Expiratory Aerosol pH: The Overlooked Driver of Airborne Virus Inactivation

Respiratory viruses, including influenza virus and SARS-CoV-2, are transmitted by the airborne route. Air filtration and ventilation mechanically reduce the concentration of airborne viruses and are necessary tools for disease mitigation. However, they ignore the potential impact of the chemical environment surrounding aerosolized viruses, which determines the aerosol pH. Atmospheric aerosol gravitates toward acidic pH, and enveloped viruses are prone to inactivation at strong acidity levels. Yet, the acidity of expiratory aerosol particles and its effect on airborne virus persistence have not been examined. Here, we combine pH dependent inactivation rates of influenza A virus (IAV) and SARSCoV-2 with microphysical properties of respiratory fluids using a biophysical aerosol model. We find that particles exhaled into indoor air (with relative humidity >= 50%) become mildly acidic (pH similar to 4), rapidly inactivating IAV within minutes, whereas SARSCoV-2 requires days. If indoor air is enriched with nonhazardous levels of nitric acid, aerosol pH drops by up to 2 units, decreasing 99%-inactivation times for both viruses in small aerosol particles to below 30 s. Conversely, unintentional removal of volatile acids from indoor air may elevate pH and prolong airborne virus persistence. The overlooked role of aerosol acidity has profound implications for virus transmission and mitigation strategies.

Automated summary

Respiratory viruses, such as influenza virus and SARS-CoV-2, can be transmitted by the airborne route. While air filtration and ventilation can reduce virus concentration, they fail to consider the impact of aerosol acidity on virus persistence. This study finds that exhaled particles in indoor air become mildly acidic, rapidly inactivating influenza virus but requiring days for SARS-CoV-2. Manipulating aerosol pH can significantly affect virus transmission and mitigation strategies.