Some Consideration About Mask Penetration: Inuence of Material Properties, Particle Size and its Morphology As Well As Particle Physical Quantities

In this work composition, size, morphology and other physical quantities dependency of the most widespread face respirators and surgery masks are investigated using different aerosol sources in the submicron size domain. The airborne ltration eciency of the sample materials are also demonstrated here. We have experimentally demonstrated high ltration eciency of NaCl, soot, DEHS and distilled water as well as airborne aerosols with limited size dependency. Using different mixing of distilled water and Sodium Lauryl Sulphate (SLS) surface tension dependency of ltration eciency have been also quantied. It is experimentally demonstrated that the ltration eciency is increased with increasing surface tension and the highest ltration eciency have been realised with salvia equivalent surface tension value regardless of the sampled respirators or surgery mask. Finally, we also experimentally demonstrated that respirators has higher ltration eciency than that of surgery mask and that the exhalation direction of the ow has negligible effect on ltration eciency in all samples.


Introduction
Triggered at the end of 2018, the current coronavirus disease  caused by the SARS-CoV-2 virus has spread over the whole planet within months. As a response of COVID-19 pandemic many countries applied severe lockdown including physical and social distancing regulations recommended or enforced by their national health authorities and politics. The person to person transmission and touching the contaminated surfaces are de ned as the primary transmission modes of viruses by The World Health Organisation (WHO) [1,2]. The rapid spread of respiratory uids and salvia including virus in distance of meters is also critical [3]. However, other alternative tracking such as airborne transition can also play an important role in virus spread too [4,5]. However, despite of its importance, the corresponding details of transition methods and e ciency as well as the e ciency of respiratory and surgery mask usage are not completely de ned yet [5,6].
The respiratory droplet transition have a typical diameter of 5 µm or larger [7]. In this regime, the transition tracking is de ned dominantly by the uid dynamics. However, the lifetime of such particles in the atmosphere is short. Therefore, besides the direct inhalation, the transition occur via deposition of virus containing respiratory uids on surfaces is dominant in this regime as well [8]. In airborne transition, the virus can be transported by itself with its characteristic diameters of about 200 nm. The airborne tracking of that virus sized aerosol sample driven mainly by diffusion. Alternatively, it can also be transferred by attachment of solid or embedded in liquid particulates dispersed in the air [9]. In this regime the particle tracking is driven by pure diffusion or uid dynamics or in other complicated ways of those.
The regime of airborne transition of virus is typically below 5µm [8]. However, the transport phenomena, the dynamics and the lifetimes of virus containing airborne particles may also depend on the size sub ranges inside that regime. In airborne transition the submicron region is also critical, not only due to its pandemic relevance but also the diverse health effect of air pollution too.
For prevention of COVID-19 transmission, besides physical distancing and hand hygiene, wearing face mask is suggested and enforced by the national health authorities and political governments in many countries. The surgery mask certi ed by different test methods like European standard EN 14683 and the lter face respirators certi ed according to test procedure of EN 149 (FFP respirators series) and §84.171 (KN respirator series). Those are primarily prescribed to wear for health workers. For general public usage, simple fabric mask with questionable ltration e ciency is at least demands in most countries [10]. However, mainly, because of the rm belief in its higher ltration e ciency and after the massive demands of medical mask or face respirators in the rst few months of pandemic period caused shortage of supply of such devices, more and more surgery masks and face respirators have been appearing in the street too. Moreover, the resident of many countries wear surgery or face respirators or even fabric masks because of the poor air quality in their countries to protect themselves from adverse health effects related to air pollution too [11,12]. It is also worth to note, that presumably because of the mask wearing, the number of medical contact associated to lung diseases has dramatically decreased in COVID-19 pandemic period. In airborne particle assembly, the major health relevant composite is the soot (IPCC, 2007). Although the soot of airborne particle is not dominant in mass, but in number it is responsible for almost all air quality relevant adverse health effects [13]. The soot originating from the incomplete combustion of fossil fuels having versatile physicochemical properties. The soot having characteristic diameter below submicron size domain with fractal morphology even have one week lifetime in the atmosphere [14]. Due its small characteristic size, high adsorption capabilities for toxic substances and effective surface area, the soot can penetrate through even in alveolus and get directly into blood too. Since the deposition map of the lungs is strongly depends on the size and morphology (especially below µm size domain) the investigation of size, morphology, composition and other physical parameter dependency of mask penetration is critical issue in the context of its airborne ltration e ciency as well.
Removal of aerosol from an airstream is mainly driven by ve different physical mechanism including gravitational settling, internal impaction, interception, diffusion and electrostatic attraction [15]. They are all shows size dependency. The rst two is more e cient in particle settling towards the smaller size, while, the latter three shows increasing e ciency with increasing size. Besides all, the test protocols for surgery or face respirators [16, 17, and 18] do not specify the size dependency of ltration but cumulative e ciency. They are also use atypical aerosol standards such as NaCl or para n oil for test protocols as well as atypical measurement condition such as constant ow with high ow rate which is far from the realistic breathing conditions. Since the penetration could also depend on other physical quantities such as viscosity, surface tension, morphology or phase of particles, the more detailed investigation of ltration e ciency even in a sub micrometre size domain is actual issues having even pandemic or more comprehensive interest todays.
In this work we demonstrate the comprehensive, description type measurement results of size dependent ltration e ciency of three widely used mask or face respirators including surgery mask, FFP2 and KN95 respirators in the submicron size domain. Using different type of aerosol standards, the phase, the morphology and the surface tension effect on size dependent ltration e ciency are also investigated. Finally, the results of size dependent airborne ltration e ciency is also demonstrated here.

