The masks being tested is fixed into an air duct and ambient particulates are flown through the facemask with a face velocity of 0.058±0.002 and 0.264±0.009 m s-1 corresponding to 20 ± 0.2 and 90 ± 1.0 L min-1 flow rates respectively. The measured ambient air temperature is 26±0.3ºC and RH% is 65±3.The measured average aerosol concentration for the upstream (Cu) and two- flow rates 20 and 90 L min-1 is presented in Fig. 3(a) (SMPS) and 4(a) (OPC) respectively. Similarly, the average aerosols concentration measured for the downstream (Cd) for two flow rate, with and without gamma sterilisation of N95 mask is shown in Fig. 3(b) (SMPS) and 4(b) (OPC) respectively. It is observed from Fig. 3 (a) and 4 (a) that, a significantly lower aerosol concentration in the upper size distribution for both the data sets, that is, for aerosol greater than 300 nm for the data measured by SMPS and greater than 5 µm for the data measured by OPC. The data were excluded above these thresholds (300 nm for SMPS and 5µm for OPC) for all results presented in the paper due to the extremely low concentration and this may increase statistical error. The pressure drop across the all facemask is measured before and after sterilisation and it is given in Table 2. The average pressure drop of surgical masks was found to be 2.45±2.03 pascal for 20 L min-1 flow rate and 7.36±2.08 pascal for 90 L min-1 flow rate, which is much less than the reported value (approximately 20 pascal at flow rate 90 L min-1). The average pressure drop of cloth masks was found to be 11.57±2.12 and 73.55±2.74 pascal for 20 and 90 L min-1 flow rate respectively. The average pressure drop of N95 mask was 31.32±2.31pascal for 20 L min-1 flow rate and 258.9 ± 4.59 pascal for 90 L min-1 flow rate. The average pressure drop of cloth masks is more than surgical masks and less than N95 masks. It is to be mentioned that the pressure drop across all the three types of decontaminated facemask showed that there is no measurable change observed i.e. change appears within the error bar values, i.e. there is no physical change in bulk density of face mask fibres/threads after decontamination. The pressure drop indicates the condition for usage during breathing and is found to be in the accepted range (inhalation and exhalation resistance limit is 343 and 245 pascal respectively) (Lin et al., 2020).
Efficiency evaluation with ambient aerosols size 0.3 ˗ 5 µm using OPC
The average FE of the cloth masks, surgical masks and N95 masks for the two-flow rate condition and before and after gamma sterilisation (15 and 25 kGy) is tabulated in Table 3. Table 3 is drawn with three particle size bins viz. 0.3 ˗ 5.0, 1.0 ˗ 5.0 and 3.0 ˗ 5.0 µm. An average FE of cloth masks vs aerosol optical diameter is shown in Fig.5 for control and decontaminated mask for two flow rates. It is observed from Fig. 5 and Table 3, in the case of cloth masks, average FE is found to be more or less same after sterilisation and found to vary from 18.84±0.33 ˗ 20.28±1.49%, 49.20±8.44˗60±7.59% and 80.66±4.28˗89.41±5.63 % for both flow rate and irradiated conditions for atmospheric aerosols of size 0.3˗5.0, 1.0˗5.0 and 3.0˗5.0 µm respectively. Similarly, from Table 3, the average filtering efficiency for the surgical mask found to vary from 15.76±0.22˗22.48±3.92%, 49.89±7.63˗59.22±9.25% and 73.15±3.73˗90.36±4.69% for the atmospheric aerosols of size 0.3˗5.0, 1.0˗5.0 and 3.0˗5.0 µm respectively. The surgical masks are found to be performed with slightly decrease in efficiency after sterilisation when compared to without gamma sterilized condition for the same particle ranges. In the case of N95 masks, the filtering efficiency is found to be reduced to 69.78±1.07% from 99.68±0.05% and about 63.12± 2.22% from 94.98±1.98% for the two irradiated conditions for 20 and 90 L min-1 respectively. The uncertainty in FE is significantly more for cloth and surgical masks in case of 1.0˗5.0 and 3.0˗5.0 µm aerosols compared to N95 masks probably indicating the texture/material quality of masks in the manufacturing stage. Further, the masks showed no measurable changes in fit or measurable structural changes when exposed up to 25 kGy dose of radiation. The International Atomic Energy Agency (IAEA) has recently indicated the same observation that, there is no significant change in the texture of the mask with respect to fit factor of the mask at 24 kGy radiation dose, which is needed to kill viruses and bacteria (IAEA, 2020).
