3.1. BFE and DP of new masks
The results of the BFE and the DP of the ten community face masks and the medical face mask when unused are presented in Fig. 3 (a and b). According to the EN 14683:2019 standard procedure, only the material constituting the masks was evaluated and leakage is not considered in this study. The horizontal dashed lines represent the EN 14683:2019 performance requirement; for type IIR (≥ 98% collection efficiency and ≤ 60 Pa.cm− 2 differential pressure), type II (≥ 98% collection efficiency and ≤ 40 Pa.cm− 2 differential pressure) and type I (≥ 95% collection efficiency and ≤ 40 Pa.cm− 2 differential pressure).
The results showed that all the masks were complaint with the breathability requirement for the various categories of medical face masks (type I, type II and type IIR) except one community face mask (i.e. CFM-B-3L) that was not in compliance with a type I or type II mask but in the limit of compliance of a type IIR mask. The medical face mask had the highest filtration efficiency of 99% and was compliant according to type II medical face mask standard. There was a variability in the filtration efficiency of the community face masks with the BFE ranging between 73% − 97%. Only 2 community face masks (i.e. CFM-A-2L and CFM-B-3L) had BFE exceeding 95%, the BFE requirement for Type I medical face mask. But all things considered, only one CFM (i.e. CFM-A-2L) is compliant with a type I medical face mask requirement, because the breathability of CFM-B-3L is well above the DP limit of 40 Pa.cm− 2. This result clearly show that a compromise has to be found between the BFE and the breathability to manufacture community face masks with excellent properties. In other words, the key technical challenge for manufacturers is to obtain community face masks with high filtration efficiency but without sacrificing their breathability.
The filtration of aerosol droplets using a face mask is governed by several mechanisms: impaction, interception, diffusion, and electrostatic attraction 9,23. The contribution of each mechanism to the filtration efficiency of a face mask depends on the materials used (porous structural differences), aerosol droplet sizes, and the operating conditions (temperature, humidity, and air filtration velocity). For aerosol droplets > 1µm, impaction and interception mechanisms are more important. For small particles < 0.1 µm, diffusion by Brownian motion is the dominant mechanism. When the mask material is charged, electrostatic forces contribute to particle capture especially for particles in the most penetrating particle size (MPPS) range of 0.1–0.5 µm (MPPS zone) 24 where no mechanism is dominant. For the average particle size of 3 µm required for the BFE, impaction and interception are the most dominant mechanisms.
The performance of the community face masks is influenced by fabric characteristics but the most influential characteristics are currently unclear25. Surface characteristics of the material used, such as the pore size, are important parameters that potentially influence the performance of the masks 26.
The representative microscopic images of the community face masks and the medical face mask are shown in Fig. 4. For brevity, only 3 out of the 10 community face masks are represented. Fibrous filter materials are usually comprised of fibers arranged in several ways. For non-woven materials, fibers are randomly oriented whilst woven and knitted materials contain yarns (bundles of fibers) that are interlaced to each other 27. The pores are formed at yarn interstices for the woven and knitted fabrics whilst they are formed by small spaces between individual fibers in non-woven filters 27. The spaces in between yarns were considered as the pores for the community face masks. Although the pore shape and size in community face masks were not uniform, we tried to extract quantitative information on the size of the inter-yarn pores by measuring the longest dimension of each inter-yarn pore using ImageJ software. The measurements provided an estimation of the size of an inter-yarn pore in each community face mask: around 150 µm, 330 µm and 900 µm for CFM-A-2L, CFM-E-3L and CFM-I-3L respectively. This could probably explain why CFM-A-2L had the highest filtration efficiency whilst CFM-I-3L had the lowest. Medical face masks are typically made up of 3 layers of non-woven polypropylene fibers (spunbound, meltblown, and spunbound layers). The pore size of the meltblown layer of the medical face mask are estimated to be around 20 µm 28,29. The small pore size of the meltblown layer compared to the different community face masks could possibly account for its higher filtration efficiency.
The number of layers of the mask, in this study, wasn’t the most influential parameter. CFM-I-3L which is a 3 layer mask had the lowest BFE whilst CFM-A-2L, a 2 layer mask, had the highest BFE. It seems in this case that layering fabrics with very high pore size doesn’t necessarily improve the BFE or DP.
Based on the results (Fig. 3), 4 categories of masks can be identified:
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Firstly, the medical face mask which has excellent BFE (> 98% (type II)) and low DP (≤ 40 Pa.cm-2) is compliant with type II medical face mask requirements.
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The CFM-A-2L, which has a good BFE (> 95% (type I)) and low DP (< 40 Pa.cm-2), that can be categorized as a type I medical face mask.
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The CFM-B-3L, which has a good BFE (> 95% (type I)) but a too high DP (≈ 60 Pa.cm-2), that cannot be categorized as a type I medical face mask since this good filtration efficiency was obtained at the expense of poor breathability properties.
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And lastly the 8 other community face masks which had inadequate BFE according to medical face masks requirements (70% < BFE < 95%) with correct DP (< 40 Pa.cm-2).
To be effective a mask needs to both filter out particles and allow a person to breathe easily. Producing community face masks typically involves a compromise between the BFE and DP and in some cases, having a high BFE comes at the cost of having a high DP leading to low breathability as seen for CFM-B- 3L. According to the results of the community face masks, we demonstrated that it is possible to have community face masks that perform similarly to a medical face mask. Indeed, out of our panel of 10 community face masks, only 1 met the BFE and DP requirements of a type I medical face masks, but could not achieve the type II requirements like the medical face masks chosen in this study. Community face masks are made to be washed and as this may alter the performances, the next part of the study seeks to evaluate the influence of the washing parameters. Only the community face masks that respected the BFE requirement for a type I (CFM-A-2L, CFM-B-3L) were chosen and compared to the medical face mask.
