The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2, has challenged healthcare providers in maintaining the supply of critical personal protective equipment, including single-use respirators and surgical masks. Single-use respirators and surgical masks can reduce risks from the inhalation of airborne particles and microbial contamination. The recent high-volume demand for single-use respirators and surgical masks has resulted in many healthcare facilities considering processing to address critical shortages. The dry heat process of 80°C (176°F) for two hours (120 min) has been confirmed to be an appropriate method for single-use respirator and surgical mask processing.
The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has focused attention on the impact of virus respiratory disease transmission.1 Regarding disease transmissibility and infectivity, the virus has many similarities with other respiratory viruses.2 Preventing disease transmission is essential, particularly in high-risk situations such as in healthcare facilities and frontline use. Infection prevention strategies based on hazard assessments and the use of appropriate administrative and engineering controls, safe work practices, and personal protective equipment (PPE) are critical in these situations.3 During the COVID-19 pandemic, the supply of disposable filtering facepiece respirators (FFRs), such as N95 respirators, and other necessary PPE has been limited. Best practice guidance in these crisis situations includes strategies to control the use of FFRs, including extending their use and even limited reuse.4
At present, FFRs are considered single-use products. As per the recommendations of the Centers for Disease Control and Prevention, the processing (or decontamination) of FFRs should only be considered in instances of critical shortage.4 The Food and Drug Administration (FDA) has issued Emergency Use Authorizations (EUAs) for decontamination processes that can be used to mitigate FFR supply shortages.5 Considering the limitations of the FFR materials and functionality after decontamination is important. Most FFRs are constructed of polypropylene and polyethylene with or without cellulose materials. Single-use respirators and surgical mask types can be categorized when evaluating performance after processing. Specific performance standards are defined in ASTM F21006 and EN 146837 to ensure safe and effective use. Performance requirements are categorized respective to the intended use but, in general, should allow the wearer to breathe while providing a tortuous pathway to prevent the ingress of microorganisms.
Initial feasibility studies with moist and dry heat treatments were conducted to determine if single-use respirators and surgical masks were physically damaged upon visual inspection following exposure. Physical damage (shrinking, hardening, deformation, and/or delamination) was noted for the moist heat conditions (i.e., 134°C for 3 min and 121°C for 15 min) and dry heat conditions (i.e., 121°C for 180 min) tested. Dry heat processing became the focus of the because of to practicality and accessibility of the equipment, which is readily available to the broader public. Physical changes decreased with lower dry heat temperatures and were nondetectable at 80°C. Therefore, in this study, more detailed testing was conducted using the dry heat process of 80°C (176°F) and time duration of 120 minutes (2 h).
All Food and Drug Administration–cleared N95 respirators are labeled as “single-use,” disposable devices.
Single-use respirators and surgical masks are essential in reducing the risks associated with respiratory infection for healthcare workers and other essential personnel.
During respiratory virus outbreaks or pandemics, single-use respirators and surgical masks might be in short supply and limited processing might be appropriate.
Dry heat processing of single-use respirators and surgical masks can be a practical and effective method.
Materials and Methods
Single-Use Respirators and Surgical Mask Test Samples
Samples tested represented the most widely used single-use respirators and surgical masks based on material composition and construction. Surrogate single-use respirators and surgical masks from two manufacturers were selected for testing to represent the three categories defined in this study (Table 1). Of note, the categories were established based on primary materials of construction, with a key differentiation being with or without cellulose. N95 respirators typically use similar raw materials; therefore, types of adhesives, numbers of filter layers, and other attributes were not considered in the categorization. One-way valved respirators were not captured in the category criteria and considered out of scope for this study.
Samples from each category were prepared based on the purpose of, and requirements for, the specific testing. For microbiological testing, material samples were prepared by cutting 1 × 1 inch2 sections of the single-use respirators and surgical masks for each category. The respirators and surgical masks used in this study were not sourced as sterile. To reduce the potential for microorganisms to be present and therefore affect the results of the test, the 1 × 1 inch2 sections were processed using a dry heat cycle of 80°C for a minimum of 40 minutes. For physical/functionality testing, the single-use respirators were marked with the sample identification, number of processing cycles, and intended test method using a permanent marker.
