key: cord-0886212-jm6gq3e9
authors: Kumar, A.; Bhattacharjee, B.; D. N., S.; V., S.; B., V.
title: Evaluation of filtration efficacy of various types of face-masks using ambient and PAO aerosols following with different sterilization methods
date: 2020-10-27
journal: nan
DOI: 10.1101/2020.10.23.20218073
sha: 024c152e6f86395e6b4e0304e812fa0c139e06ba
doc_id: 886212
cord_uid: jm6gq3e9

Due to the ongoing pandemic, various types of facemasks such as N-95, surgical mask and cloth masks are being used as an essential protective measure. The filtration efficiency of these masks were tested before and after sterilization by different methods for two flow rates conditions corresponding to normal breathe rate (20 lpm) and during sneezing (90 lpm). Sterilization techniques used here are autoclaving (30 and 60 minutes), dry oven heating (30 and 60 minutes), gamma irradiation (15 and 25 kGy), hot water washing with and without detergent and immersing in 10% concentration of liquid hydrogen peroxide for 30 minutes. As expected, the N-95 is the greatest filtering efficiency among all the other type face masks. The best method to sterilize N-95 masks without affecting its performance is by using dry heat with temperature ranging from 70-80oC. The cloth masks and surgical mask are performed more or less same for both flow conditions. As an affordable sterilization method hot water washing is highly recommended which does not deteriorate the efficiency of the masks and can be used for the general public. The use of double or triple layer cotton cloth masks in the general public serves fit for the purpose than surgical masks. The surgical mask can be sterilized only few times with the help of dry heat, hot water wash and/or autoclave.

Since the outbreak of the novel corona virus disease i.e., COVID-19, one of the essential Personal Protective Equipment (PPE) used in this pandemic is face masks which offers protection against covid-19 through droplet and aerosols transmission. The uses of facemasks have become compulsory in India and other countries across the world. Hence, it is become mandatory to wear a facemask by public and workers in the front line working against Covid19

pandemic. Good quality masks can prevent transmission of virus particles from one person to another. Starting from just 1 case in 30 th January 2020, we now have more than 7 million cases as of 15 th October 2020 (https://www.covid19india.org/). Frontline workers in India and around the world are facing shortage of masks because of the increase in number of the usage. To solve this crisis, masks are being reused multiple times. But using a used facemask for multiple times without killing the pathogens (if any) can result in allowing any virus or bacteria on its surface to directly or indirectly enter into our respiratory system. Here the various sterilization/decontamination processes comes into play. Sterilization can be defined as the procedure of destroying all microorganisms in or on a given environment in order to prevent the spread of infection. Under these scenario, multiple potential methods for sterilization have begun to be explored (Juan et al., 2020) .The thermal heat, radiation or chemical agents usually do this.

Many potential methods of sterilization have been explored already. They are based on chemical and physical methods. Chemical methods include hydrogen peroxide, chlorine dioxide, bleach, alcohol, soap solution and ethylene oxide, ozone decontamination etc and physical methods include dry/steam heat treatment, UV light sterilization, electron beam and gamma irradiation etc (Kumar et al., 2015; Liao et al., 2020) . All above sterilization methods having advantages and disadvantages from one than others (Cramer et. al., 2020; Juan et al., 2020) . Some of the methods such as soaking/dipping in alcohol and others damage the fibers of face masks and significantly degrade the filter efficiency. This happens because many masks use fibers that have a static charge which are removed by dipping in alcohol. In the present work, we used some of the methods mentioned above to sterilize the masks and evaluated for filtration efficiency. Those methods include autoclaving for 30 and 60 minutes, gamma irradiation (15 and 25 kGy), dry heat (30 and 60 minutes), detergent wash with hot water, hot water wash and soaking in 10% concentration of hydrogen peroxide for 30 minutes.

In this situation, mostly three types of masks are being used widely in most of the countries namely N-95 (health care professionals), non-woven fabric surgical masks (front line workers) and cloth masks (common publics). While N-95 and surgical masks are being used by the frontline workers during this pandemic, but not all can afford the expensive N-95 face masks, and the public often uses mostly cotton masks as Government of India made it mandatory to wear masks in public places. So the usage of cloth masks by the public has increased rapidly.

