A detailed investigation of N95 respirator sterilization with dry heat,hydrogen peroxide,and ionizing radiation

Abstract

In the present ongoing pandemic, the N95 respirator is an essential protective barrier to suppress the spread of the SARS-Cov-2 virus and protect the frontline worker from exposure. The N95 respirators are meant for single usage; however, they can be used after sterilization, considering the economy and shortfall in availability. At this juncture, the performance of the respirator after various types of sterilization and usage condition is required to be analyzed in detail. With this motto, this work has proceeded. The respirator’s filtration efficiency (FE), breathing resistance, and quality factor are evaluated for two face velocities (5.8 ± 0.2 and 26.4 ± 0.9 cm/s). Sterilization techniques used here are dry air oven heating at 70–80 ± 3°C for 30 and 60 min, gamma irradiation for cumulative dose 15 and 25 kGy, and soaking in liquid hydrogen peroxide for 30 min. The filtration performance and electrostatic surface charge density are used to determine the facemask’s efficacy after sterilization. The respirator’s physical, chemical, and morphological degradation were investigated using materials area density, microscopic analyses, FTIR, Raman spectroscopy, EPR, and TGA analyses. The highest reduction in filtration efficiency is 29.36 ± 0.49–36.08 ± 1.78% after irradiation due to a reduction in the charge density (71–133%) of the respirator layers. However, the FE does not reduce significantly (0.39 ± 0.52 to −2.46 ± 0.60) for dry air heat and H₂O₂ sterilization despite a change in charge density (0.4–53%), but there is no direct correlation with FE. Electrostatic charge measurement of the filtration layer is a crucial indicator of FE degradation. Hence, dry air heat and H₂O₂ soaking are found to be the most suitable sterilization methods. No significant degradation was observed on the physical, chemical, and morphological properties of respirators layers after sterilization.

Keywords

N95 decontamination Dry heat sterilization Surface charge density Personal protective equipment

Introduction

Personal Protective Equipment (PPE), such as respiratory facemasks, offers protection against the spread of COVID-19 through a droplet and aerosol transmission.¹ When an infected person breathes (exhale), speaks, sneezes, and coughs, the virus-containing droplets (>5.0 μm) and aerosols (<5.0 μm) will be excreted, remain suspended up to a few hours in the environment, and travel a longer distance in the air with the wind.^2,3 N95 is an essential PPE to protect the wearer with at least 95% FE for Most Penetrating Particle Size (MPPS) 0.1–0.3 μm and higher efficacy for particles smaller or bigger than MPPS.^4,5 Hence, the N95 facemask offers excellent protection when it correctly fits over the face. N95 masks are made of filter fabric, which consists of nylon, cotton, polyester, and polypropylene.⁶ Though the N95 facemask was designed for single-use, the recent pandemic crises made it necessary to seek emergency alternatives for decontamination and reuse procedures. Globally, frontline workers are facing a shortage of certified masks because of the sudden increase in demand.^7–9 Toward this crisis, facemasks are being reused multiple times after sterilization with the appropriate method without compromising their effectiveness.

Several non-traditional sterilization/decontamination methods have been evaluated with various degrees of success.^9,10 Many potential sterilization methods have been explored already, and they are based on chemical and physical processes. The chemical methods include immersing the mask with liquid/vapor hydrogen peroxide, chlorine dioxide, ethylene oxide, chlorine dioxide, sodium hypochlorite, bleach, alcohols, soap solution, and ozone decontamination.^11–14 The techniques such as soaking/dipping in alcohol, chlorine, soap, and bleach damage the fibers of facemasks and significantly degrade the respirator’s filtration ability.¹⁵ The liquid/vapor hydrogen peroxide and ethylene oxide are reported to preserve filtration characteristics, but it needs complicated equipment, extensive procedure, low throughput, and trained human resource.^12,14 The physical methods include treatment with dry/steam heat, microwave oven, hot water, UV light sterilization, electron beam, and ionizing radiation.^16–18 Although UV radiation does not affect the filtering efficiency, it seldom penetrates the respirator’s filter layers, resulting in partial sterilization.¹⁹ Sterilization by ionizing radiation and the electron beam is a well-known technology with large throughput for PPE and other medical equipment; however, recent studies show that the FE of N95 facemasks significantly deteriorates after gamma sterilization.^20,21 Nonetheless, it was helpful for surgical and textile facemasks.^9,20 The dry/steam heat treatment has emerged as a promising and straightforward decontamination method for respirators with good scalability and uniformity in disinfection.^17,22 The steam sterilization (generated by autoclave and microwave) was found helpful as far as FE, but it may deteriorate fabric materials of the N95 mask.^9,23 However, understanding the physical and chemical changes induced by the various treatments remain unexplored or incomplete in a few cases. Therefore, examining the potential changes in physical structure and in particular, the electrostatic charge density status of decontaminated respirators should be carried out.²⁴

Hence, the effect of decontamination processes on the electrostatic charge status, physical and chemical change in the fiber of the N95 respirator have been studied in the current work. The sterilization methods used in this work are (i) cumulative ionizing radiation dose of 15 and 25 kGy, (ii) dry heat (the temperature at 70–80°C) for 30 and 60 min, and (iii) liquid soaking in hydrogen peroxide (10% concentration) for 30 min. The respirators were evaluated for filtration performance (FE, breathing resistance, and quality factor) before and after the decontamination. Here, we carefully studied the FE and surface charge density on different layers of the facemask to notice a relation to the electrostatic filtration before and after various sterilization. In addition to that, the physical and chemical changes to the respirator fiber material were examined using an optical microscope, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), electron paramagnetic resonance (EPR), Raman spectroscopy, and thermogravimetric analyses (TGA). Most of the recent studies are focused exclusively on the respirator’s filtration performance.^{14,17,22,25,26} Nonetheless, the current work aimed to investigate the effectiveness of the N95 respirators after decontamination by various methods and, most importantly, the origin cause for degrading respirator performance. The detailed experimental materials and methods, data analysis, results and discussion, conclusion are described in this article.

