How shear and thermal buoyancy affect airflows and aerosol transmission in waiting lines (queues)

Waiting in a line (or a queue) is an important, often unavoidable social interaction that occurs frequently in public spaces like grocery stores, health clinics, or airports. Despite its wide prevalence and rich parametric variability, few studies have addressed the risks of airborne infection while waiting in a line. Here we use a combination of laboratory experiments and direct numerical simulations to assess the air flow patterns in a simplified waiting line setting. From observations of the transport of breath-like expulsions, we show the presence of fluid dynamical counter-currents in waiting lines, due to the competing effects of line kinematics and thermal gradients. This work highlights the need for updated transmission mitigation guidelines in settings where, in addition to physical separation, interaction duration and periodicity of dynamic movements are factors.