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.