Materials and methods

Protective respirator

The N95 respirator (Model: N95001 and Approved No.: 84-6969) was approved by the National Institute for Occupational Safety and Health (NIOSH) and manufactured by Magnum Health & Safety Pvt. Ltd, India, is used in the present studies.²⁷ Detailed specifications of respirators along with the optical image are summarized in Table 1. The inner and outer layers of the respirator are made of spun-bond polypropylene fiber mat, while the filter layer is made of the melt-blown polypropylene fiber mat. The NIOSH-certified N95 mask could filter out all particles, including bacteria and viruses, more than 95% for MPPS.^20,27,28 [21, 28, 29].

Table 1.

Detailed specifications of N95 respirators along with an optical image.

S. N.	Properties of respirator	Description
1	Optical image
2	Certification and standard	NIOSH (42 CFR 84)
3	Type and color	Flat fold shape (V shape/duckbill shape) and white
4	Layer of construction	3 layers: Outer layer for fluid repellent, filter layer made of two sub-layers (primary and secondary), inner layer
5	Material	Polypropylene (PP), outer and inner layer is made of spun-bond PP, and filter layers are made of melt-blown PP
6	Areal density (mg/cm²)	15.74 ± 0.55
7	Fiber diameter range (μm)	20.19 ± 1.35 for the outer and inner layer and 3.58 ± 1.3 for the filter layer

Filtration test setup

The test setup consists of a stainless-steel cylindrical duct of 8.5 cm internal diameter with upstream and downstream parts and a middle region to connect filter media. The total length of the test rig is 190 cm. The upstream and downstream concentrations were measured at 45 cm away from the test specimen to get uniform concentration along the entire duct. The schematic diagram of the test setup was presented in our earlier articles.^9,20,29 However, the schematic diagram of the stainless-steel cylindrical duct system is shown in the Supplementary material (Figure S1). The tested specimen (without cutting the respirator) connected between two flanges fitted with O-rings and mounted in the duct without any air leakage. The desired airflow rate is attained through suction by an air displacement pump and flow meter. The aerosol concentration is sampled before and after the specimen when the fluid (aerosol-air suspension) passes through the respirator. The details of the experimental setup, necessary data acquisition systems, and filtration evaluation of respirators for ambient and laboratory-generated aerosols are explained in earlier work.^9,20 In addition, the detailed description of the equipment used, and their main measured characteristics are described in Table 2. The aerosols suspended in ambient air are constantly introduced at the inlet of the test rig. The aerosol sample was taken from the upstream and downstream at the same time using two Optical Particle Counter (OPC). The OPC has a limitation in measuring the particle size of less than 0.3 μm.

Table 2.

The details of instruments used in the present study, their description, and main measured parameters.

S. N.	Equipment name	Description of specifications (make and model)	Measured parameters/characteristics
1	OPC	OPC (Model: 1.108, M/s Grimm Aerosol Technik, Germany) works on the single-particle light scattering principle. The sample flow rate of OPC is 1.2 ± 3% L/min, and reproducibility is >97% for the entire measuring range	Aerosols number concentration and size in a range of 0.3–20 µm
2	Flowmeter	Flow controller (M/s. Weiser)	To control and monitor the air flow rate during the experiment
3	Air displacement pump	Make M/s. Rotovac and CG commercial motor	For suction of the air from the test rig
4	VelociCalc air velocity meters	Multifunction tester for temperature, air velocity, humidity, and differential pressure (make and model: Velocicalc, TSI, USA, and 9565P143003)	Air velocity monitoring, pressure drops across, relative humidity and temperature during the masks testing, and room environment
5	Gamma irradiator	Gamma chamber (Model and Make: GC500 and M/s BRIT, India), the source is Co-60 (gamma energy 1.17 and 1.33 MeV), dose rate −1.94 kGy/h	Mask is irradiated for a cumulative dose of 15 and 25 kGy
6	Hot air oven	Labtherm scientific products, Sr No.0013, the air temperature can be varied from 25 to 200 °C	Masks are sterilized at 70–80 °C temperature
7	Microbalance	(M/s AND cooperation, Japan, and model number: GR 202), gravimetric analysis	Weight of the masks
8	Optical microscope	Magnified image of an object specimen, the image has been taken differential interference contrast (DIC) modes (Model and Make: Carl Zeiss, Axio Vert.A1)	Optical image of the various layers of the facemask
9	Keithley electrometer	Make M/s. Tektronix USA and Model: 6517B	Used for surface charge measurement of the various layers of respirators
10	ATR-FTIR	Make: ABB and Model: MB 3000 with a deuterated triglycine sulphate detector and analyzed by Horizon MB software	For a chemical change of surface layers of respirators
11	Raman spectroscopy	Renishaw micro-Raman (model inVia) spectrometer working in the backscattering geometry	To evaluate any chemical change of filter layers of respirators
12	EPR	Make and model: Bruker EMX X-band EPR spectrometer with TE₁₀₂ rectangular cavity	Sterilization-induced radicals and their effect on a respirator
13	TGA	Make and model: TA instrument, TGA Q500 V20.10 build 36	Thermal properties of respirator

Sterilization/decontamination methods

Three sterilization methods are employed, viz. ionizing irradiation, dry heat, and soaking in H₂O₂ liquid. The antibacterial and virus decontamination test has been performed in many laboratories worldwide, and few researchers have reviewed and compiled the literature.^9,30–33Nevertheless, the relevant antibacterial test and virus deactivation studies pertinent to the scope of the present paper, along with pros and cons, have been summarized in Table 3. The sterilization methods have been chosen based on the large throughput and easy availability of sterilization infrastructure at an affordable cost for various people. The respirators were sterilized with ionizing radiation of two doses, 15 and 25 kGy, using a gamma chamber having a dose rate of 1.94 kGy/h in air. The required time needed to deliver the desired dose is 12.89 h for the highest irradiation integral dose. Gamma irradiator mainly consists of a set of stationary Cobalt-60 sources placed in a lead-shielded cylindrical cage (17.2 cm diameter and 20.5 cm height) for uniform dose rate throughout. The dose variability inside the chamber was ≤10%, based on the most recent calibration.²⁰ Before irradiation, each mask was packed into an airtight plastic bag to prevent any contamination during irradiation.

