  The Covid-19 pandemic has focused attention on airborne transmission of
viruses. Using realistic air flow simulation, we model droplet dispersion from
coughing and study the transmission risk related to SARS-CoV-2. Although most
airborne droplets are 8-16 $\mu$m in diameter, the droplets with the highest
transmission potential are, in fact, 32-40 $\mu$m. Use of face masks is
therefore recommended for both personal and social protection. We found social
distancing effective at reducing transmission potential across all droplet
sizes. However, the presence of a human body 1 m away modifies the aerodynamics
so that downstream droplet dispersion is enhanced, which has implications on
safe distancing in queues. Based on median viral load, we found that an average
of 0.55 viral copies is inhaled at 1 m distance per cough. Droplet evaporation
results in significant reduction in droplet counts, but airborne transmission
remains possible even under low humidity conditions.
