Estimating time and length scales of indirect transmission of respiratory diseases

The recent pandemic has highlighted several open questions across many disciplines. One of the main epidemiological uncertainty relates to the mechanisms of pathogen transmission between individuals. In particular, a dichotomy exists between direct (short-range) transmission and indirect (long-range) transmission.  Direct transmission is associated with larger droplets falling immediately in the proximity of the infected person and spreading the virus with deposition on other individuals or objects. On the contrary, indirect transmission is related to smaller droplets that evaporate faster and can remain suspended in the air for longer times as aerosol nuclei reaching longer distances from the emission point. Here, we will use Direct Numerical Simulations (DNS) to assess the indirect transmission in a poorly ventilated room to quantify the relevant resident times and distances of respiratory droplets. We will show that droplets with an initial diameter of less than 30 microns can remain suspended for more than an hour, reaching distances even of the order of 10 meters. Assuming typical values of viral loads, we can explain the occurrence of superspreading events and evaluate the efficiency of non-pharmaceutical interventions such as social distances and masks.