Table 3.

Sterilization methods used in the present studies and their antibacterial and virus deactivation test parameters.

Sterilization methods	Sterilization effect on bacteria and viruses	Advantages	Disadvantages
Gamma irradiation for cumulative dose 15 and 25 kGy	• Gamma radiation dose <10 kGy effectively deactivates pathogenic microorganisms such as Salmonella and Campylobacter³⁴ • Gamma sterilizations inactivate many pathogens, including SARS-CoV-2, even at a low dose of 10 kGy.^24,35–37 • The irradiation dose between 20 and 30 kGy dose is more frequently used to treat food that is intended for consumption³⁴ • ISO (2013) recommends a sterilization dose of 25 kGy for single-use medical devices.³⁸	• Pre-existing large-scale facilities • Better penetration and certainty of sterility of densely packed respirators • Independent from temperature and pressure conditions • Minimum exposure during transportation and sterilization^{24, 39}	• May damage the physical properties of respirator materials • Enhance polymer oxidation, chain scission or cross liking, surface charge modification of electret material • Required skilled person and training to operate sterilization facility^{24, 39}
Dry heat for 30 and 60 min	• Heating at 70^°C for 5 min decontaminate the virus SARS-CoV-2,⁴⁰ • Heating at 80^°C for 20 min inactivates 49–65% of the virus,⁴¹ • 60 min exposure to 70°C dry heat inactivation of SARS-CoV-2 in of N95 respirators • Heat treatments at 75°C for 30 min or 85°C for 20 min at 100% RH resulted in efficient sterilization from the fabric of SARS-CoV-2, human coronavirus NL63 (HCoV-NL63), and another enveloped RNA virus, chikungunya virus vaccine strain CHIKV-181/25⁴²	• Leaves no toxic residue and is inexpensive • Used for sterilization of equipment, which is heat and moisture resistant • Can fully be penetrant to the porous layers of mask surface • Widely available in commercial kitchens, laboratories, and medical facilities⁹	• Material (fabric) may deteriorate if sterilization temperature is high⁹
Liquid H₂O₂ soaking or submersion	• Vaporized hydrogen peroxide exposure inactivates SARS-CoV-2 and other pathogens in N95 respirators,⁴³ • Effective inactivation of coronavirus with 0·5% hydrogen peroxide within 1 min,⁴⁴ • 3–6% liquid hydrogen peroxide would deactivate SARS-CoV-2 if N95 submerged for 8 to 30 min at 20^°C.^45,46	• Fast and effective against a large set of microorganisms • Each hospital may have the infrastructure • Low-temperature sterilization and safe to the environment^{9, 46}	• Degassing and removal of H₂O₂ vapors is difficult and time-consuming • Need complicated equipment, trained technicians, and may not thoroughly sterilize^{9, 46}

The dry heat sterilization of respirators was carried out for two different time durations of 30 and 60 min using a hot air oven with air temperature ranging from 70 to 80 ± 3^°C. The sterilization temperature and duration are chosen accordingly to deactivate SARS-COV-2 without damaging the facemask fiber integrity.⁴⁷ The liquid H₂O₂ (M/s Thermo Fisher Scientific Pvt. Ltd., India, and brand name is Qualigens) is diluted with distilled water at a 1:9 ratio of H₂O₂ and water. The mixed solution is stirred for about 10 min to homogenize the solution, and then the respirator is submerged for 30 min. The H₂O₂ concertation and duration of soaking are selected for better sterility based on the literature. Special care has been taken in each sterilization process not to bend or damage the respirator to avoid reducing FE, decreasing surface charge density, and mechanical properties. The facemasks were dried overnight after soaking in H₂O₂ before performing the FE testing in the room environment. The ambient temperature and humidity are in the range of 25–30°C and 60–80%, respectively. The measuring range equipment for air temperature and RH are 10 to 60 ± 0.3^°C and 5 to 95 ± 3%, respectively.

Test conditions

All respirators, pristine and sterilized were tested for FE under fully sealed conditions. The effective area of the tested masks in the present work was 56.74 cm² which was in line with previous results. The effective face area used for testing of facemask/filter were ranges from 6.4 to 78.5 cm².^48–51 To better simulate various human breathing efforts like regular breathing rate and moderate/strenuous exertion, the respirator was tested for two flow conditions, 20 and 90 L/min. The face velocity of 5.8 ± 0.2 and 26.4 ± 0.9 cm/s corresponds to low and high flow rates. NIOSH-approved respirators are certified under 85 ± 4 L/min flow.⁵² The European standard is followed where material penetration is evaluated at a flow rate of 95 L/min.⁵³ Many investigators in different countries have used the flow rate in the range of 20–30 L/min and 80–100 L/min.^48,50,54 FE also depends on the environmental conditions (temperature and RH%) apart from air stream flow rate, aerosol characteristics, and respirator fiber characteristics. During the tests, the measured ambient air temperature is 27 ± 0.3^°C, and relative humidity is 60–70 ± 3%, which is the dominant atmospheric condition in our region. Interestingly, it is worth mentioning that the masks are conditioned at 85 ± 5% RH to simulate the actual usage condition during breathing because breathing creates high humidity.^55,56 Before starting each test, the tested facemask is conditioned for about 30 min. A summary of the test conditions is given in Table 4. Each type of respirator was tested for two face velocities and four replicates to minimize the experimental error.

Table 4.

Summary of the experimental test conditions.