Efficiency evaluation with PSL aerosol size of 1.0 µm
The performance evaluation of mask for atmospheric aerosols may give more statistical error due to low aerosols number concentration (less than 101 ˗ 102 L-1) for 1.0 µm and above. In this regard, all three types of masks are tested for laboratory generated PSL aerosols of size 1.0 µm. This kind of added information may be very useful for some specific context and application. For example, the average number of aerosols generated per cough by Influenza patient is 7.5*104 and count median diameter (CMD) of cough generated aerosols/droplet were in between 0.6 to 0.9 µm with Geometric Standard Deviation (GSD) 1.53 to 2.28 (Lindsley et al., 2012). Similarly, another recent study suggests that, the average number of droplet/aerosols expelled per cough by a person having respiratory infection is 4.9*106 with most of the aerosols are less than 5.0 µm and aerosol number becomes less when person recovered from the infection (Lee et al., 2019). Another study shows that 80% of droplets/aerosols are centred in the range of 0.74 ˗ 2.12 µm during coughing and sneezing (Yang et al., 2007). Further, the detailed transmission of SARS-CoV-2 virus is not well understood till now; aerosols less than 5.0 µm are considered as the primary source of transmission of respiratory infection (Doremalen et al., 2020; Wang et al., 2020). In this context, the filtering efficiency of the mask needs to be tested for high aerosols concentration (105 ˗ 106 L-1) in the 1.0 µm range. Fig. 6 shows the filtering efficiency of these three types of filters for PSL particles of size 1.0 µm with and without sterilisation and for the two flow rate conditions. It is observed from Fig. 6 that filtering efficiency for N95 mask shows greater than 96.25±0.67% even after sterilization for both the flow rate conditions. The filtering efficiency of surgical mask is found to be in the range of 71.12±2.09 ˗ 83.95±1.04% before sterilisation and it reduced to 43.2±5.65 ˗ 56.58±1.69% after gamma sterilisation. The measured filtering efficiency of cloth mask in the range of 55.82±4.56 ˗ 63.89±4.44% before sterilisation and 45.07±6.69 ˗ 59.68±0.79% after sterilisation. The cotton mask showed not much variation after sterilisation. The lower efficiency is attributed to density of fibres in the non-woven type masks while the density of wrap and weft per unit area in the case of cloth mask. However, these masks are effective for the particles in the size range of 1.0 µm and above (about 50% efficiency). It is also to be noted that the filtration efficiency of all three types of mask is more for 90 L min-1 flow rate for 1.0 µm aerosol when compared to the 20 L min-1. This is due to higher filtration capability for micron sized aerosols by impaction at large flow rate condition.
Filtering efficiency evaluation of facemask with 102.7 nm aerosol
The FE of N95 facemask has been evaluated for nano-sized laboratory generated PSL aerosols. This gives added information and performance of masks in terms of FE for nano-sized aerosols. This is also relevant in the present pandemic situation too, because transmission vector of Covid- 19 infection takes place in ultrafine aerosol size and corona virus itself is in the range of 60 ˗ 140 nm (Kim et al., 2020; Cascella et al., 2020). The N95, cloth and surgical facemasks are exclusively tested for 102.7 nm aerosols with concentration in the order of 104 cm-3. Fig. 7 shows an average FE of N95 mask for both un-irradiated, gamma sterilized and for two flow rate conditions. It can be seen from the figure that, the filtering efficiency is more than 98.02±0.15% for un-irradiated condition and for both flow rates. The efficiency found to be reduced from 99.83±0.09% to 96.17±0.99% for the flow rate of 20 L min-1 and 98.02±0.15% to 80.29±1.13% for 90 L min-1 under gamma-sterilized condition. The filtering efficiency for 90 L min-1 is found to be less when compared to 20 L min-1 condition (opposite to 1.0 µm aerosols) where nano-sized particles are carried away due to high flow rate by the flow gas streamlines in the filtering media. Further, FE of the cloth and surgical masks are also evaluated for nanoparticles. The FE of cloth masks is 20.59±3.69% and 17.67±5.01% for 20 L min-1 and 90 L min-1 respectively. The variation in the efficiency is not significant. The FE of surgical masks is 45.12±4.61% and 10.13±3.39% for 20 L min-1 and 90 L min-1 respectively. The variation in the efficiency is significant due to unstructured packing of fibres in the surgical mask which paves way for particles to trace the gas streamlines at higher flow rate. The FE of surgical masks is more than cloth masks for 102.7 nm aerosols for 20 L min-1 while in case of 90 L min-1 flow rate, the FE of cloth masks is more compared to the surgical masks.