Experimental Set-up, Methods And Sample
Three simple experimental con gurations for comprehensive investigation of three different type of mask samples were used in this work (Fig. 1). The set-up presented schematically in Fig. 1a adopted two type of liquid nebuliser successively. For generating NaCl and water aerosol standards TOPAS ATM 220 liquid nebuliser was used, while for producing monodisperse liquid aerosol in the higher size range condensation based technique based on the Sinclar Lamer principle was applied (model: MAG3000, Palas, GmbH). In this procedure, NaCl aerosol was used to initiate the DEHS (Di-Ethil-Hexil-Sebacate) condensation onto its surface resulted in monodisperse aerosol assembly in the most relevant accumulation mode of the atmosphere. In this con guration diffusion dryer was used to stabilise the yield of particles. For generating soot fractal aggregates spark discharge generator (model: DNP2000, Palas, GmbH) was applied (Fig. 1b).
In this con guration two ejector diluter were implemented into the set-up to avoid the excess of concentration limit de ned by the SMPS. For the investigation of airborne relevance of ltration e ciency ambient aerosol was used which entered the room through open window of the laboratory ( Fig. 1.c). The aerosol standards were drawn through the sample xed onto a cylindrical ange. The characteristic parameter of the ange were ID=40 mm and length of 200 mm respectively. The ow was maintained by the SMPS pump ensuring the atmospheric pressure condition in this con guration.
The number concentration and the size distribution of the emitted particle assembly were measured by a Scanning Mobility Particle Sizer (SMPS, TSI Inc., model: 3938) The SMPS consist of two parts. A differential mobility analyser (DMA) which is responsible for the size separation of the sampled particle assembly (DMA, model 3081, with Kr85 neutraliser model 3077). The Condensation Particle Counter (CPC, model 3752) is responsible to count the separated particles in this con guration. The sheath and aerosol ow rates were set to 2.3 and 0.23 litre per minute, thus the SMPS measured the size distribution over the range of 16-930,6 nm with resolution of ±0.5 nm at 16 nm to ±30 nm at 930 nm. In order to minimize the shielding artefact, coinciding correction of the measured data was also applied. This technique provides mobility particle diameter (d p =d mob ). This parameter set was used throughout this work. Both in the  Table 1. The aerosol standards using this study is including NaCl, distilled water, DEHS, soot and surface tension modi ed distilled water using Sodium Lauryl Sulphate (SLS). patents as well as a surgery mask using clinical circumstances de ned by EN 14683. From all mask and respirator materials tted area was cut, prepared and implemented onto the ange. In order to minimize the mask material uncertainty each measurement was repeated three times using freshly mounted different samples from the same mask type for all aerosol standards. Overall, the measurement cycle was the following: 30 minutes measurement of ange transition (6 scan in SMPS), than 30 minutes measurement of lter transition, nally 30 minutes measurement of ange transition. These are repeated at three times of each freshly mounted sample. As the measurement uncertainty 1-sigma standard deviation of the average was calculated.