Cloth masks are preferred for community use because they also reduce the non-biodegradable waste from more use of surgical masks. At this juncture, the reusability of these cloth masks is being carried out, by adopting simple method of sterilization, which is less cost and affordable by the common public. Hence it is very important to study the effectiveness of sterilization by simple methods for these cotton masks. Besides, the effectiveness of different types of cloth masks in blocking respiratory droplets depends on the weave and number of layers. Earlier studies have also shown that the mask efficiency was directly related to the closeness of the mesh and the number of thicknesses of gauge. We have taken two variants of cloth masks, one being two layer cloth mask of same fabric (cotton) and other two layer cloth mask of different fabric (cotton and gada). Since, N-95 is expensive cannot be used by everyone (due to different . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint economic strata of India), a common person can afford cloth mask and surgical mask for protection. The filter efficiencies of these masks were first tested and then were sterilized using the above-mentioned methods and then efficiency was evaluated. The detailed experimental method, results and discussion, conclusion and recommendations are described in this article.

The evaluation of Filtration Efficiency (FE) of different types of masks are carried out in a test facility called HEPA Filter Testing Laboratory, Radiological and Environment Safety Division, Indira Gandhi Centre for Atomic Research (IGCAR) consisting of stainless steel cylindrical duct which has provision for fixing the facemask without any air leakage. The upstream and downstream aerosol concentrations are counted and the FE of mask is determined before and after various sterilization methods. The details of the experimental setup, types and quantity of masks, necessary data acquisition systems, sterilization methods and evaluation of FE of masks for ambient and laboratory generated aerosols are explained in this section.

The schematic diagrame of experimental setup for face mask testing is shown in Fig.1 . It consist of a cylindrical tube of 8.5 cm internal diameter (D) with upstream and downstream segment and a middle region to connect facemasks media. The upstream length is more than 15D and the concentration measurements in the upstream and downstream are taken at 5D away from the connection flange to get uniform concentration along the entire duct. The experimental setup also includes aerosols generator, aerosols diagnostic instruments, velocity meter, differential pressure monitor and relative humidity and temperature monitoring system of air stream. The test piece (without cutting the facemask) is connected in between two flanges fitted with O-rings and . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint mounted in the duct and it is perfectly found to be fitted without any air leakage (the mask edges were completely sealed).The desired airflow rates are achieved through suction by using an air displacement pump (Make: M/s. Rotovac and CG commercial motor) and flow controller (M/s. Weiser). The aerosols are sampled before (upstream) and after (downstream) of the specimen when aerosol-air suspension passes through the facemask. 

All types of facemasks are tested for FE with ambient and laboratory generated aerosols. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint instrument working principle and specification can be found elsewhere (Kumar et al., 2020and Usha et al., 2019 .The reproducibility of the OPC is ± 3% for the entire measuring range of OPC (0.3 to 20 µm). The acceptable accuracy in measurement of aerosols number concentration from OPC is ± 3%.

The three types of facemasks viz. N-95 (certified facemasks), non-woven fabric (equivalent to surgical mask) and self-made double layer cotton cloths (general public choice during the All the facemasks when fitted to the sampler holder care have been taken that masks should not stretch and there should be no leakage from the edges.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

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The following sterilization methods namely, gamma irradiation, dry heat, autoclave (steam), hot water with and without detergent wash and soaking in H 2 O 2 liquid were carried out for various types of facemasks employed in this study and they are summarized in Table 2 

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The dry heat decontamination of face masks for 30 and 60 minutes is achieved by hot air oven (Labtherm Scientific Products, Sr. No.0013) with air temperature ranging from 70-80°C. Steam sterilization has been achieved by autoclave machine and sterilization process takes place at a pressure of 103 kPa and temperature of 121°C. Masks were subjected to the autoclave for 30 minutes and 60 minutes of each type of masks (Make: Nat Steel semi-automatic autoclave, Mumbai India). The facemasks sterilization has been also carried out with detergent cum hot water, hot water and 10% concentration of H 2 O 2 (diluted with distilled water) by soaking for 30 minutes and dried before testing the face masks.

The pressure drop or differential pressure (ΔP) was measured using a manometer (Make and model: TSI, USA and 9565P143003) across the tested masks. The measuring range and error in reading in Δ P is -381 to +381 and ±1% of reading + 0.13 mm of H 2 O respectively. The differential pressure is an indicator for condition of comfort and breathability of facemask. The face velocity in the test section is measured with a velocity meter (Make and model: Velocicalc, TSI, USA and 9565P143003). The measuring range and error in reading in air face velocity is 0 50 m s -1 and ±3% respectively. The K type thermocouple and humidity monitor are used for the measurement of temperature (T) and relative humidity (RH) of air stream respectively. The measuring range along with error in the measurement of T and RH are -10 to 60 ºC± 0.3 ºC and 5 to 95% ± 3% respectively.

