Simulation-based suggestions for lockdown rules in dense urban areas considering indoor-outdoor droplet transmission under natural ventilation conditions

Lockdown rules have been arbitrarily applied to curtail respiratory disease outbreaks without being certain about indoor-outdoor droplet transmission mechanisms and associated infection risks, causing enormous economic losses and arousing social unrest. This study addressed these issues by proposing lockdown rules for dense urban areas using computational fluid dynamics (CFD) simulations of indoor-outdoor droplet dispersion originating from various source locations in single-sided and cross-ventilated spaces of a six-story building in idealized urban areas with plan area densities (λ_p) of 0.25, 0.44, 0.59, 0.69, and 0.82. The findings suggest issuing lockdown orders based on the relative locations of the patient and vulnerable persons. For example, droplets exhaled by a patient on the upper and middle floors can enter the apartments on one or two floors below under single-sided ventilation, while the droplets are transmitted to the same level downstream spaces by cross-ventilation. Although pedestrians and building occupants pose no infection risk to each other in urban areas with λ_p = 0.25, they are both exposed to high droplet concentrations in dense urban areas with λ_p = 0.69 and 0.82. Therefore, this study recommends area lockdowns for dense urban areas to protect residents and pedestrians from virus-laden droplets carried by indoor-outdoor droplet transmission.