Results And Discussion
The size distribution and its characteristic parameters measured with empty ange and with lter material in both inhalation directions (in and exhalation) using different aerosol sources are shown and summarised in Fig. 3 to 6. and Table 2 respectively. The related ltration e ciency as a function of particle diameter deduced from the measured data are also presented in those gures (left side). The NaCl is the dominant aerosol standard for verifying the ltration e ciency [16,17]. The size distribution of empty ange and ange with lter material both in and exhalation directions using NaCl aerosol shows signi cant ltration e ciency regardless of the type of mask and direction of inhalation throughout the emitted particle size range (Fig. 4.).
Take into account the two order of differences in transmission (relative to the empty ange transmission) moderate size dependency of ltration e ciency have been found in all samples (Fig. 3). The maximum cumulative ltration e ciency of 6% and 5% are de ned in FFP2 and KN95 respirators. In the dominant size range in number-concentration (below about 200nm) the FFP2 and KN95 respirators shows negligible differences in transmission (< 0.2%) regardless in the ow direction. Although at the higher size of the particles (> 200 nm) the differences are increased it is remained below 1% regardless of the direction of ow. However, it is worth to note that above 200 nm the number concentration of the emitted particulate is below 1% (zoomset of Fig. 3.). The surgery mask shows smaller but also signi cant ltration e ciency without negligible size dependence. The ltration e ciency is below or about 6% even in the higher size range. The cumulative ltration e ciency of surgery mask is below 2%, which is much better than the de ned value of the respiratory protocols. The deduced transition function of empty and sample material implemented ange in bot ow direction shows also high e cient ltration when spark discharge generated soot particle assembly is used (Fig. 3.). Smaller than 2% cumulative ltration e ciency have been realized in the whole emitted size domain regardless of the type of mask or ow directions. However, KN95 respirator shows signi cant differences in ow direction resulted in U-shape curve (in both ow directions) (Fig. 3). Nearly one order of magnitude higher ltration e ciency has been realised in exhalation than in the reversed ow direction. Although this deviation is signi cant the e ciency of soot ltration is remarkable and ful l the patent requirement not only in cumulative aspect but also in any size sequent even in both ow directions. The FFP2 sample shows high similarity in cumulative e ciency of ltration and also that of size dependency resulted in a similar U-shape (below 3%). However, in exhalation ow direction bimodal size distribution have been measured in FFP2 mask sample. Due to its unexpected measurement results, the whole measurement cycle was repeated at three additional freshly mounted sample, but provides the same distribution curves in all measurement cycle. The surgery mask sample provides also high e cient ltration of soot particles having cumulative ltration e ciency below 3%, but with decreasing tendency towards the higher sizes with the minimum of about 200-300 nm size domain.
The NaCl particles have a compact morphology with sharp lamella-like boundary conditions, while the soot has a morphology of fractal aggregates including spherical primary particles with irregular spatially arrangement [14]. The differences in morphology in the overlapping characteristic size domain makes possible to investigate the morphology dependence of the ltration e ciency. From the measurement data one can conclude that morphology cannot affect the ltration e ciency of the investigated mask sample in the 20 nm to about 600 nm size domain. Moreover, since the combustion generated soot fractal aggregates are responsible many divers health effects, the high e ciency in soot ltration highlight the advances of mask wearing even over the pandemic period too. The liquid phase ltration e ciency were modelled with DEHS aerosol condensed onto a salt core particle and distilled water aerosol assembly.
The size distribution of transmission and the size dependent ltration e ciency of DEHS are shown in Fig. 4.
Using DEHS particle emission all the investigated sample provides high e cient ltration throughout the emitted size domain for all type of masks and for both ow directions. In all con gurations less than 1% of transmission e ciency was realised with this aerosol. It is also provides distribution of transmission modelled with bimodal distribution function in inhalation direction in case of KN95 sample. However, it also worth to note, that due to the extremely high ltration e ciency, it is not affect anyhow the ltration behaviour of the sample. Surgery mask provides also remarkable, but worse ltration e ciency for DEHS aerosol too (Table.2). Some microphysical properties of the respiratory uids and salvia were modelled at rst approximation with distilled water aerosol generated by liquid nebuliser (Fig. 4). Using distilled water aerosol high e cient ltration e ciency was realised too. Both respirators provide less than 1% transition throughout the emitted size domain, while surgery mask provides less than 2% cumulative ltration e ciency with worst value of that about 5% at around 200nm size. Using water aerosol source we have also realised measurable differences in ow direction in some cases. However, besides high e cient ltration of water, it is not affected the quality of the investigated materials anyway. The surface tension of salvia is differ from that of distilled water. Therefore to investigate the surface tension affect for ltration e ciency and for approximate the measurement condition to more realistic circumstances emulsion of distilled water and Sodium Laurate Sulphate (SLS) was created at two different mixing ration (100 ppm and 200 ppm SLS). The salvia equivalent surface tension was realised by 200 ppm SLS mixing rate. The cumulative ltration e ciency of distilled water (ST=71.99 mN/m), 100 ppm (ST=65,33 mN/m) and 200 ppm SLS (ST=48.93 mN/m) mixing are shown in Fig. 5. The surface tension dependency of cumulative ltration e ciency is clearly shown that with increasing surface tension the ltration e ciency is increased.
The highest ltration e ciency is realised at 100 ppm and 200 ppm SLS sources which is more or less is the same as surface tension of salvia (ST~60 mN/m) [19].
Finally, ambient aerosol sources was also used to mimic the airborne transition of mask samples. The size of the sampled airborne particles fall between 10 nm and 950 nm diameter. The measurement results is shown high ltration e ciency for all type of mask and ow directions (Fig. 6). The cumulative ltration e ciency of less than 1% have been realised with moderable size dependency throughout the whole emitted size range in all samples.