The FE of three different types of facemask is calculated using the following formula:

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. has been calculated based on the following individual uncertainty (accuracy in measurement, resolution and calibration of instrument). After that, the combined uncertainty has been estimated and then expanded uncertainty derived from combined uncertainty by multiplying the coverage factor (95% confidence level).

Before start the testing of facemasks, the experimental setup has been evaluated. The facemask experimental test rig is tested for zero efficiency condition using ambient aerosols. The upstream . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) 1 1 and downstream counts are recorded without facemask. The average upstream and downstream aerosol counts as a function of aerosols size is given in Fig. 2 . It is observed from the Fig. 2 that, the difference in the upstream and downstream counts ranges 10 -30 #/L up to aerosol size 0.7 µm and after that difference is very less (1 -3 #/L) observed. is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint this threshold (10 µm) for all results presented in the paper due to the extremely low concentration and this may upsurge statistical error. 2 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 

The measured pressure drop (ΔP) across the various facemasks before and after sterilization is given in Table 4 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

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The FE of various types of control facemasks are determined with ambient aerosols for particle and for two flow rates and given in Fig.4 . It is observed from the Fig.4 that, the mean FE of . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted October 27, 2020. for low flow rate (20 lpm) than high flow rate (90 lpm) and FE of the green cloth masks is more than red cloth mask and less than surgical mask for both flow rates. The red cloth mask is performing slightly lower efficiency than green cloth masks. The higher FE of green cloth mask is attributed to use of same fabric of good thread counts, which helps in trapping the particles efficiently.

As expected, the FE of certified N-95 masks is significantly greater than the surgical and cloth masks for all aerosol measured size ranges. Even MPPS were filtered more than97.37±0.5% CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) 1 8 to be least for certified N-95 masks. Further, statistical variability in FE of all type of face masks is more for 90 lpm flow rate than 20 lpm. . It is known that, least efficiency is associated with particles in the range of 0.3 0.5 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±0.09% to . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) 1 9 63.73±1.09%. In the case of greater than 1.0 µm particles, the efficiency is not found reduced even after gamma irradiation. The other three types of N-95 face masks (magnum cup shape, V-44+ and 210307) were sterilized with autoclave for 30 minutes only and FE is given in Fig.7 . It is observed from the 

The surgical facemasks are sterilized with the help of steam (autoclave) for 30 and 60 minutes, gamma irradiation for 15 and 25kGy dose, 10% H 2 O 2 concentrated liquid soaking, hot water 1 E . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint soaking with and without detergent; after that FE is calculated for two flow rates and shown in CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint increases with larger aerosols size for both face velocity. The lower efficiency is attributed to density of fibers in the non-woven type masks. However, these masks are effective for the particles in the size range of 3.0 µm and above (about 60% efficiency). 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. The microscopic image of filter media of the control and processed masks (15 kGy and 25 kGy) is given in Supporting Information (SI). It is observed that, the structure of the filter fibre media is bonded togeather by using chemical adhesive, appeared entangled structure and there is no observable change in the structure of the filter fibre after gamma sterilisation.

The homemade double layer cloth facemasks (green color and same fabric)are sterilized with the help of autoclave for 30 and 60 minutes, gamma irradiation for 15 and 25 kGy dose, hot water soaking with and without detergent; after that FE is calculated for two flow rates and shown in is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint 5 FE is relative less for red colored different layered cloth masks than with that of green colored same fabric double layered mask. The reason is attributed to texture (less thread counts) of second layer in the red colored mask. The uncertainty in FE is significantly more for cloth and surgical masks in case greater than 0.3 -0.5 µm aerosols compared to certified N95 masks probably indicating the texture/material quality of masks in the manufacturing stage.

The performance of various types of masks after different sterilization methods has been evaluated by laboratory generated aerosols (PAO) and FE has been compared from ambient aerosols. This type of analysis may be very useful for some specific context where a person exposed to the higher aerosols concentration. For example, the average number of aerosols generated per cough by Influenza patient is 7.5*10 4 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*10 6 with most of the aerosols are in sub micron (Lee et al., 2019) . Another study shows that 80% of droplets/aerosols are centered in the range of 0.74 2.12 µm during coughing and sneezing (Yang et al., 2007) . In this context, the filtering efficiency of the mask needs to be tested for high aerosols concentration (10 5 10 6 #/L). Table 5 shows the FE of various types of facemasks for ambient and PAO aerosols after different sterilization methods, for the two flow rate conditions and for MPPS (0.3-0.5) aerosol optical diameter. It is observed from the Table 5 that, certified N-95 mask shows slightly greater FE for laboratory generated aerosols (high . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint aerosols concentration) than ambient aerosols even after sterilization for both flow rate conditions. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint 7

The FE of surgical mask is found to be reduced for high aerosols concentration. However, the cloth masks (green and red color), the FE has been increased for laboratory generated aerosols. It is also to be noted that the FE of all three types of mask is less for 90 lpm flow rate when compared to the 20lpm.This is due to higher filtration capability for sub-micron sized aerosols by diffusion and interception at low flow rate.

