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Riboflavin based setup as an alternative method for a preliminary screening of face mask filtration efficiency – Scientific Reports

With the Sars-CoV-2 virus diffusion, considering the significative reduction of virus transmission associated with the mask wearing, great attention was focused on new masks development as well as on the mask filtration performance assessment15,16,17,18. In this context, the aim of proposed study is to develop a new tool for a green, easy and rapid preliminary screening of mask and related materials fabrics filtration efficiency. In particular, the present method should be used to distinguish between masks or materials (i.e. fabrics, tissue non-tissue or their combinations) with high (i.e. > 90%) and low (i.e. < 90%) filtration efficiency, therefore, not to replace the regulation approaches.

The proposed setup is composed of a commercial aerosol generator commonly used for aerosol therapy, custom components, such as the aerosol chamber and the sample holder fabricated using 3D printing as a labware prototyping technology, collection filters, a standard glass filtration assembly and a vacuum pump with a flow rate of 28.3 L/min. In addition, the airborne transmission is simulated using a riboflavin aqueous solution which is an autofluorescent, eco-friendly and easily-to-handle biomolecule solution. The pore size, 5 µm, of the collection filter employed for riboflavin detection downstream of the mask sample is a key aspect of the proposed setup because pores of lower diameter did not allow to maintain the right pump flow rate (i.e. 28.3 L/min) during the test and higher pore size could cause loss of riboflavin solution and therefore of detected information.

Moreover, preliminary experiments (data not shown) were conducted in order to assess the extraction efficiencies of riboflavin from the collection filter in 30 min of continuous agitation demonstrating that the filter is able to release the total collected riboflavin amount when submerged in water. As indicated in UNI EN 14683:2019, each test was carried out by generating the aerosol for 1 min while the vacuum pump was active for two minutes with the recommended flow rate of 28.3 L/min. The sample size (12.5 cm2), which is strictly correlated to the available glass filtration assembly (diameter 47 mm), combined with 28.3 L/min of flow rate generates a face velocity of 40 cm/s with respect to a maximum of 10 cm/s indicated by regulation. Schilling et al. (2021) investigated the role of face velocity in filtration efficiency demonstrating that increased face velocities (up to 25 cm/s) resulted in a reduced filtration efficiency, but faster flow rates may provide more potential to discern between masks19. Moreover, in FDA-BFE, only the flow rate of 28.3 L/min without sample dimensions specification is reported20.

Surgical masks can be classified as type I for BFE values ≥ 95% and type II or IIR for BFE values ≥ 98%. Among the tested masks of this study, only mask A is compliant with the UNI EN 14683:2019. The same result was obtained for the filtration efficiency assessed using the proposed riboflavin-based setup. Aerosol particles are blocked by masks according to different mechanisms, such as straining, inertial impaction, interception, diffusion, and electrostatic attraction21. However, filtration efficiency is strongly correlated with mask fiber diameter and porosity. The high filtration efficiency of mask A (i.e. 97.5% as BFE or 99.2% as Riboflavin filtration efficiency) is related to the meltbown layer which is composed of micro- and nano-fibres with fibre density clearly higher than the other two exterior layers22. Mask D inner layer is composed by microfiber of lower diameter respect to the same layer of mask A but the fiber diameter combined with the higher thickness (about 400 µm) of the layer which corresponds to layer thickness used in commercially available FFP2 mask23, allowed obtaining good filtration efficiency also for the mask D (i.e. 91.4% as BFE or 93.9% as Riboflavin filtration efficiency).

Despite BFE test represent the standard for filtration performance assessment, several scientific questions remain unanswered and possible modernization of the test methods should be considered such as the mask fits on the face to prevents leakage24,25.

In literature, there are studies focused on alternative methods to BFE for filtration efficiency assessment especially on the use of inert particles instead of biological ones17,26,27,28,29 but only few of them considered the comparison with BFE1,11,17. In this study, the comparison between the filtration efficiency data obtained by both methods on the same samples was performed with the aim to determine if BFE could be predicted from riboflavin filtration efficiency. If only the average filtration efficiency is considered, the proposed method allowed to obtain data quite similar to the BFE. Riboflavin filtration efficiency is higher than the BFE for all tested samples with an average difference of 2.1%. Despite this, no statistically significant differences were observed for the mask A, B and C while the data obtained for the mask D are statistically significant (p < 0.05) confirming that riboflavin filtration efficiency can predict BFE but not replace it.

He et al. in their study reported that the filtration efficiency of surgical mask is affected by aerosol particle dimensions17. In the present study, riboflavin aerosol particle size is more distributed with respect to the S. aureus aerosol (0.9–5.1 μm vs 3.0 ± 0.3 μm). However, the combination of riboflavin aerosol with higher face velocity respect to BFE allowed obtaining data quite similar to regulation approaches.

Moreover, the aerosol chamber of the proposed setup has a lower length respect to the chamber described in UNI EN 14683:2019. The reduced distance between aerosol generator and sample could be responsible of high measured value of filtration efficiency.

The proposed riboflavin-based setup has the main advantage of using the aerosol of a natural, safe eco-friendly, and easy-to-handle solution of riboflavin respect to the bacteria aerosol required by UNI EN 14683:2019. In addition, the riboflavin detection downstream of the mask sample can be performed using a filter and a plate reader respect to the complex equipment required by setup which employs solid or inert aerosol particles17,26,27,28,29. This approach can be beneficial also during materials manufacturing, before mask assembly, to quickly evaluate if the fibre network and the filtrating power is ensured for every batch, allowing prompt corrective actions when needed.

Considering the good correlation between the averaged values of riboflavin filtration efficiency and BFE, the proposed method can be used as an alternative method to BFE to predict filtration efficiency for masks with high (> 90%) aerosol retention capacity such as masks A and D as well as for masks with lower (< 90%) retention capacity such as masks B and C. In particular, the riboflavin-based setup can be employed for a preliminary screening of masks and related materials. In conclusions, the proposed setup can be easily implemented purchasing on the market the riboflavin tabs at low price as well as the other components of the setup are easily accessible. Moreover, the riboflavin filtration efficiency assessment is more rapid than BFE, 30 min vs. 24 h, and can be performed in general lab without the needing of biosafety laboratories and specialised operators.

More investigations should be performed on samples which have values of riboflavin filtration efficiency similar to the cut off of 95% and 98% indicated by UNI EN 14683:2019 for type I and type II surgical mask, respectively. As BFE assessment, we evaluated the filtration efficiency on mask material without considering the face mask fitting which is crucial for real protection. An improvement of the proposed setup could consider face mask wearing simulation like in standard EN 149.