Variables	Quantity/levels
Respirator	Pristine (4 Nos.), ionization radiation (each 4 Nos. For 15 and 4 for 25 kGy doses), dry heat (each 4 Nos. for 30 min 60 min), and soaking in H₂O₂ (4 Nos.) = 24
Face velocity	Each respirator tested at constant face velocity for 5.8 cm/s (20 L/min) and 26.4 cm/s (90 L/min)
Challenge aerosol	For atmospheric aerosol particulate >0.3 µm
Total experiment runs	24 respirator × 2 face velocity = 48

Data acquirement and analysis

The performance of the tested respirator was assessed in two aspects: (i) the FE, which was determined as % of aerosol particle retained by the respirator, and (ii) the comfort of the mask, which was represented by the breathing resistance (R_f). A good respirator model should combine high efficiency with low resistance; however, the resistance and filter efficiency often conflict with each other. Comprehensively considering these two parameters, the overall performance of a respirator can be assessed by the quality factor (Q_f). A higher Q_f value corresponds to the better overall performance of the respirator.

Filtration efficiency (FE %)

FE of the respirator is determined before and after sterilization using the following formula

F E (%) = (1 - \frac{C_{o u t}}{C_{i n}}) * 100

(1)

where C_in and C_out are the average aerosol concentrations in counts/liter of upstream and downstream.

Breathing resistance (R_f)

R_f was determined as the pressure difference between the upstream (P_in) and downstream (P_out) side of the respirator when applied constant airflow

R_{f} = P_{i n} - P_{o u t}

(2)

where P_in and P_out in Pa.

Quality factor (Q_f)

A commonly used relation for filtration quality factor to determine the performance of the respirator is defined as^15,57,58

Q_{f} = - l n (1 - 0.01 * F E) / R_{f}

(3)

When the FE is ≥95%, the quality factor is more than 0.031 Pa⁻¹ and 0.022 Pa⁻¹ for inhalations and exhalation resistance limit at 345 and 245 Pa breathing resistance. A higher FE and lower R_f yield a higher filtration quality factor, that is, better performance of the respirators.

The sampled data has been recorded for about 2–3 minutes in 6 s frequency of each tested condition from OPC (total sample record 20–30). FE estimated by taking the average of the collected data (upstream and downstream) for each mask for the tested period. FE averaged for all similar test conditions (face velocity and sterilization methods) and four replicates. To evaluate the statistical difference in the FE, R_f, and Q_f after various sterilizations, one-way ANOVA was performed for two-face velocities. p-values < 0.05 were considered statistically significant. The FE, R_f, and Q_f were calculated by equations (1)–(3) to assess the respirator’s particulate protection, breathing comfort, and overall performance. However, investigating the deterioration of electrical and material properties of polypropylene fiber needs further investigation.

Electrostatic charge measurement

The electrostatic surface charge density is determined by measuring the surface charge on the respirator layers. The Keithley electrometer used for charge measurement of the respirators and the schematic presentation is given in Figure S2 (Supplementary material). The electrometer’s input is a three-lug (Triax) connector, with the red-colored innermost wire (input high) as the charge-sensing terminal. The charge-sensing terminal is connected to the metal surface (Copper plate). The metal noise shield (another copper enclosure) is connected with black wire (input low), and the third wire (green color) is grounded. Both copper plate and noise shield are electrically isolated by introducing an insulating Teflon sheet in between. Resolution of charge measurement capability is few femtocoulombs (fC) with an accuracy of 0.4% of the measured value. The test sample is a piece that has an area of 10 cm² (4 cm x 2.5 cm) cut from each respirator. Five samples were analyzed for each test case, and the average value with standard deviation was reported.

Raman spectroscopy

Each respirator sample was analyzed using a micro-Raman spectrometer working in the backscattering geometry and excited with a diode pump solid-state (DPSS) laser operating at a wavelength of 514 nm. The laser beam was focused through a 20× microscope objective. The power of the laser at the sample was approximately 2 mW. The scattered light was recorded using a Peltier-cooled CCD. Raman spectra were recorded on multiple fibers at various locations of each filter layer of a respirator.

Fourier transform infrared analysis

The respirators have been analyzed before and after decontamination by Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR). All the spectra of respirator layers were recorded from 4000 to 500 cm⁻¹ wavenumber with a deuterated triglycine sulphate detector and analyzed by Horizon MB Software. All measurements were made using 50 scans and 4 cm⁻¹ resolutions.

Electron paramagnetic resonance analysis

Electron paramagnetic resonance analysis of pristine and sterilized filter layer of the respirator was carried out using Bruker EMX X-band EPR spectrometer with TE₁₀₂ rectangular cavity. All the EPR spectra records were performed at room temperature using 9.84 GHz microwave frequency, 6.23 mW microwave power, 100 kHz modulation frequency, 8G modulation amplitude, 40.96 ms time constant, and 41.94 ms sweep time. Approximately 20 mg filter layer samples were taken in a quartz tube (3 mm inner diameter) for EPR analysis.

Thermogravimetric analyses analysis

A thermogravimetry instrument was used to study the sterilization effect on the thermal properties of the respirator’s fiber mat. All thermal analysis experiments were carried out in argon atmosphere from room temperature to 700^°C with a 10^°C/min heating rate.

Respirator structure and microscopic analysis

The material area density (fabric mass per unit area) is determined by weighing a specified area of 1 inch² (2.54 × 2.54 cm²) of each layer of respirator material. The fabric mat weight has been calculated based on each layer. An analytical microbalance with an accuracy of 0.1 mg was used to weigh the masks, and the error in the dimension of the length of the respirator specimen is 0.1 cm. Further, macro and microscopic assessment of the physical structure of the respirator was examined by the optical microscope. The average diameter of fibers of different layers of respirator is assessed using Image J software.

Results and discussion

FE for aerosol optical size >0.3 µm

The measured upstream aerosols concentration, for ambient aerosols of <1.0 μm size, ranges from 97.45 to 97.75% and >1.0 μm, ranges from 2.55 to 2.25% for the face velocity of 5.8 cm/s. At the higher face velocity (26.4 cm/s), it ranges from 98.03 to 98.22% and 1.97 to 1.78% for aerosols <1.0 μm and >1.0µm, respectively.⁹ The highest aerosol concentration is observed to be in the range of 0.3–0.5 μm. A significantly lower aerosol concentration is observed in the upper size range >1.0 μm for both face velocities. The data analysis was excluded for separate size bins because as size increases statistical error will be upsurged due to lower concentration. In addition, it is to be noted that, the separate size bin analysis of FE of N95 masks is greater than 99% for particles higher than 1 μm after various sterilizations.⁹ Nevertheless, the foremost objective of the present work is to investigate the root cause of FE degradation based on the physical and chemical characterization of the various layers of mask media rather than size-dependent FE. Hence, the FE evaluation of the facemask for two face velocities and four replicates assessed for aerosol diameter >0.3 μm and is briefed in the Supplementary material (Table S1).