3.2. Influence of wash cycles on the performance of the masks
To evaluate the effect of the wash cycles, the masks were washed 10, 30 and 50 times at 60°C with the laundry detergent. The results of the BFE and DP are shown in Fig. 5. From the graph (Fig. 5b), it is observed that washing didn’t significantly impact the differential pressure of the medical face mask and the community face masks.
Concerning the community face masks, the washing cycles didn’t impact in a significant manner the BFE and thus they were able to maintain their performance up to 50 washes. This was in accordance to previous study by Sankhyan et al. 30, who found that community face masks could be washed 52 times without significant loss in particle filtration efficiency. For the medical face mask, the BFE decreased by 1% when the masks were washed but its BFE remained constant up to 50 washes. Alcaraz et al. 17 also observed a slight decrease in BFE of medical facemasks when washed but concluded that they could be washed up to 10 times without further degradation of the filtration or breathability properties. The reason for the decrease in efficiency when the medical face mask is washed is as a result of the loss of electrostatic charges which will be explained in section 3.4.
SEM images of the new and washed medical face mask (meltblown layer) and the community face masks are presented in Fig. 6. The new community face masks exhibited fiber bundles (yarns) that were globally intact with relatively smooth texture. After 10 washes, there was some liberation of individual fibers from the fiber bundles and there was some deconstruction of the individual fibers which increased slightly after 50 washes (Fig. 6a and b). This however didn’t seem to impact the performance of the masks, as despite the liberation and deconstruction, the fiber bundles remained globally intact. For the medical face mask, very few meltblown fibers exhibited breakages (Fig. 6c).
3.3 Influence of temperature on the performance of the masks
The effect of the wash temperature on the performance of the masks was studied by varying the temperature at 30°C and 60°C whilst the number of wash cycles was kept at 10 and detergent used for each wash. The results of the BFE and DP for the masks are shown in Fig. 7a and b.
With regards to the community face masks, the temperature didn’t seem to influence greatly their performances (BFE and breathability). The SEM images (see Supplementary Fig.S1 a and b) showed similar deconstruction levels of the washed fibers which is attributed to the mask being washed 10 times rather than the temperature. The fiber bundles were globally intact in all cases.
For the medical face mask, there was a decrease in the BFE of the washed masks compared to the new mask, however, the wash temperature didn’t seem to influence its BFE. The meltblown layer of the medical face mask is charged electrostatically by corona effect to increase particle collection efficiency. The charge stability can be affected by temperature. Liu et al. 31 subjected the electret meltblown layer to heat treatment at several temperatures at various times (1–24h) and noticed that below 70°C the effect on the filtration efficiency was minimal up until 24 hours of treatment but when the temperature was increased to 90 or 110°C, the filtration efficiency decreased significantly with the increase of the treatment time. They attributed it to the fact that higher temperatures led to higher charge escape/loss which subsequently led to a reduction in electrostatic effect. The temperatures studied in this work were not high enough to impact the charge stability of the electret layer and may explain why there was no impact on the BFE. SEM images (see Supplementary Fig.S1 c) also show that the temperature didn’t affect the fiber morphology. Finally, the DP of the medical face mask wasn’t also impacted by the temperature change.
3.4. Influence of the use of detergent on the performance of the masks
The masks were washed 10 times, at 60°C with and without detergent to determine the influence of the use of detergent on their performance. The results of the BFE and DP are shown in Fig. 8a and b.
Figure 8: Influence of detergent on (a) Bacterial Filtration Efficiency (%) and (b) Differential Pressure (Pa.cm− 2) for the medical face mask and the community face masks. Average values (N = 5) ± standard-deviation.
The presence of the detergent didn’t seem to impact significantly the BFE and DP of the community face masks. SEM analysis (see Supplementary Fig.S2 a and b) also showed that the fiber morphology was not significantly impacted by the use of a detergent and once again the deconstruction of the fibers was attributed to the number of wash cycles. The fiber morphology of the medical face mask was also not significantly impacted by the use of a detergent as shown in Supplementary Fig.S2 c.
Concerning the medical face masks, the BFE for the mask washed without detergent was similar to that of the new mask but the BFE was decreased when the mask was washed with the detergent. This shows that the presence of the detergent is probably responsible for the loss in BFE for the medical face mask. The washing agents present in the detergent are likely to bind to the surface and cause a loss of electrostatic charges of the electret meltblown layer 32–35. This observation was also highlighted by Charvet et al. 16 and Alcaraz et al. 17. The reduction of efficiency was observed only for the submicron particles (impaction plate collection size between 1.1 and 0.65 µm) as shown in Fig. 9. Inertial impaction and/or direct interception are the dominant particle capture mechanisms for particles > 1 µm but for submicron particle sizes other mechanisms particularly the electrostatic mechanism play an important role. Therefore the loss of electrostatic charges caused by the detergent tends to reduce filtration efficiency for the submicron particles. Charvet et al. 16 and Alcaraz et al. 17 mimicked the loss of electret effect by discharging a medical face mask in isopropanol. Their results showed that spectral filtration efficiency of a mask discharged by immersion in isopropanol was similar to that of a washed mask.