Evaluation of Filtering Efficiency of N95, cloth and surgical masks for aerosols from 10 nm to 5.0 µm
The filtering efficiency of N95, cloth and surgical masks are examined by combining data of OPC and SMPS covering from the range of 10 nm to 5 µm sized aerosols. The evaluation in the lower size range may be useful for the users, who are working or interested in filtration of nanoparticles in the facility like microelectronics fabrication environment. Further, as per ASTM standard, 2018, it is required to study the performance evaluation with respect to filtering efficiency less than 100 nm (ASTM standard, 2018). The FE of N95 mask under all conditions (un-irradiated, gamma sterilised and for two flow rates) is shown in Fig. 8. The filtering efficiency of N95 mask shows a typical conventional U shaped curve for aerosols ranging from 10 nm to 5.0 µm with minimum efficiency of control mask is 99.68±0.05% for 20 L min-1 and 94.98±1.98% for 90 L min-1 for 300 nm aerosols. The minimum efficiency of sterilized N95 mask is 69.78±1.07 % for 20 L min-1 at 300 nm sized aerosols while 64.25±2.05% for 90 L min-1 in the range of 200 - 300 nm sized aerosols. The filtering efficiency is more for 20 L min-1 than 90 L min-1 in all the three respective conditions i.e. for the control, 15kGy and 25kGy. It is known that, the aerosol filtration takes place by five basic mechanisms viz. gravitational settling, inertial impaction, interception, diffusion and electrostatic interaction (Hinds, 1999 and Vincent, 2007). The gravitational and impaction settling play a major role in filtering the aerosols larger than 1.0 µ m. As aerosol size decreases in the range of 0.1 to 1.0 µm, the Brownian diffusion and mechanical interception are the predominant mechanisms and for aerosols size less than 0.1 µm, which can easily slip from filtering media, are captured predominately by electrostatic attraction in addition to the mechanical processes. The N95 mask consists of electrostatic filtration media which encompass a broad class of materials that are capable of capturing and retaining fine air borne particulates through electrostatic interaction (Coulomb and dielectrophoretic forces) in addition to mechanical processes (Impaction, settling, Interception and Diffusion) (Myers and Arnold, 2003). It is known that, least efficiency is associated with particles in the range of 0.1 ˗
0.3 µm that is bigger for diffusion and smaller for interception; hence, the 99% efficiency is achieved for this range by electrostatic interaction. When the media loses its charges, the particles are captured only by mechanical processes where the efficiency is reduced from 99% to 65% (Fig.8 and table 3). In the case of 1.0 µm particles, the efficiency is not found reduced even after gamma irradiation (Fig. 8 and table 3). Further, the N95 masks are designed for filtering efficiency more than 95% of aerosols size greater than Most Penetration Particle Sizes (MPPS), and therefore, their underperformance below MPPS is not surprising results (Balazy et al., 2005 and 2006). The FE of mechanical filters has least efficiency at MPPS and increases with increase or decrease of aerosol size from MPPS.
The FE of cloth and surgical masks has been investigated for two flow rates and is shown in Fig. 9. The FE of both cloth and surgical masks shows a typical conventional filtering curve for aerosols ranging from 10 nm to 10 µm. The FE is less for 90 L min-1 flow rate when compared to the 20 L min-1 for both cloth and surgical masks. The minimum FE of cloth masks is 14.27±2.85% for 90 L min-1 and 4.32±1.35% for 20 L min-1 for the aerosols in the range of 50˗150 nm and 60˗80 nm respectively. Similarly, the minimum FE of surgical masks is 22.59±3.05% for 90 L min-1 and 12.54±1.51% for 20 L min-1 for 300 nm and 200 nm aerosols respectively. The FE of cloth and surgical masks increases with increase of optical diameter and decrease of mobility diameter from least efficient MPPS and the value of MPPS is lower for high flow rate compared to the low flow rate.