The average FE of the pristine and sterilized respirator was determined for the particle of optical diameter >0.3 μm for two face velocities and summarized in Table 5. As expected, the FE is 99.73 ± 0.08 for low face velocity and 95.31 ± 0.45 for high face velocity. The FE of respirators remained the same after dry heat and H₂O₂ sterilization for low face velocity. However, the FE slightly increased after dry heat sterilization and remained the same after H₂O₂ sterilization for high face velocity conditions. The FE decreased significantly (p < 0.05) after gamma irradiation for both flow conditions. The reduction in FE after radiation sterilization is 29.36 ± 0.49–31.12 ± 0.30% for low face velocity and 34.26 ± 0.69–36.08 ± 1.78% for high face velocity. The N95 mask has electret fibers layers that are efficient at low airflow velocity rather than high airflow velocity.⁵⁹ The reported data in Table 5 suggest that the tested mask has limited protection against aerosols after ionizing radiation sterilization, which is consistent with previous studies.^{9,20,21,24,39}

Table 5.

Filtration efficiency, breathing resistance, and quality factor of respirator for two face velocity, control, and after decontamination.

Sterilization methods	Face velocity: 5.8 ± 0.2 cm/sec			Face velocity: 26.4 ± 0.9 cm/sec
Sterilization methods	Mean FE ± SD (%)	R_f ± error (Pa)	Q_f ± 0.001 (Pa⁻¹)	Mean FE ± SD (%)	R_f ± error (Pa)	Q_f ± 0.001 (Pa⁻¹)
Control	99.73 ± 0.08	31.38 ± 2.16	0.188	95.31 ± 0.45	258.90 ± 4.61	0.012
Heat (30 min)	99.30 ± 0.54 (p = 0.1647)	30.60 ± 2.26	0.162	97.65 ± 0.50 (p = 0.0004)	264.98 ± 3.73	0.014
Heat (60 min)	99.34 ± 0.46 (p = 0.1514)	30.65 ± 2.25	0.164	97.55 ± 0.46 (p = 0.0004)	265.21 ± 3.69	0.014
H ₂ O ₂	99.15 ± 0.38 (p = 0.0239)	30.40 ± 2.45	0.157	95.88 ± 0.61 (p = 0.1851)	238.11 ± 3.53	0.014
15 kGy	68.69 ± 0.37 (p = 0.0000)	30.89 ± 2.64	0.038	60.92 ± 1.50 (p = 0.0000)	257.71 ± 3.59	0.004
25 kGy	70.44 ± 0.54 (p = 0.0000)	30.90 ± 2.65	0.040	62.66 ± 0.40 (p = 0.0000)	257.75 ± 3.63	0.004

Breathing resistance (R_f)

R_f is an important parameter to characterize the performance of the respirator. The measured R_f across the respirators before and after sterilization for two face velocities is given in Table 5. As expected, the R_f across respirators increases with an increase in face velocity. The average R_f of control respirators is 31.38 ± 2.16 and 258.90 ± 4.61 Pa for low face velocity and high face velocity correspondingly. The R_f across respirators remains similar after dry heat, H₂O₂, and ionizing radiation sterilization for low face velocity. In the case of high face velocity, the R_f almost remained the same after ionizing irradiation; however, it slightly increased after dry heat sterilization and decreased after H₂O₂ disinfection. For all tested conditions, the R_f indicates the condition for usage during breathing (even for high face velocity), which is in the accepted range, 345 and 245 Pa for inhalation and exhalation resistance limit, respectively.⁶⁰ These findings reveal that the tested N95 mask did not deteriorate as far as breathing resistance was concerned after sterilization.

Quality Factor (Q_f)

Based on the FE and R_f reported in Table 5, the Q_f was calculated for two face velocities to assess the overall performance of the tested respirator. The results are presented in Table 5. The higher Q_f value corresponds to the better overall performance of respirators. The Q_f of pristine and decontaminated respirators ranges from 0.188 to 0.157 Pa⁻¹ for 5.8 cm/s face velocity and 0.012–0.014 Pa⁻¹ for 26.4 cm/s face velocity for dry heat and hydrogen peroxide sterilization. In the case of gamma radiation, the Q_f becomes 0.04 and 0.004 Pa⁻¹ for low and high face velocity, respectively. The decrease of Q_f value with an increase of face velocity is consistent with the literature. The Q_f values of respirators at low face velocity and after sterilization are more than defined values for inhalation and exhalation resistance. However, it is found less than the defined limits (0.031 Pa⁻¹ and 0.022 Pa⁻¹ for inhalations and exhalation) for high face velocity due to high-pressure drop across the mask (except gamma sterilization). In the case of ionizing radiation decontamination, the Q_f value is 3–5 times lower than the pristine respirator, which was mainly a reduction in FE. The overall performance of the tested respirator based on FE, R_f, and Q_f shows that the respirator was worsened only after gamma sterilization.

Electrostatic surface charge density (σ)

The electrostatic charge on respirators is measured before and after sterilization to evaluate the effect of surface charge density on FE. The average surface charge on a full respirator (without cutting and separating each layer) is measured and summarized in Table S2 (Supplementary material). The average surface charge on the control respirator is 1.031 ± 0.277 nC, which is in line with other literature.⁶¹ The polarity of the surface charge on the respirator changed after sterilization; however, the magnitude of charge has increased despite sterilization methods. The variation in the average surface charge between pristine and sterilized respirators ranges from 2 to 7 times. The highest variation was observed after gamma sterilization, while the lowest was in dry heat sterilization. Here, we observed that the full respirators charge measurement does not convey any correlation with FE. However, FE has decreased after gamma sterilization, but the average charge on the respirator surface increased.

