The effect of the ventilation rate on exposure to SARS-CoV-2 in a room with mixing ventilation

Throughout the COVID-19 pandemic, guidance was to increase ventilation as a way to reduce the risk of transmission. While the benefits of ventilation, when it is used to supply fresh air or to remove virus laden air from a space, is indisputable, we show that in some circumstances it can also enhance the transport of virus from the infected to the uninfected. We used computational fluid dynamics to study exposure to SARS-CoV-2 from a person coughing, in a mechanically ventilated room with mixing ventilation, over short time periods. Models were run with three ventilation rates and two definitions for how the virus is distributed within different size droplets. These showed that up to 3 m from the person (the largest distance assessed in this work), the median exposure had a statistically significant increase as the ventilation rate was increased. For example, as the room air change rate was increased from 0.5 h−1 to 5 h−1, the median exposure after 5 min increased by a factor of 7 or 134 depending on the model settings specified. The models showed that the negative impact of mixing ventilation on exposure (i.e. increased ventilation rate leading to increased exposure) reduced with time, which brings the predictions in line with the general guidance. Ventilation measures are therefore most likely to have the greatest impact on reducing transmission in spaces where people spend longer periods of time.