Short-range airborne transmission of respiratory infection

Abstract

A susceptible person experiences the highest exposure risk of respiratory infection when he or she is in close proximity with an infected person. The mechanism of exposure to droplets expired at close contact, however, remains unexplored. In this thesis, we aimed to investigate short-range exposure to respiratory viruses from an engineering perspective by examining airflow, droplet, and occupant characteristics.

An analytical model was developed to study the exposure to exhaled droplets during close contact (< 2 m) via both the short-range airborne and large droplet sub- routes. The short-range airborne route was found to dominate at most distances studied during both talking and coughing. The large droplet route only dominated when the droplets were larger than 100 μm and when the subjects were within 0.2 m while talking or 0.5 m while coughing. The smaller the exhaled droplets, the more important the short-range airborne route. The large droplet route contributed less than 10% of exposure when the droplets were smaller than 50 μm and when the subjects were more than 0.3 m apart, even while coughing.

Numerical simulations considering detailed facial membranes, including eyes, nostrils, and a mouth were carried out via computational fluid dynamics (CFD). We considered two scenarios (with two spheres and two human manikins), source– target distances of 0.2 to 2 m, and droplet diameters of 3 to 1,500 μm. The CFD results confirmed the predominance of the short-range inhalation route beyond 0.2 m for expiratory droplets smaller than 50 μm during talking and coughing. A critical droplet size of 87.5 μm was found to differentiate droplet behaviors.

A dynamic meshing method in CFD was used to simulate the head movement of a human-shaped thermal manikin. Droplets were released during the expiration periods of the source manikin, during which it was either not moving its head, was shaking its head or was nodding its head, while the head of a target manikin standing face-to-face with the source manikin remained motionless, as it was in listening mode. The target manikin had a high level of exposure to the source manikin’s respiratory droplets when the source manikin was not moving its head, whereas the target manikin’s level of exposure to these droplets was significantly reduced when the source manikin was shaking or nodding its head. People’s level of exposure to infectious pathogens via respiratory droplets in the close-contact conversational scenarios was found to be highly variable.

Finally, we considered the long-range airborne transmission as an extended short-range airborne route, which reconciled the link between short- and long-range airborne routes. The effective short-range distance was defined as the distance in short range at which long-range route had the same volumetric exposure value as that due to short-range route. A decrease in ventilation rate or room volume per person, or an increase in the ratio of the number of infected to susceptible people reduced the effective short-range distance. Effective environmental prevention strategies for respiratory infections require appropriate increases in the ventilation rate while maintaining a sufficiently low occupancy.