The effect of local ventilation on a spatiotemporal model of airborne disease transmission in indoor spaces

Source abstract

Abstract We incorporate local ventilation effects into a spatially dependent generalisation of the Wells–Riley model for airborne disease transmission and consider a room in which an air purifier supplements the ventilation provided by a poorly functioning air-conditioning (AC) unit. Aerosol production and removal through ventilation, biological deactivation and gravitational settling as well as transport around a recirculating AC flow and global turbulent mixing are modelled using an advection–diffusion–reaction equation. This local ventilation model is compared with a global ventilation model, where all ventilation is treated as a global sink. We undertake this comparison for a weak purifier (clean air delivery rate of 140 m cubed 3 $^3$ h Superscript negative 1 − 1 $^{-1}$ ) and a strong purifier (clean air delivery rate of 1,000 m cubed 3 $^3$ h Superscript negative 1 − 1 $^{-1}$ ). For each purifier, we determine a total effective air exchange rate and compare against the global model with equivalent ventilation. We find that, as expected, the purifier removes fewer aerosols when the distance between the infectious person and the purifier is increased, resulting in a greater average aerosol concentration in the room. Moreover, the concentration is generally lowest when the infectious person is upstream of the purifier, located in regions where the airflow streamlines are directed into the purifier. For these infectious source locations, the global ventilation model significantly overestimates the concentration throughout the room. For infectious sources outside of these regions, there is generally good agreement between the models, particularly for the weak purifier. We also studied, for a fixed distance from the purifier, how the infection risk to a susceptible person varies as the infectious person changes location. A susceptible person faces the highest infection risk when they are directly downstream of the infectious person, where the aerosol concentration is the highest. There is better agreement between local and global ventilation models for the weak purifier than for the strong purifier since the weak purifier has less impact on the airflow in the room.