  Airborne transmission has been recognized as an important route of
transmission for SARS-CoV-2, the virus responsible for the COVID-19 pandemic.
While coughing and sneezing are spectacular sources of production of infected
aerosols with far-reaching airflows, the prevalence of asymptomatic
transmissions highlighted the importance of social activities. Gathering around
a table, a common scenario of human interactions, not only fixes a typical
distance between static interlocutors, but influences airborne transmission, by
serving both as a flow diverter and a surface for droplet deposition. Here, we
use high-fidelity large-eddy simulations to characterize short-range airborne
transmission when two people face each other at a typical table. We show that
compared to the natural distance travelled by free buoyant puffs and jets, the
distance between the table and the emission constitutes a new length scale that
modifies downward exhaled flows, common during nose breathing, speech, and
laughter. When the table is close to the emitter, its main effect is to
restrict the forward spread of emitted particles. However, if the distance
between individuals is too short, particles reaching the recipient become more
concentrated, rising transmission risks. Additionally, simulations of forceful
exhalations, like laughter, demonstrate that the table acts as a filter that
collects medium-sized particles that would have remained in the free jet
otherwise, but can in that case be involved in the fomite transmission route.
The table introduces a cut-off size for particles that depends on the inertia
of the exhaled material, thereby modifying the size distribution of particles
suspended in the air.
