

The team's far reaching research provides useful information on the behavior of viruses in indoor air, putting an emphasis on the need for well-ventilated spaces and effective air filtration systems to minimize the risk of viral transmission. According to the release, the study's findings have been recently published in the journal Environmental Science & Technology.