A critical note on filtering-face-piece filtration efficiency determination applying EN 149
Introduction
Filtering-face-piece (FFP) respirators protect their wearer from inhaling harmful aerosol particles and limit the amount of exhaled particles in the close environment of the wearer. Especially during the current COVID-19 pandemic, filtering-face-pieces, surgical masks and homemade face-masks are used to reduce the risk of infection (Hao et al., 2020; O’Dowd et al., 2020; Maurer et al., 2021). In order to guarantee a specific minimum quality of filtering-face-pieces, there are several international standards, which specify testing methods and parameters. Development of such standards started at the beginning of the 19th century to protect miners from hazardous dust (Spelce et al., 2018). Today, filtering-face-pieces are tested for example regarding filtration performance, leakage, inhalation resistance, exhalation resistance, dead space, visual field, flammability, cleaning and/or disinfection, and skin compatibility. If a filtering-face-piece respirator pass the tests, it can be certified for example as FFP2 (EU), KN95 (China) or N95 (US) (DIN EN 149, 2009; DIN EN 13274-7, 2019; GB 2626, 2019; NIOSH – 42 CFR 84, 1995). A respirator does not need to fulfil the mentioned standards in order to be on the market. However, these standards guarantee a certain level of quality and laws and regulations may refer to them (Stintz et al., 2015).
There are many factors influencing the overall filtration performance of filtering-face-pieces in a specific test (or application) scenario, which can be divided in properties of the aerosol, design parameters of the filtering-face-piece and operational conditions. Aerosol properties, which influence the overall filtration performance are the type and material of aerosolized particles (Bałazy et al., 2006; Gao et al., 2015; Rengasamy et al., 2008), the particle size (Rengasamy et al., 2011b; Rengasamy et al., 2017; Serfozo et al., 2017b; Bałazy et al., 2006; Mostofi et al., 2011; Brochot et al., 2020), the electrical charge of the particles (Rengasamy et al., 2011b; Oh et al., 2002) and relative humidity (Mahdavi et al., 2015). Design parameters of the filtering-face-piece are pore size (Lawrence et al., 2006), fiber size and orientation (O’Dowd et al., 2020; Essa et al., 2021; Hao et al., 2020), fiber electric charge (Oh et al., 2002), thickness of the filtering face piece (Hao et al., 2020), geometrical design (Lei et al., 2013; Li et al., 2019) and the variation of filtering-face-pieces of the same type and model within the sample (CDC/NIOSH/NPPTL/RB, 2020). Operation conditions are the flow velocity (Mostofi et al., 2011; Gao et al., 2015), mounting conditions (Serfozo et al., 2017a), leakage (Rengasamy et al., 2011a; Grima-Olmedo et al., 2014; Lei et al., 2013), detachment of deposited particles (Qian et al., 1997; Fisher et al., 2012), the particle load of the filter (Mostofi et al., 2011; Kasper et al., 2009; Wang, 2001) and pretreatment (for example disinfection with steam or ethanol) of the filtering face piece (He et al., 2020; Tombolini et al., 2016; O’Dowd et al., 2020).
In (DIN EN 149, 2009) filtering-face-pieces are classified according to the total filtration efficiency of their filter media for a specified test aerosol, for example FFP3 masks have a filtration efficiency of 99%, FFP2 masks of 94% and FFP1 masks of 80%. The standard does not define explicitly if the number, mass or intensity (photometric) based filtration efficiency is meant, although the resulting values end up being entirely different. In literature, the total filtration efficiency is defined as the percentage of particles deposited in the filter media during filtration (Hinds, 2012) and it is differentiated between number, mass or intensity (photometric) based filtration efficiency, because particles of different sizes are weighed differently in the determination of differently-based efficiencies. Often, the total penetration is used to characterize filtration performance instead of the total filtration efficiency. The total penetration is the percentage of all particles which pass through the filter medium (Hinds, 2012), and it can also be based on different types of quantities.
The filtration efficiency is determined using NaCl and oil test aerosol separately (DIN EN 149, 2009; DIN EN 13274-7, 2019). The number based median diameter of the NaCl test aerosol should be in the range of 0.06–0.1 μm with a geometrical standard deviation of 2.0–3.0. For the oil test aerosol, the number based median diameter should be in the range of 0.29–0.45 μm with a geometrical standard deviation of 1.6–2.2. This range of allowed test aerosols result in a range of determined total filtration efficiency, which may cause that one and the same filtering-face-piece either passes or fails certification in dependence of the test aerosols particle size distribution applied.
This note investigates the range of determined total filtration efficiency values caused by the range of allowed test aerosol particle size distribution and different definitions of the total filtration efficiency, using size resolved penetration curves of FFP2 masks from literature and own measurements to calculate the total filtration efficiency for different test aerosol particle size distributions, thus emulating a multitude of different test conditions all within the range of the standard. Only oil test aerosol is regarded in this study.
Section snippets
Methods
Particle size resolved penetration curves from literature and from own measurements are used to calculate the range of total filtration efficiency/total penetration of oil test aerosol in dependence of the range of particle size distribution.
The particle size resolved penetration curves determined in this study are measured using a filter test rig (PMFT 1000, Palas GmbH, Karlsruhe, Germany) with electrical neutral oil aerosol ( = 19.5 mg/m³, DEHS, CAS-number 122-62-3, Palas GmbH, Karlsruhe,
Results and discussion
Total penetration/total filtration efficiency ranges were calculated for a variety of FFP2-mask size resolved penetration curves from literature and for penetration curves measured in this study. The size resolved penetration curves are shown in Fig. 2.
All penetration curves from literature (except of (OFI 2020a) and (OFI 2020b)) were measured using a differential mobility analyser and a condensation particle counter with an electrically neutralized aerosol and hence not in accordance to (DIN
Conclusion
Reliable classification of filtering-face-pieces is crucial to guarantee a certain level of protection for the wearer. The total filtration efficiency or the related total penetration of the filter media is one key parameter to characterize and classify filtering-face-pieces. There are several drawbacks in total penetration determination according to the standard. This study investigates two of these drawbacks, namely the lack of an exact definition of the total penetration and a too broad
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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