Transmission of the SARS-CoV-2 Virus

The transmission of SARS-CoV-2 virus is regarded to take place mainly through droplets generated when an infected person coughs, sneezes, or speaks. It is commonly considered that these droplets are too heavy to remain suspended in the air and fall quickly on to grounds and surfaces. On this basis, the infection is considered to take place by breathing in the virus in the vicinity of an infected person (typically within 1 metre) or by contact with contaminated surfaces and then touching eyes, nose or mouth. However, the recent study of Wuhan hospitals(1) reports the airborne virus cluster size is in the range 0.01 to 10 µm, with the mode of the distribution in the range 0.25 to 1 µm, depending on the hospital zone. This strongly suggests that SARS-CoV-2 virus could spread primarily through dispersion, as an aerosol which travels through air at a size of less than a few micrometres. For the fines fraction corresponding to the mode of the size distribution reported above, there are currently no masks commonly available to the public which can filter out these aerosols efficiently. In laboratories, particle technologists use FFP1, FFP2 and FFP3 grade filters when handling benign powders. However, all three FFP grade masks have low filtration efficiency for the particle size range around 0.09-0.16 µm(2). There is extensive literature on filter performance evaluation, and as commonly available filters are all fabric filters, they all suffer from poor performance for the most penetrative particle size. This depends on the filter material, but is typically in the sub-micrometre range. So, considering the average size of a single virus, measured by transmission as well as by scanning electron microscopy(3), ranging from 0.08-0.22 µm, they could penetrate through filters even when in clusters if they are suspended in air.