To investigate more insightful information on the surface charge on each layer of respirator, the surface charge is measured by separating each layer of the filter media with the sample area 10 cm². Care has been taken not to damage the filter media while cutting otherwise which would alter the electrostatic charge measurement. The σ (nC/m²) calculated for each respirator layer from measured charge and surface area is summarized in the Supplementary material (Table S3) and presented in Figure 1. The measured σ of pristine respirator is negative polarity for the outer (−1927 ± 662 nC/m²) and the inner layer (−2220 ± 203 nC/m²), while for the filter layer, it is positive (+3530 ± 750 nC/m²). The electrostatic charge on pristine masks showed that each layer has a significant amount of surface charge density. The charge density imposed on the respirators layer is the same order of magnitude as other reported values.⁶² It appears that charge density on each layer gets reduced after sterilization, and polarity has changed for the filter layer after gamma irradiation. After sterilizing the respirator by dry heat and H₂O₂, the polarity of charge density remains similar, but the magnitude persists for H₂O₂ and decreases for dry heat decontamination. After gamma irradiation, the polarity of charge density on the filter layer changes while remaining identical for the inner and outer layers; however, a significantly considerable reduction in charge density. The measured σ has a significant statistical variation, and absolute values are uncertain; however, the substantial change in charge density is detected qualitatively. The present observation on electret electrostatic charge in line with other literature^21,61,62 shows inconsistent variation.

Figure 1.

Electrostatic surface charge density on each layer of a pristine and sterilized respirator.

The percentage change in FE (ΔFE%) and surface charge density (Δσ%) after different sterilization from pristine facemasks are summarized in the Supplementary material (Table S4). The ΔFE of the facemask from pristine to dry heat and H₂O₂ sterilization is ranged from less than 0.57% and - 2.46% for low and high face velocity. However, FE changes around 29.36 ± 0.49–36.08 ± 1.78% for gamma sterilization for both flow conditions. The ΔFE% is higher for gamma sterilization and further enhances for higher face velocity. The Δσ on the outer and inner layers ranges from 70.98 to 86.13%, while for filter layer ranges from 122.69 to 133.67% for gamma sterilization. In case of dry heat sterilization, the Δσ on the outer, inner and filter layer ranges from 3.10 to 53.10%, and for H₂O₂ sterilization, the Δσ ranges from 0.35 to 8.70%. The most significant reduction in charge density observed after gamma sterilization correlates with the decrease in respirator FE. The FE does not affect much after dry heat and H₂O₂ sterilization despite the change in surface charge density. There is a qualitative correlation between σ and FE; masks with higher reduction in charge density show a higher decrease in FE. Nevertheless, the finding is consistent with other studies that have observed the degradation in the FE of respirator associated with filter media electrostatic charge degradation. Previous studies also demonstrated similar observations.⁶²

It is known that aerosol filtration occurs by five basic mechanisms, viz. gravitational settling, inertial impaction, interception, diffusion, and electrostatic force.^63,64 The N95 mask consists of electrets fibers media capable of capturing and retaining fine particles through electrostatic and mechanical filtration processes.⁶⁵ However, the mechanical filtration was not influenced by electrostatic force, that is surface charge density. The least efficiency is associated with particles from 100 to 500 nm, more prominent for diffusion and lesser for interception; hence, the 95% efficiency achieved for this range of particles by electrostatic force and dielectrophoretic force. When the electret media modified its charges (σ), the efficiency may also get altered. It is found from the present studies that each layer of respirator has a different charge density and different polarity. The filter layer is composed of opposite charge layers, which gives rise to a strong electric field gradient inside the filter layers. In the absence of a durable inhomogeneous electric field, the respirator’s filtration properties are reduced, which is observed in ionizing radiation sterilization.^{9,20,21,24,39,61} The gamma irradiation produced significant bipolar ions (positive and negative ions) in the surrounding environment and in the respirator fiber material (electrons and/or holes). These ions attracted, trapped, de-trapped, and recombined in the PP fiber structure and modified their surface charge density. The electrons and/or holes produced inside the PP fibers during irradiation also contributed to reducing surface charge density.⁶⁶ The surface charge density decreased rapidly at the initial stage of irradiation exposure and got saturated when charge density fell below the value at which significantly ion recombination occurred. In the case of dry heat sterilization, the reduction in charge density is due to the de-trapping of electrons and holes, which depends on the sterilization temperature. With increasing temperature, trapped charged on PP layer de-trapped easily, that is, the charge dissipation flux is more prominent at a higher temperature.⁶² In the case of H₂O₂ sterilization, no significant reduction in charge density is observed.

Evaluation of physical, chemical, and morphological properties of facemask structure

N95 respirator comes in many brands and models that use different materials and methods to meet industrial filtration requirements. Most of the N95 respirators use a melt-blown electret material that employs electrostatic forces to increase the filtration for MPPS. Ionizing radiation and other sterilization methods are known to modify the properties and structure of the polymer. They can also adversely affect filter fiber in several ways, such as enhancing polymer oxidation, chain scission, cross-linking, and charge neutralization of electret material.^67,68 To check any significant change of the respirator, an investigation is carried out for physical, chemical, and morphological properties caused by sterilization. The respirator was analyzed using Raman spectroscopy, FTIR, EPR, TGA, optical microscopy, and gravimetry analysis.

Raman analysis

Changes in chemical structure due to sterilization were investigated by recording Raman spectra of pristine and sterilized respirators (Figure 2). Raman spectrum of pristine respirator exhibits multiple bands due to CH₂, CH₃ groups frequencies, and their coupled modes corresponding to isotactic polypropylene (iPP).⁶⁹ Bands in the 800 to 1200 cm⁻¹ range are attributed to the 3₁ helix, a regular conformation of isotactic polypropylene polymorphs.⁶⁹ No modification was observed in the number of peaks and peak positions, confirming that iPP retains its chemical structure even after various sterilizations. In addition, no chemical changes were observed in Raman analysis of gamma-irradiated facemasks up to 25 kGy. Although few studies show that >50 kGy gamma irradiation forms carbonyl and hydroxyl groups in iPP fiber,⁷⁰ however, in the present study, corresponding peaks were not present, which is in line with other work.²¹

Figure 2.

