3.1 Testing with ambient aerosols size ≥0.3µm
The filtration efficiency of masks is determined for atmospheric aerosols of size ≥ 0.3 µm and is shown in Fig. 3.The pressure drop across the filter is measured and is given in Table 2. The pressure drop indicates condition for usage during breathing and is found to be in the accepted range (inhalation and exhalation resistance limit is 35 and 25 mm w.g. respectively). It is observed from Fig. 3 that particulate filtering efficiency is less for high flow rate (90 lpm) when compared to the low flow rate (20 lpm) for all three types of mask. The surgical and cloth mask are performed more or less same or slightly decrease in efficiency after sterilisation when compared to without gamma sterilized condition.
The filtering efficiency found to vary from 18% to 22% in both un-irradiated and irradiated condition for the cumulative particles of size ≥0.3µm.In the case of N95 masks, the filtering efficiency is found to be reduced to 70% from 99.6% and about 62% from 95% for the two irradiated conditions for 20 lpm and 90 lpm respectively. Further, it is also noticed that there exists about 1-2% reduction in efficiency for all the three types of masks between 15 kGy and 25 kGy exposed doses.
Table 2. Pressure drop across the filter for two-flow rate condition.
S. N.
|
Type of Mask
|
Flow rate (lpm)
|
ΔP (mm of water gauge)
|
1.
|
N95
|
20
|
3.15
|
90
|
26
|
2.
|
Surgical
|
20
|
0.25
|
90
|
0.75
|
3.
|
Cloth
|
20
|
1.18
|
90
|
7.5
|
3.2 Testing with aerosol size 1.0µm
Fig.4 shows the filtering efficiency of these three types of filters for mono dispersed PSL particles of size 1.0µm with and without sterilisation and for the two flow rate conditions. The filtering efficiency for N95 mask shows>96%even after sterilization for both the flow rate conditions. The filtering efficiency of surgical mask is found to be in the range of 70-83% before sterilisation and reduced to 43-56% after gamma sterilisation. The filtering efficiency of cloth mask showed 55-63% before sterilisation and 45-59% after sterilisation. The cotton mask showed not much variation after sterilisation. Upon comparison with filtering efficiency of surgical and cloth masks between the particles in the size ≥0.3µm and mono dispersed particles of size 1.0 µm, it is observed that filtering efficiency for the surgical and cloth masks is found to be more for 1.0µm particles for both flow rates under sterilized or non-sterilized condition. It is to be noted that, that cumulative efficiency for particles ≥0.3µm is shown in Fig. 3 and about 95% of the particles are in the range of 0.3 and 0.5 µm in the ambient aerosols for which efficiency is about 20%, whereas, this test is carried out with mono dispersed particles of 1.0µm, for which the efficiency is found to be higher. It is also to be noted that the filtration efficiency of all three types of mask is more for 90 lpm flow rate for 1.0 µm sized particles when compared to the 20 lpm. This is due to higher filtration of micron size aerosols by impaction at large flow rate condition.
3.3 Testing with aerosol size 100nm for N95 mask
The N95 facemask is exclusively tested for1 02.7nm laboratory generated PSL aerosols in line with size range of viruses (70-100 nm) (Kim et al., 2020).Fig. 5 shows filtration efficiency of N95 mask for both un-irradiated and gamma sterilized and for two flow rate conditions. It can be seen from the figure that, the filtering efficiency is more than 95% for un-irradiated condition and for both flow rates. The efficiency found to be reduced from 99% to 94% for the flow rate of 20lpm and 95% to 72% for 90lpm under gamma-sterilized condition. It can be seen from Fig. 4 that, the filtration efficiency of N95 mask is decreased for 90 lpm flow rate when compared to the 20 lpm for gamma sterilisation conditions. This is opposite to1.0 µm aerosols where the particles are carried away due to high flow rate by the flow gas streamlines in the filtering media.
3.4 Filtering Efficiency for N95 masks for particles from 25nm to 5.0µm
The filtering efficiency of N95 mask is examined by combining data of OPC and SMPS covering from the range of 25nm to 5 µm sized aerosols, because, coughs and sneezing actions produce droplets < 100 nm (<0.1 µm) and bulk of droplets > 100 nm (>0.1 µm) (Lee et al., 2019).The particle generated from coughing/sneezing ranges from 0.1 µm to 100 µm (Yang et al., 2007, Gralton et al., 2011 and Lindsley et al., 2012).The filtration efficiency of N95 mask under all conditions (un-irradiated, gamma sterilised and for two flow rates) is shown in Fig. 6. It is observed from Fig. 6 that, the filtering efficiency under un-irradiated condition is greater than 99% for the all the particles except in the range of 0.2-0.3 µm where it is reduced by about 3-8%. Under the gamma sterilized condition the filtration efficiency falls down to about 70% for aerosols in the size range of 0.2-0.3µm for 20 lpm and it is reducing further for high flow rate (90 lpm).
It is known that N95 filter 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) (David L. Myers and B. Dean Arnold, 2003). The reduction in efficiency due to radiation sterilization is attributed as follows: Upon irradiation, the static charges associated with fibrous filters become neutralized (David L. Myers and B. Dean Arnold, 2003).It is known that, least efficiency is associated with particles in the range of 0.2- 0.3µm that is bigger for diffusion and smaller for interception; hence, the efficiency is increased for this range by electrostatic interaction. When the media lost its charges, the particles are captured only by mechanical process where the efficiency is reduced from 99% to 70%. The fall of efficiency confirmed from Fig.3 and Fig.5, in the case of 1.0µm particle, the efficiency is not found reduced much i.e. 99.9% to 97% even for gamma irradiation.
3.5 Comparison of N95 mask filtering efficiency from literature
A comparison of filtering efficiency of N95 respiratory mask with various works from the literature under gamma-sterilized condition is made and summarized in table 3. It can be seen from table that, the irradiation dose for mask sterilization is varied from 10 to 50kGy and mask efficiency has been tested for aerosols ranging from 0.1 to 1.0µm. 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 Avilash et al., 2020 showed highest reduction in efficiency under gamma sterilized condition. This is may be due to their filtering media as control mask itself is showing 5-15% less efficiency from others works. Further, the filtering characteristics of mask much depends on fibre diameter, packing density, charge on fibres and filter thickness, as well as aerosol characteristics like diameter and density of aerosols and airflow velocity (flow rate). The filtering efficiency of present work is more comparable with Man et al., 2020, but here also, at what flow rate that masks have been tested is not given. It is noted that in all the works, the testing flow rates has not been mentioned. However, it is important to test the masks under breathing rate condition towards fit for the purpose.