Comparison of N95, cloth and surgical mask filtering efficiency for total test aerosols concentration of sizes < 300 and > 300 nm
The filtering characteristics of any mask depends much on aerosol characteristics like diameter, charge and density of aerosols, concentration of aerosols and airflow velocity (flow rate) apart from filter characteristics. Here, we have compared the FE of N95 mask for total test aerosols concentration in sizes less than 300 nm and greater than 300 nm. The total measured aerosols number concentration is in the order of 103 cm-3 and 104 cm-3 for aerosol sizes >300 nm (0.3 ˗ 5.0 µm) and <300 (10 ˗ 300 nm) nm respectively. Table 4 gives FE of N95 masks for two-flow rate conditions for the masks including control and after sterilization. The FE is found to be ≥ 99% for particles sized >300 nm and <300 for both the flow rates. The FE reduces after sterilisation from 99.98±0.01 to 84.28±1.68% and 99.89±0.03 to 99.04±0.11% for particles >300 nm and <300 nm respectively for fixed 20 L min-1 flow rate. In the case of 90 L min-1 flow rate, the FE of mask is reduced after sterilisation from 99.31±0.05 to 89.07±1.15% and 98.92±0.12to 95.49±0.89% for particles >300 nm and <300 nm respectively. The reduction is found to be more for particles>300 nm aerosols when compared to that of <300 nm aerosol for both flow rates. The N95 mask performed most efficiently for low flow rate and nano-sized aerosols even after gamma sterilization. The electrostatic filters are efficient for nano-sized aerosols at low airflow velocity (flow rate) rather than high airflow velocity (Colbeck and Lazaridis, 2014).
The FE of cloth mask is 46.76±2.58% and 36.58±2.87% for 20 and 90 L min-1 respectively for particles <300 nm. Similarly, the FE of surgical masks is 63.82±1.78% and 28.75±1.52% for 20 and 90 L min-1 respectively for particles <300 nm. The FE of surgical masks is more compared to the cloth mask for <300 nm aerosol size and 20 L min-1 flow rate while for 90 L min-1 FE of cloth mask is more. The FE of cloth and surgical masks is more for <300 nm aerosol size compared to the >300 nm for both flow rates.
Observation of morphological change by optical microscope
The fiber structure of N95 and surgical masks are examined in the optical microscope (Make and Model: Axioplan 2 Imaging, Metasystems, Germany) under 100X magnification. The image taken by optical microscope for N95 and surgical mask is shown in Fig. 10 and 11 respectively. The dimension of each image is 65 µm x 65 µm and the filter fibre diameter is in sub-micrometre range for N95 mask while micrometre range for surgical mask. It is observed from Fig. 10 (a), (b) and (c) that, no observable and significant change in morphology of filter fibre is found with gamma sterilization upto 25 kGy dose. Similar observations were reported by Scanning Electron Microscopy (SEM) up to 61 kGy dose (IAEA technical report). The surgical mask consists of three layer viz. repellant the outer colored layer, the filter medium in the middle and absorbent at the innermost layer. Fig. 11 (a), (b), (c) are the microscopic image of repellant, filter media and absorber of the control mask respectively and Fig. 11 (d) and (e) belong to the filter medium exposed to 15 kGy and 25 kGy. It is observed from Fig. 11 (a), (b) and (c) that, the structure of the filter fibre media is bonded togeather by using chemical adhesive and appeared entangled structure. It is observed from Fig. 10 (d) and (e) that, there is no observable change in the structure of the filter fibre after gamma sterilisation.
Comparison of N95 mask filtering efficiency from literature after gamma sterilisation
A comparison of filtering efficiency of N95 respiratory mask with various works from the literature under gamma-sterilized condition is summarized in table 5 (Cramer et al., 2020; Man et al., 2020; Lin et al., 2020). It can be seen from the table that, the irradiation dose for mask sterilization is varied from 1.0 to 50 kGy and mask efficiency has been tested for aerosols ranging from 0.1 to 1.0 µm. The reduction in efficiency is found to be more for most penetrating particle size (0.3 µm) in all the cases. Among all the works carried out, the work of Cramer et al. (2020) showed the highest reduction in efficiency under gamma-sterilized condition. This may be due to their filtering media and relatively large face velocity (0.4 m s-1), as the control mask itself is showing 5-15% less efficiency from others works. The filtering efficiency of present work and by Man et al.(2020) is found to have similarity for particles > 0.3 µ m, but here also, at what flow rate that masks have been tested is not mentioned. It is noted that in all the works, the testing flow rate has mentioned in few works. However, it is important to test the masks under breathing rate condition towards fit for the purpose and that condition and that is followed in our work. The Lin et al. (2020), has presented his work for aerosol size distribution ranging from 7 ˗ 882 nm with CMD around 75 nm and FE of N95 mask was found to be 44 ˗ 77 % after gamma sterilization, which is much less than that of present work.