Summary And Conclusion
The respective transition of number concentration and the size distribution of different aerosol sources covered the submicron size domain were measured at three different widely used respirators such as FFP2 and KN95 and surgery mask de ned by EN 149, §84.171 and EN 14683 respectively. The ex and inhalation direction of ow effect on transition was also investigated. Finally, airborne aerosol sources was used to measure the ltration e ciency in the air quality relevant submicron size domain.
Applying different sources makes possible to investigate the size, phase, surface tension and viscosity dependency of ltration even in the overlapped size domain. Using NaCl particles which is the most relevant aerosol standards for quantifying ltration e ciency and the spark discharge generated soot aerosol allow the investigation of morphology dependency of ltration. The compact sharped layer bounded NaCl aerosol provides the similar ltration e ciency than that of soot particles having fractal-like morphology regardless of the mask types and ow directions. Therefore, one can conclude, that the morphology of the ltered aerosol is not affected the ltration e ciency in this regime. It is experimentally demonstrated that liquid phase aerosol sources having different microphysical properties provides different ltration e ciency. The distilled water aerosol source was used to model more realistic physical parameters of respiratory uids or salvia. All the investigated mask or respiratory materials shows high e ciency of ltration for water aerosol in the investigated size domain. Although liquid aerosols having different viscosity were tested in this study, the correlation between the viscosity and ltration e ciency was not realised. However, the measurement of particles transition generated from distilled water and SLS mixing at different SLS mixing rates shows de nite surface tension dependency on ltration. The higher ltration e ciency have been reached with 100 ppm and 200 ppm SLS concentration which is well modelled the real surface tension condition of salvia (~ 60 mN/m). It is also experimentally demonstrated here that although the different aerosol sources have spread from around 10 nm to 950 nm size domain with overlapping subdomains the size dependency of ltration e ciency for the whole size range cannot be quanti ed in an additive way, because of the signi cant, phase, composition and other microphysical dependency of those.
Finally, both the arti cially generated soot and the ambient transition measurement have also veri ed experimentally that both the respirators and the surgery mask are also a useful tool for airborne relevant health diseases protection and the usage of those goes beyond those pandemic relevance. Overall, the ltration e ciency of the surgery and face respirators is depends strongly not only the composition of the aerosol standards but its phase and other microphysical parameters too. Therefore, the pandemic relevance of its usage quanti ed by the modelled aerosol standards and measurement protocols are needs more detailed investigations. However, all type of mask provides remarkable ltration of e ciency of the airborne particles in the health relevant submicron size region. Therefore, the usage of face respirators or surgery masks in the ambient having poor air quality is useful for protection of health diseases associated to air pollution too.

Declarations
Experimental set-up for ltration e ciency measurement of different sample materials: (a) the experimental set up using two type of liquid nebulizer to generate, NaCl, DEHS, distilled water and para n oil aerosols, (b) experimental setup for generating spark discharged soot aerosol assembly, (c) experimental set up for ambient relevance measurement of ltration e ciency.

Figure 2
Size distribution of different aerosol sources drawn through the ange without lter material.      Measured size distribution of different samples for airborne aerosols.