Three types of facemasks viz. certified N-95, surgical equivalent and self-made double layer cloth masks have been tested for particulate FE and pressure drop across the masks for two-flow rate condition viz. 20 and 90 lpm before and after various sterilization using ambient aerosols in the size range of 0.3 10 µm and laboratory generated aerosols (PAO) of size 0.3 -0.5 µm. As expected, the pressure drop increases with increasing face velocity, this reflect that, it is harder to breathe through facemask when respiratory flow is large. The measured pressure drops during masks testing were found to be accepted range (the inhalation and exhalation resistance limit viz. 35 and 25 mm of H 2 O respectively). The FE of surgical masks is more or less same as green colored cloth masks (double layer of same fabric) for 0.3 to 10 µm optical diameters for 20 and 90 lpm flow rate. However, the FE of red cloth masks (double layer with different fabric) is less compared to the surgical masks. The FE of cloth and surgical masks increases with increase of optical diameter from MPPS and the efficiency is less at high flow rate compared to the low flow rate. In the case of N-95 mask, the FE is found to be more than 95% for control masks, for both flow rates and ambient aerosols (0.3 10µm). The FE reduces to about 70% for most penetrating aerosols (0.3 µm) after gamma sterilization for 20 lpm flow rate and still lesser with higher flow rate (90 lpm). The results found that the cloth masks that underwent autoclave for duration of 60 minutes have shown better efficiency. The increased in FE of surgical masks for autoclave of 60 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint minutes duration is because the mask is subjected to steam for prolonged time, which shrink the fiber of the facemask but same time destroying of the microbes and melted the elastic cords of surgical masks to a slight extent.

Face masks can provide additional protection when some measures, particularly social distancing is difficult to maintain. The effectiveness of facemasks depends on consistent and proper use.

Though the surgical masks have slightly better efficiency than the double layer cloth masks, we assume that the efficiency can be matched with it if the cloth masks have preferably three layers preferably with fabric having good thread counts (fabric with adequate droplet blocking efficiency). Thus, we can recommend the use of cloth mask for the common public provided there are automatic hand sanitizers installed before entering a crowded place or an office. It has to be made sure that the cloth masks are sterilized with any of the method above (preferably hot water sterilization in houses) before subsequent uses. The FE of N-95 masks is slightly decreases after autoclave for 30 and 60 minutes duration because the mask is subjected to steam of 120 o C for prolonged time which results in destroying of the microbes. At the same time, the process melted the elastic cords of the N-95 and surgical masks to a slight extent. The FE of the N-95 masks remain almost the same compared to the control condition after the H 2 O 2 sterilization but faded the names in ink in the masks. The FE of certified masks more or less same before and after dry heat sterilization for 30 and 60 minutes duration. Thus, it is recommended that, the N-95 masks can be sterilized in a few times with dry heat and H 2 O 2 vapors. Further, the autoclave sterilization methods can be also used but temperature of stream should be kept less (< 100 o C) so that the elastic cords of the masks remained in good condition.

We highlight few recommendations from the present studies:

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The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint • The breathability test (pressure drop) and filtering efficiency conveys that, the cloth masks with double or triple layer with tight weaves, low porosity and high thread count are potential substitute for medical or surgical masks (not for certified N-95 masks).

• The cotton mask is a potential substitute for the public instead of a surgical mask due to its cost effectiveness and it could be reused even after decontamination by any above methods (preferably 80-90 o C hot water wash).

• We recommend that hot water washing with or without detergent is quite good for sterilization of cloth masks which is very much affordable for the common public and it is very effective.

• N-95 masks, which are made of electrets filtering media, are not recommended for sterilisation or decontamination by ionising radiation, it will compromise the filtering efficiency. However, it is recommended that, masks can be sterilized in a few times with dry heat and H 2 O 2 vapors.

The authors acknowledge the Dr. S. Athamalingam, Associate Director, Health, Safety and Environmental Group (HSEG), Dr. R. Venkatesan, Head, Radiological and Environmental Safety Division (RESD) and Dr. C. V. Srinivas, Head, Radiological Impact Assessment Section (RIAS) for their encouragement and support to carry out this work. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted October 27, 2020. ; https://doi.org/10.1101/2020.10.23.20218073 doi: medRxiv preprint

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