In the ongoing COVID-19 pandemic situation, exposure assessment and control strategies for aerosol transmission path are feebly understood. A recent study pointed out that Poissonian fluctuations in viral loading of airborne droplets significantly modifies the size spectrum of the virus laden droplets (termed as “virusol”). Herein we develop theory of residence time of the virusols, as contrasted with clean droplets in indoor air using a comprehensive “Falling-to-Mixing-plateout” model that considers all the important processes. This model fills the existing gap between Wells falling drop model and the stirred chamber models. The effect of various parameters on mean residence time are examined in detail. Significantly, the mean residence time of virusols is found to increase nonlinearly with the viral load in the ejecta, ranging from ~125 s at low viral loads (<104/mL) to about 1150 s at high viral loads (>1011/mL). The implications are further discussed.