Raman spectra of pristine and sterilized filter layer of the respirators.

Fourier transform infrared analysis

Typical FTIR spectra of respirators filter layer with and without decontamination are shown in Figure 3. ATR-FTIR spectrum of the pristine respirator exhibits four IR peaks in the range of 2750–3000 cm⁻¹ due to stretching vibrations of CH₂ and CH₃ groups. Peaks at 2950 and 2871 cm⁻¹ are due to CH₃ asymmetric and symmetric stretching vibrations, respectively, while peaks at 2908 and 2833 cm⁻¹ are due to CH₂ asymmetric and symmetric vibrations, respectively.^71,72 ATR-FTIR spectrum of respirator also contains two intense peaks and various small peaks in 1500 to 750 cm⁻¹. The peak at 1457 cm⁻¹ is attributed to CH₃ asymmetric deformation or CH₂ scissor vibrations, while 1373 cm⁻¹ is due to CH₃ symmetric deformation vibrations.⁷¹ Absorption peaks displayed at 1162, 997, 971, and 899 cm⁻¹ are assigned to CH₃ rocking vibrations, while peaks at 843 and 805 cm⁻¹ are assigned to CH₂ rocking vibrations. Also, the peaks at 1162, 971, and 899 cm⁻¹ can be assigned to C-H wagging, C-C asymmetric stretching, and C-C symmetric stretching vibrations, respectively.^71,73 ATR-FTIR spectra of respirators confirm that the filter layer is made of polypropylene fiber. The peak position and number of peaks remain the same before and after various sterilization methods (Figure 3). It can be concluded that gamma irradiation and other sterilization methods have not caused any significant changes in the molecular structure of iPPs.

Figure 3.

ATR-FTIR spectra of pristine and sterilized filter layer of the respirators.

Thermogravimetric analyses analysis

Thermal properties of pristine and sterilized filter layer of respirator were analyzed with TGA and presented in Figure 4. The thermal properties of the pristine and sterilized respirators are identical. TGA curve and its derivative for the filter layers for pristine and sterilized masks showed a significant weight loss with a peak degradation temperature of 460^°C. Besides, no significant changes were observed in the TGA curve of the sterilized respirator layer, confirming no change in the thermal properties of facemasks after sterilization.

Figure 4.

TGA analyses (left figure) and derivative of TGA (right figure) curve of filter layer of respirators.

Electron paramagnetic resonance analysis

Sterilization of respirators can generate radicals (electrons and/or holes) that may lead to a change in the properties of the respirator’s fiber materials. EPR analysis was carried out to investigate the behavior of sterilization-induced radicals and their effect on respirator performance. EPR spectra of the pristine and sterilized respirator are shown in Figure 5. No change was observed in the EPR spectra of pristine, dry heat, and H₂O₂ treated filter layers. However, a gamma sterilized facemask exhibits three EPR signals at g₁ = 2.035, g₂ = 2.008, and g₃ = 2.005, and the signal intensity of these radicals increases with an increase in radiation dose. These radicals are attributed to stable peroxyl radicals (ROO^•) formed by exposure to carbon-based radicals (generated during irradiation) with atmospheric oxygen.^74,75

Figure 5.

EPR spectra of pristine and sterilized filter layer of the respirators.

Gamma irradiation-induced radicals may lead to further modifications in the respirator fiber material. The stability of radicals was investigated by storing the irradiated respirator at room temperature and recording the EPR spectra at different time intervals of up to 30 days. The signal intensity spectra and peak intensity variation with storage time for 25 kGy are shown in Figure 6, and for 15 kGy are given in the Supplementary material (Figure S3). The EPR peak signal intensity decreases with time for both radiation doses (Figure 6 and S2), and the signal fades 95–97% after 30 days from the radiation exposure. The radiation-induced radicals react with the polymer backbone via chain scission or cross-linking, which leads to a decrease in radical concentration. However, as revealed by FTIR and Raman studies, the dose used for sterilization (up to 25 kGy) is insufficient to induce any materials properties changes in the respirator layer. The iPP has excellent resistance to a radiation dose up to ∼10 kGy widely accepted and performed acceptably up to ∼40 kGy for year storage.⁷⁶

Figure 6.

Electron paramagnetic resonance signal intensity spectra and peak intensity variation with storage time.

Physical and morphological properties

The N95 respirators were disassembled to understand their structural components and possible physical and morphological degradation of the respirator. The masks are made of three primary structure layers: outer, filter, and inner layer, and the filter layer is composed of two sub-layers. The filter layer of the respirator is sandwiched between the outer and inner layers. The macro and microscopic analysis of the respirator physical structure was carried out using gravimetric analysis and an optical microscope. The material areal density of different layers of respirator is investigated and given in Figure 7. The measured average material areal density of the outer, filter, and inner layer of the respirator is 5.97 ± 0.29, 5.83 ± 0.31, and 4.06 ± 0.26 mg/cm², respectively. The fabric material area density is 30–32% higher for the outer and filter layer than the respirator’s inner layer. The material areal density remained almost the same before and after decontamination, and the variation was insignificant. The combined three layers of mask material’s area density are 15.74 ± 0.55 mg/cm².

Figure 7.

Comparison of material area density for pristine and sterilized respirator layers.

Further, optical microscopic analysis of polypropylene fiber layers of respirator has been conducted, and optical images are shown in Figure 8. There is no observable and significant change found in the morphology of the fiber after various decontamination. The respirator fiber diameter of the inner and outer layer is 20.19 ± 1.35 μm, and the filter layer is 3.58 ± 1.31 μm. The filter layer has less mean fiber diameter compared to the outer and inner layers.

Figure 8.

Optical image of three-layer certified mask: outer, inner, and filter layers of control and sterilized. The scale bar represents 200 μm in length for outer and inner layers and 100 μm for filter layers. Merge corner image is in higher optical magnification.

