Fluid dynamical pathways of airborne transmission while waiting in a line

Waiting in a line (or a queue) is an unavoidable social interaction that occurs frequently in public spaces. 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 flow patterns in a simplified waiting line setting. From observations of the transport of breath-like expulsions, we reveal the presence of fluid dynamical counter-currents -due to the competing effects of line kinematics and thermal gradients. Depending on the walking speed, an intermediate temperature range can potentially heighten the infection risks by allowing the breath plume to linger; however, colder and warmer ambients both suppress the spread. Current guidelines of increasing physical separation appear to have a limited impact on reducing aerosol transmission. This work highlights the need for updated transmission mitigation guidelines in settings where physical separation, interaction duration, and periodicity of movements are factors.