No abstract

stimulate revision of the science in the light of current evidence. Six 'myths' are presented, explained and ultimately refuted on the basis of recently published papers and expert opinion from previous work related to similar viruses. There is little doubt that SARS-CoV-2 is transmitted via a range of airborne particle sizes subject to all the usual ventilation parameters and human behaviour. Experts from specialties encompassing aerosol studies, ventilation, engineering, physics, virology and clinical medicine have joined together to produce this review to consolidate the evidence for airborne transmission mechanisms, and offer justification for modern strategies for prevention and control of COVID-19 in health care and the community.

The occurrence of close proximity infection for many respiratory diseases is often cited as evidence of large droplet and/or close contact transmission. We explored interpersonal exposure of exhaled droplets and droplet nuclei of two standing thermal manikins as affected by distance, humidity, ventilation, and breathing mode. Under the specific set of conditions studied, we found a substantial increase in airborne exposure to droplet nuclei exhaled by the source manikin when a susceptible manikin is within about 1.5 m of the source manikin, referred to as the proximity effect. The threshold distance of about 1.5 m distinguishes the two basic transmission processes of droplets and droplet nuclei, that is, short-range modes and the long-range airborne route. The short-range modes include both the conventional large droplet route and the newly defined short-range airborne transmission. We thus reveal that transmission occurring in close proximity to the source patient includes both droplet-borne (large droplet) and short-range airborne routes, in addition to the direct deposition of large droplets on other body surfaces. The mechanisms of the droplet-borne and short-range airborne routes are different; their effective control methods also differ. Neither the current droplet precautions nor dilution ventilation prevents short-range airborne transmission, so new control methods are needed.

The influence of the human exhalation on flow fields, contaminant distributions, and personal exposure in displacement ventilated rooms is studied together with the effects of physical movement. Experiments are conducted in full-scale test rooms with life-sized breathing thermal manikins. Numerical simulations support the experiments. Air exhaled through the mouth can lock in a thermally stratified layer, if the vertical temperature gradient in breathing zone height is sufficiently large. With exhalation through the nose, exhaled air flows to the upper part of the room. The exhalation flow from both nose and mouth is able to penetrate the breathing zone of another person standing nearby. The stratification of exhaled air breaks down if there is physical movement in the room. As movement increases, the concentration distribution in the room will move towards a fully mixed situation. The protective effect of the boundary layer flow around the body of a moving person disappears at low speed, and is reduced for a seated person placed nearby due to horizontal air movements, which can also cause rebreathing of exhaled air for the seated person. The results indicate that the effect of the exhalation flow is no acute problem in most normal ventilation applications. However, exhalation and local effects caused by movement may be worth considering if one wishes to contain contaminants in certain areas, as in the case of tobacco smoking, in hospitals and clinics, or in certain industries.

As one of the major potential sources for infectious droplet nuclei in a hospital environment, exhalation flows of an infected patient can interact with the respiratory activities of other close individuals and with the room ventilation systems. Our latest results provide information on the penetration of exhalation jets into the ambient environment in different ventilation systems. This work is useful in identifying an appropriate and effective ventilation method for removing droplet nuclei more effectively, and thus minimizing the risk of cross-infections in hospital wards with multiple beds.