World Monitor Magazine, №1/2020 WM_March 2020_FOR WEB (12.03.) | Page 46

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FIGURE 5. Filtering effect for small droplets (aerosols) by various masks; home-made of tea cloth, surgical mask (3M “Tie-
on”) and a FFP2 (N95) respirator mask. The numbers are scaled to the reference of 100 (source of droplets) for illustrative
purposes, calculated from the PF (protection factor) values in Table 2 of van der Sande et al, 2007. Measurement was per-
formed with a Portacount counter that registers particles in the air with sizes in the range between 0.02 and 1 micrometer
at the end of a 3-hour wearing period with no physical activity. The number for the protection are medians of 7 (or 8) adult
volunteers per group. Protection at the beginning of the test was similar for the Tea Cloth and Surgical mask, but for FFP2
the protection was double. Children experienced substantially less protection (see van der Sande et al 2007)
the nasopharynx can be stopped by any physical barrier, such
as simpler surgical or dust masks.
Of course many aerosol droplets in the exhalation or cough
spray may not contain the virus, but some will do. In the case
of the SARS-Cov-2 virus it is not known what the minimal
infectious load is (number of viral particles needed to start
the pathogenesis cascade that causes a clinical disease). But
we begin to appreciate whether the small aerosol or large
projectile droplets are more relevant.
The tacit notion at the CDC that the alveolae are the destina-
tion site for droplets to deliver the virus load (the alveolae are
after all the anatomical site of life-threatening pneumonia),
has elevated the apparent importance of N95 masks and led
to the dismissal of surgical masks. Nuances do not translate
to the lay people (as well as many arm chair experts) who
now, owing to message binarization, think that masks are
useless.
Even with respect to the small aerosols we must not forget
that the partial filtering provided by surgical masks is better
than nothing. In an experimental simulation of the filtering
capacity of masks in 2008, van der Sande and her colleagues
in the Netherlands compared the ability of three masks: (i)
home-made (DYI) of tea cloth, (ii) standard surgical masks
and (iii) FFP2, the European equivalent of N95 masks, with
respect to their ability to stop small aerosols in the range 0.2
to 1 um –droplets that reach the lower lung.
What the authors found for inward protection warrants some
questioning of CDC’s message that surgical masks are “not
effective”: While FFP2 (or N95) masks indeed filtered out
>99% of particles (thus, reducing the aerosol load by 100-
fold), the surgical masks lowered the number of aerosol drop-
lets behind the mask still by a substantial 4-fold compared to
outside of the mask. It is plausible that for larger spray drop-
lets from cough expulsions the difference between surgical
masks and the F95 respirator masks would be even smaller.
Interestingly, for outward protection, the effectiveness and
differences are much smaller (see numbers in the FIG. 5).
These results raise the urgent question: If all we want is to
mitigate the pandemic, that is, to “flatten the curve”, how
much does a 4-fold reduction of particles that reach the lungs
decrease transmission from person to person? Intuition sug-
gests that even an imperfect mask may offer some protection
that is at least in the range of the recommended separation
by more than 6 feet in social interactions or wasing hands
or not touching your face — all recommendation based on
mechanistic plausibility without strong epidemiological sup-
port.