To mitigate the spread of a viral disease, it is crucial to understand the factors that influence airborne virus transmission. However, the micro-environment to which the virus is exposed in expiratory aerosol particles is highly complex. The relative humidity, the aerosol particle size and composition, and the air composition affect virus infectivity by modulating the salt and organic concentrations within the particle, as well as the phase state. A parameter that has been overlooked is the aerosol pH. Several viruses are sensitive to acidic pH; for example, the inactivation of influenza A virus becomes very fast at pH 5.5 and below, a threshold that is quickly reached in an expiratory aerosol particle exhaled in a typical indoor environment. Therefore, aerosol acidity plays a sig-nificant role in controlling the persistence of airborne, acid-sensitive viruses such as influenza virus, and aerosol pH control could be applied to limit the risk of airborne virus transmission.