Pathogen transport and air exchange during short conversations

The scenario where two unmasked individuals are in proximity having a face-to-face conversation is increasingly common and may contribute to the spread of the SARS-CoV-2 virus. We conduct flow visualization experiments and direct numerical simulations of colliding respiratory jets to study the airflow and exchange between two individuals during a short conversation. We observe that the vertical offset between the mouths is largely governed by the three-dimensional spread of the jets. At low offsets, the jets collide and interact such that the susceptible speaker is temporarily protected from the pathogen-loaded saliva droplets in the jet, and is termed as the ‘blocking effect’. At large offsets, the interaction between the jets is minimal and the jet spreads axially without any hindrance. At intermediate offsets, the respiratory jets interact such that jet entrainment and inhaled breath, assist the pathogen-containing jets to propagate towards the susceptible speaker's mouth. Thus, the interaction of the respiratory jets permits air exchange to a varying degree depending upon the effectiveness of the blocking effect and jet entrainment.