Airborne transmission of viral respiratory infections from an aerosol physics perspective

Airborne transmission of viral diseases is caused by infectious material being carried by droplets of different sizes through respiratory activities such as breathing, talking, singing, coughing and sneezing. For common activities such as breathing, talking and singing, these droplets leave the host at relatively low speeds, but the process is maintained for a long time. For coughing and sneezing, however, the droplets are thrown out at high speeds but with short durations. In the latter two cases, the droplets are surrounded by a strong turbulent jet that is capable of transporting the smaller droplets well away from the host. Large droplets will fall to the ground, while the smaller droplets may be readily transported by air currents.

This report assesses the mechanisms behind airborne transmission of viral respiratory infection from an aerosol physics perspective. Physical processes and concepts central to aerosol dispersion, mainly associated with Stokes numbers, fall times and evaporation, have been discussed to nuance the understanding of aerosol dispersion as a phenomenon. The report elucidates effects of various aerosol sizes and atmospheric conditions such as humidity and temperature, especially with regard to the aerosols’ fall and evaporation times. This is achieved by means of a literature review combined with aerosol physics modeling.

The literature review reveals several knowledge gaps that need to be filled in order to improve our understanding on how viral respiratory infections are transmitted through an airborne route. There are three main areas that stand out. The first is virus-specific empirical data, and includes topics such as how much infectious material is available per volume of droplet, how long it survives in dried states and on various surfaces, and which dosage is needed to be infected. The other area is empirical knowledge on respiratory droplets. Here, it is necessary to characterize the size distribution of droplets in different respiratory activities as well as to be able to determine solute and solid particle content. The latter is important to accurately model evaporation. The third area that has knowledge gaps is droplet and droplet nucleus transport modeling. In literature, simplified models have primarily been used to describe droplet transport, but the use of high-fidelity computational fluid dynamics models will yield more reliable results. Combined with good empirical data, high-fidelity computational fluid dynamics models will provide sound science for assessing various emergency response measures.

This report shows that respiratory droplets smaller than 50µm can remain suspended as droplet nuclei for a long time and thus be transported far away from the source. If the virus has a low infectious dose and long enough survival time, these droplets can be an effective route of infection, often referred to as airborne route of transmission. Droplets larger than 150 µm) have the potential to contain much more virus, but will rarely spread beyond 2 m by normal breathing, talking, singing or coughing. However, in the case of violent sneezes they can be spread farther away from the source. These droplets represent the so-called droplet/contact route of transmission. Whether medium droplets, typically in the size range 50 µm to 150 µm, constitute the airborne route of transmission or the droplet/contact route of transmission, depends on air temperature and humidity