The current pandemic has added to the growing evidence that respiratory viruses can be transmitted by the airborne route, yet the parameters modulating the infectivity of viruses in aerosol particles are poorly understood. Aerosol particles in the natural environment can by highly acidic, and acidic pH is known to reduce the persistence of some respiratory viruses. Yet, the pH of expiratory aerosol particles in indoor air and its effect on virus transmission remains unknown.

In this work, we assessed the role of aerosol pH in virus inactivation, and we evaluated if pH control of indoor air can reduce airborne virus transmission. The inactivation of influenza A virus (A/WSN/33) and two coronaviruses (hCoV-229E and SARS-CoV-2) was monitored in surrogate lung fluid (SLF) over a pH range of 2.1 to 7.4. The physicochemical properties of micron-sized SLF droplets, such as the water diffusion coefficient, were determined by injecting a fluid droplet into an electrodynamic balance (EDB) and measuring the relative changes in mass and volume upon changes in relative humidity. The physicochemical and virological data were then integrated into a biophysical aerosol model, to determine inactivation as a function of relative humidity, air composition and aerosol particle size.

Influenza virus was found to be readily inactivated at pH < 5, leading to airborne inactivation over the course of hours to minutes, depending on the size of the carrier particles. In contrast, coronaviruses are much more stable and can remain infectious within aerosol particles for days. The model results suggest that the addition of 50 ppm nitric acid to indoor air lowers the aerosol pH below 2 and thereby causes a dramatic decrease in inactivation times for influenza virus and SARS-CoV-2, and to a lesser extend for hCoV-229E. Alternatively, air acidification can be achieved by scrubbing of gaseous ammonia, though the resulting reduction in aerosol pH is insufficient to reduce coronavirus persistence. Finally, the model also suggests that the enrichment of air with nitric acid is more effective than ventilation or air filtration to limit the relative risk of virus transmission in indoor environments. Consequently, pH control may be an effective strategy to limit the transmission of a disease in indoor environments such as hospitals and schools.