Conclusions

During the initial stage of the pandemic, it was unclear whether the supply chain of respirators could adequately respond to the demand. This motivated the present study for the reusability of respirators after sterilization. The N95 respirator was evaluated for particulate FE, breathing resistance, quality factor after sterilization using dry heat, H₂O₂ soaking, and ionizing radiation for a single cycle and two face velocities 5.8 and 26.4 cm/s. As expected, the breathing resistance increases with increasing face velocity; this reflects that it is harder to breathe through a facemask when the respiratory flow is high. The measured breathing resistance of the tested facemask is in the accepted range for inhalation and exhalation resistance, that is, breathing resistance was not changed significantly. The FE does not deteriorate considerably after dry heat for 30 and 60-min duration and H₂O₂ sterilization. However, in the case of gamma sterilization, FE reduces to 68.69 ± 0.35–70.44 ± 0.54% for low face velocity and is still reduced with higher face velocity. The quality factor of facemasks under-investigated sterilization conditions is in the acceptable range except for ionizing irradiation.

Further, the surface charge density on respirator layers was investigated and found to be changed 71–133% during the first cycle of irradiation sterilization. In the case of dry heat and H₂O₂ treatment, the surface charge density dropped 3–53% and 0.35–8.7% during the first cycle, respectively. The EPR analysis reveals that the ionizing radiation has generated substantial radicals inside respirator fiber media; however, in the case of dry heat and H₂O₂ sterilization, there are no significant radicals observed. The FTIR, Raman, and TGA spectra of the respirator’s layer revealed that irradiation and other sterilization methods have not caused any considerable chemical and thermal changes on the fiber materials. The morphological and materials area density analysis of the respirator layers indicates that no significant deterioration has occurred after sterilization. The most significant reduction in charge density observed after gamma sterilization correlates with reducing respirator FE. However, the FE does not affect much after dry heat and H₂O₂ sterilization despite the change in surface charge density. Thus, it is recommended that the N95 respirator may be sterilized a few cycles with dry heat and H₂O₂ soaking. N95 masks, made of electret filtering media, are not recommended for sterilization by ionizing radiation; it will compromise the FE, that is, quality factor. Nonetheless, few researchers advocate the reusability of masks post gamma sterilization by recharging the masks that recover the FE.^21,61

This study comprises electrostatic charge measurement to detect a change in FE after various sterilization methods. Like some other tests (the face fit test) is meant for control materials and can only determine the respirator’s fitness appropriate to the user. They cannot be able to conclude to what degree and why the masks are degraded after sterilization. Nonetheless, the fit test is important to check if the mask has any physical modifications following the decontamination process, which will be attempted in the future course of action. Most of the recent works focused on measuring the respirator’s surface potential and correlated it to the filtration performance.^21,62 Electrostatic charge measurement of the filtration layer is a crucial indicator of FE degradation. The present study was used to understand the degree of degradation under gamma sterilization and correlate with electrostatic charge properties on each respirator layer. However, it is worth noting that the large variability in charge measurement made it challenging to obtain high certainty in observations. Nevertheless, the finding is consistent with other studies that have observed the degradation in FE of masks associated with filter media electrostatic charge degradation.^{21,24,39,61,62} Further, the N95 respirators are available in many brands and models that use different materials and manufacturing methods to meet industrial filtration requirements. Additionally, electret charge measurement of other brands of respirators and each cycle of sterilization can also be assessed for their reusability and various sterilization methods.

Supplemental Material

sj-pdf-1-jit-10.1177_15280837221090663 – Supplemental Material for A detailed investigation of N95 respirator sterilization with dry heat, hydrogen peroxide, and ionizing radiation

Supplemental Material, sj-pdf-1-jit-10.1177_15280837221090663 for A detailed investigation of N95 respirator sterilization with dry heat, hydrogen peroxide, and ionizing radiation by Amit Kumar, Shailesh Joshi, Subramanian Venkatesan and Venkatraman Balasubramanian in Journal of Industrial Textiles

Footnotes

Acknowledgements

The authors acknowledge Director, IGCAR, Shri S. Athamalingam, Associate Director, Radiological and Environmental Safety Group (RESG), and Dr R. Venkatesan, Head, Radiological and Environmental Safety Division (RESD), for their encouragement and support to carry out this work. Dr O. Anna Lakshmi and Dr S. Balakrishnan were acknowledged for providing EPR and TG/DTA analyses facility. Dr Soumee Chakraborty and Dr Ch. Kishan Singh was admitted for Raman analysis and optical microscope of various layers of the mask. The authors would like to thank the reviewers for their valuable comments and suggestions, which significantly enhanced the article’s value.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Amit Kumar

Supplemental Material

Supplemental material for this article is available online.

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Supplementary Material

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A detailed investigation of N95 respirator sterilization with dry heat,hydrogen peroxide,and ionizing radiation

Abstract

Keywords

Introduction

Materials and methods

Protective respirator

Filtration test setup

Sterilization/decontamination methods

Test conditions

Data acquirement and analysis

Filtration efficiency (FE %)

Breathing resistance (Rf)

Quality factor (Qf)

Electrostatic charge measurement

Raman spectroscopy

Fourier transform infrared analysis

Electron paramagnetic resonance analysis

Thermogravimetric analyses analysis

Respirator structure and microscopic analysis

Results and discussion

FE for aerosol optical size >0.3 µm

Breathing resistance (Rf)

Quality Factor (Qf)

Electrostatic surface charge density (σ)

Evaluation of physical, chemical, and morphological properties of facemask structure

Raman analysis

Fourier transform infrared analysis

Thermogravimetric analyses analysis

Electron paramagnetic resonance analysis

Physical and morphological properties

Conclusions

Supplemental Material

sj-pdf-1-jit-10.1177_15280837221090663 – Supplemental Material for A detailed investigation of N95 respirator sterilization with dry heat, hydrogen peroxide, and ionizing radiation

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

Supplemental Material

References

Supplementary Material

Breathing resistance (R_f)

Quality factor (Q_f)

Breathing resistance (R_f)

Quality Factor (Q_f)