Morphology of the Face Mask
Figure 6 shows the morphology of the outer, middle, and inner 3-ply face mask layers, which are composed of non-woven fibres. Based on the figure, it is observed that the fibre structure of the outer and inner layers is similar. Fibre diameters in the range of 20.4 to 21.5 µm were used for these two layers, and the arrangement is less dense compared to the middle layer (Fig. 6b). The middle layer consists of finer fibre with diameters of 2.6 to 3.0 µm. The presence of three layers, with the middle layer having a dense structure and small pores, shows that the 3-ply face mask is efficient in blocking the particulate matter. The morphology of the cotton fabric face mask is shown in Fig. 6d. The yarn sizes used to produce the fabric face mask are bigger than those of the fibres present in the three layers for the 3-ply face mask. This leads to large gaps between the warp and weft yarns. The arrangement of the yarns is totally different if compared with the randomly arranged microfibre in the nonwoven fibres in the 3-ply layers.
Measurement of the filtration efficiencies was carried out to investigate the effect of different mask types and double masking, and the results are shown in Fig. 7. Among the single face mask tests, the 3-ply face mask was found to have vastly superior filtration efficiencies in the range of 87‒95.6% as compared to the fabric mask, which was in the range of only 7.5‒14.26%. This could be due to the big size of the voids existing in the woven structure of the fabric face mask, which provided less of a barrier to the aerosol particles. The randomly arranged nonwoven fibres present in the three layers of the 3-ply mask provide excellent filtration compared to the fabric mask.
The variation in the filtration efficiencies of the fabric face mask could be due to the differences in the tightness of the weave, which depends on the threads per inch of the fabric used for making the mask . The result from single masking demonstrates that the fabric mask provides very little or no protection, especially in places where there is a high risk of infection. The combination of a fabric mask with a 3-ply mask in the double mask tests indicates a significant 500% improvement in filtration efficiencies. The reason for the improvement can be attributed to the better fit and compactness obtained after using the combination of the two face masks [10, 23]. The maximum filtration efficiency of 95.65% was obtained by double masking using FM2CM3, which is similar to that of the N95 respirator and shows that double masking is the easiest and low-cost alternative for medical professionals where there is a shortage of N95 respirators.
Breathing resistance is the measure of comfort properties, which affect the inhalation and exhalation of the wearer. The breathing resistance, which is measured by the pressure drop, is an important parameter that influences the filtration efficiency . An increase in breathing resistance results in a greater physical effort to breathe through the face mask. In other words, the wearer’s lungs have to work harder in the breathing process. In order to keep the breathing resistance as low as possible, the face mask must be air-permeable on the one hand while still filtering particles on the other hand. Figure 8 shows the breathing resistance for single and different combinations of double face masks. The breathing resistance value shown by the N95 face mask is used as a comparison. As expected, the breathing resistance was found to be much higher for double masking as compared to the single mask, which indicates a tighter and leak-free fit in double masking.
Among the double masking variants, the lowest breathing resistance is observed in the combination of FM1 and fabric face masks (CM1, CM2, and CM3). This indicates that this combination is suitable and provides better breathing comfort for the wearer compared to the other FM2 and FM3 double maskings. The highest breathing resistance was found in the FM3 double masking, for example, FM3CM2, FM3CM3 and FM3CM, with breathing resistance values of 0.20, 0.19 and 0.17, respectively. A constant breathing resistance of 0.17 was found for all combinations of FM2 double masking, which demonstrated that the combination of different 3-ply face masks and fabric face masks does not have a significant effect on the breathing resistance. The breathing resistance test is congruent with the previous report by Vrielink and Meijer , in which they showed that increasing the number of layers will lead to an increase in the differential pressure. The values of breathing resistance less than 0.2 mbar shown by the double masks (Fig. 8) are comparable with the value measured for the N95 face mask.
As for breathing resistance, all of the double masks are at a tolerable level, where the results are comparable with the Malaysian Standard MS 2323:2010 and European Standard EN 149:2001 + A1:2009. According to both standards, the breathing resistance should not exceed 0.7 mbar. In this case, wearers will not have difficulty in breathing or feel suffocated when applying double masking. Besides, these findings are in line with the findings from a study conducted in previous work .
Carbon Dioxide Content
The carbon dioxide (CO2) content is crucial in defining the comfort properties of the mask. The accumulated CO2 has a detrimental effect on the wearer, especially in health care settings where the health professionals have to wear masks for prolonged periods. An ideal mask should release the CO2 gas exhaled by the wearer, but most of the mask accumulates the CO2 gas, which is further inhaled by the wearer, causing severe headaches, fatigue, and loss of content . The CO2 content of various masks with double masking combinations was found and is plotted in Fig. 9. For the single mask test, the lowest CO2 content of 0.48 was found in a fabric mask (CM1), whereas the highest of 0.61was found for a 3-ply mask (FM1).
The fabric mask was found to retain less of the CO2, which could be due to the larger voids present between the weft and warp weave as compared to the compact packing of randomly arranged fibres in the case of the nonwoven filter layers present in 3-ply masks. The testing performed with double masking was found to retain a higher CO2 content, which was expected due to the presence of more barriers, which do not allow passage for exhaled air to leave in the atmosphere. Among all the results for double masking, the combinations of FM1CM1 and FM2CM1 produced the lowest CO2 content (0.58%), which indicates that these combinations are good to reduce the accumulation of CO2. The CO2 content of these double masking combinations was lower compared to that of N95, which showed a CO2 content of 0.68%. However, the highest CO2 content of 0.89% was obtained by the combination of FM3CM3, which is the result of the tight fit and higher barrier created by the combination of the two masks. Based on the results, it is found that all combinations of double face masks are acceptable, as the CO2 content is still lower than the maximum CO2 content, which is 1.0%. The results on carbon dioxide content shown by double face masks are at a tolerable level, < 0.90%. According to the Malaysian Standard MS 2323:2010 and European Standard EN 149:2001 + A1:2009, the maximum permitted average for CO2 content is no more than 1.0%. Although a significant increase in CO2 contents is noted routinely with just one face mask, the levels still remain within the standard limits.