Filtration performance of certified face masks with respect to usage in the COVID-19 pandemic context

Abstract

During the COVID-19 pandemic, the wearing of certified respiratory masks emerged as a key protective measure against direct and indirect infections from virus-laden aerosols. However, standardised test procedures lack relevant aspects to evaluate the filtration performance with respect to respiratory aerosol particles. The presented work deals with two of these under-represented factors, namely the filtration efficiency depending on the nature and size of exhaled aerosol particles and the filtration performance associated with facial leakage considering a differentiated distinction between self-protection and third-party protection. Therefore, the fractional efficiency and the net pressure loss are experimentally determined within a screening of different surgical masks (DIN EN 14683 [1]) and filtering face pieces (FFP2, FFP3) (DIN EN 149 [2]). While the certification of surgical masks and filtering face pieces either uses bacteria-laden aerosol particle distributions with a mean diameter of 3 µm (DIN EN 14683) or liquid paraffin oil droplets and solid-phase sodium chloride particles (DIN EN 149), that neither represent the species nor the size of exhaled droplet nuclei, we use aerosol particles generated from artificial saliva. A comparison of both artificial and actually exhaled aerosol particles shows that the modes of the particle size distributions are in the order up to 0.4 µm and the size distributions are similar with most particles smaller than 2 µm. In the frame of mask performance evaluation, an additively manufactured test head based on ISO/TS 16976-2 is used to vary the flow direction and fit of the different mask models. In the self-protection arrangement without facial leakages, all investigated samples deposit by count more than 85% of artificial saliva aerosol particles, whereas in third-party protection most masks tend to show similar efficiencies but lower pressure losses. This deviation tends to be significant primarily for masks with thin filter layers like surgical masks or nano-fiber-containing filtering face pieces and is shape-dependent. Taking facial leakages into account, both filtration efficiencies and pressure losses are strongly interdependent and significantly lower. The total filtration efficiencies range between approximately 30-85%, indicating a much greater influence of the fit of the mask than the filter material. Furthermore, the net pressure loss varies at similar filtration efficiencies between different masks, which points out the influence of the material and the filter area on pressure loss. In natural conditions of use, it is generally found that masks tend be more effective in self-protection than in third-party protection, but this is inversely correlated to the pressure loss.

Full text

FILTRATION PERFORMANCE OF CERTIFIED FACE MASKS WITH
RESPECT TO USAGE IN THE COVID-19 PANDEMIC CONTEXT
Simon Berger, Marvin Mattern, Jennifer Niessner
Institute for Flow in Additively Manufactured Porous Media (ISAPS)
Heilbronn University of Applied Sciences
Max-Planck-Straße 39 – 74081 Heilbronn - Germany
ABSTRACT
During the COVID-19 pandemic, the wearing of certified respiratory masks emerged
as a key protective measure against direct and indirect infections from virus-laden
aerosols. However, standardised test procedures lack relevant aspects to evaluate the
filtration performance with respect to respiratory aerosol particles. The presented work
deals with two of these under-represented factors, namely the filtration efficiency
depending on the nature and size of exhaled aerosol particles and the filtration
performance associated with facial leakage considering a differentiated distinction
between self-protection and third-party protection. Therefore, the fractional efficiency
and the net pressure loss are experimentally determined within a screening of different
surgical masks (DIN EN 14683 [1]) and filtering face pieces (FFP2, FFP3) (DIN EN
149 [2]).
While the certification of surgical masks and filtering face pieces either uses bacteria-
laden aerosol particle distributions with a mean diameter of 3 µm (DIN EN 14683) or
liquid paraffin oil droplets and solid-phase sodium chloride particles (DIN EN 149), that
neither represent the species nor the size of exhaled droplet nuclei, we use aerosol
particles generated from artificial saliva. A comparison of both artificial and actually
exhaled aerosol particles shows that the modes of the particle size distributions are in
the order up to 0.4 µm and the size distributions are similar with most particles smaller
than 2 µm. In the frame of mask performance evaluation, an additively manufactured
test head based on ISO/TS 16976-2 is used to vary the flow direction and fit of the
different mask models. In the self-protection arrangement without facial leakages, all
investigated samples deposit by count more than 85% of artificial saliva aerosol
particles, whereas in third-party protection most masks tend to show similar efficiencies
but lower pressure losses. This deviation tends to be significant primarily for masks
with thin filter layers like surgical masks or nano-fiber-containing filtering face pieces
and is shape-dependent. Taking facial leakages into account, both filtration efficiencies
and pressure losses are strongly inter-dependent and significantly lower. The total
filtration efficiencies range between approximately 30 – 85%, indicating a much greater
influence of the fit of the mask than the filter material. Furthermore, the net pressure
loss varies at similar filtration efficiencies between different masks, which points out
the influence of the material and the filter area on pressure loss. In natural conditions
of use, it is generally found that masks tend be more effective in self-protection than in
third-party protection, but this is inversely correlated to the pressure loss.
KEYWORDS
Filtration Performance, Artificial Saliva, Masks, Filtering Face Pieces DIN EN 149,
Medical Masks DIN EN 14683, COVID-19