Dry Heat Equipment and Exposure Parameters
All processing cycles were performed using a Blue M oven Thermal Product Solutions (model no. DCW-336-G-SP-GOP; Thermal Product Solutions, New Columbia, PA). Temperature distribution was confirmed using a thermocouple placed in contact with a single-use respirator or surgical mask surface within the processing load, with measurements recorded every minute of the cycle. The Blue M oven was preheated to 80°C. Single-use respirators and surgical masks were placed in the preheated oven, and the time for the items to reach 80°C was approximately 13 minutes prior to starting exposure for the specific testing performed. Single-use respirators and masks were exposed to 80°C (176°F) for two hours (120 min) for the microbial inactivation studies or three hours (180 min) as a maximum exposure time for 1, 3, 10, and 20 cycles for the functionality studies. Postexposure, samples were immediately removed from the Blue M oven and allowed to cool to ambient temperature on the bench countertop prior to testing.
Five test microorganisms were used to show inactivation (i.e., Mycobacterium species, two Gram-positive and two Gram-negative vegetative bacteria). The selection of these microorganisms was consistent with the FDA EUA guidance and represented the hierarchy of relative resistance to decontamination processes, of which enveloped viruses (e.g., SAR-CoV-2) were rated as least resistant.5,,15 The specific microorganisms used were Mycobacterium terrae ATCC 15755, Staphylococcus aureus ATCC 6538, Klebsiella pneumoniae ATCC 13883, Pseudomonas aeruginosa ATCC 9027, and Escherichia coli ATCC 8739. M. terrae was cultured in Middlebrook broth (Hardy Diagnostics, Santa Maria, CA) at 37°C for 14 days under a capnophilic environment (3–10% CO2 [Gas Pack System; BD, Franklin Lakes, NJ). All other strains were cultured in Trypticase soy broth (TSB [BD]) at 32°C for 36 hours.
A suitability test (i.e., bacteriostasis and fungistasis testing) was performed in accordance with ISO 11737-18 for each test microorganism and the extraction eluent (peptone Tween with sodium chloride) to ensure the mask materials did not inhibit microbial growth. In addition, bioburden recovery efficiency was calculated for each surrogate single-use respirator and surgical mask category and microorganism type by extracting inoculated mask samples in triplicate. Inoculated mask samples were extracted in 100 mL using a handshaker set to the maximum rotation setting (~100 rpm) for 10 minutes. Eluents were filtered through a 0.22-μm filter, placed onto Trypticase soy agar plates (or Middlebrook agar for mycobacteria), and incubated at 32°C for 36 hours (or 37°C for 14 days for mycobacteria). A correction factor for the microorganism recovery efficiency was determined by comparing the inoculate estimate with the number of colony-forming units (CFUs) recovered per first extraction, as compared with the total CFUs recovered from four extractions. Four extractions were deemed suitable to meet the criteria for extraction efficiency calculations per ISO 11737-1.8
For the inactivation studies, three test mask samples of each respirator and surgical mask category were prepared for each of the five microorganisms. Microorganisms were tested individually. Mask samples were prepared by inoculation with 100 μL for each microorganism from broth medium and allowed to dry for one hour. The inoculated mask samples then were placed into the oven and processed at 80°C (176°F) for two hours (120 min). Surviving microbial counts for each mask sample were determined using the bioburden extraction procedure previously determined, in compliance with ISO 11737-1.8 Microorganism inactivation was calculated by subtracting the calculated recovery (plate count times the correction factor) from the starting titer.
Single-Use Respirator Functionality
The surrogate N95 single-use respirators (categories 1 and 2) were tested for functionality. The surrogate surgical mask (category 3) was not tested for functionality, as it consisted of the same materials as the N95 single-use respirators. Surrogate single-use respirators were evaluated for performance using bacterial filtration, breathability, cytotoxicity, band attachment, and fit tests after multiple cycles. No physical manipulations of the respirators were performed between dry heat cycles. As these respirators are not typically reused, no standard method exists to consistently deliver worst-case physical conditions that might be observed during use. The bacterial filtration and breathability tests are standard methods used for challenging respirator function per ASTM F21019 and EN 14683.7 Cytotoxicity was performed to evaluate the potential for the dry heat process to liberate any harmful chemicals from the respirator materials. Band attachment testing was performed to evaluate for potential degradation over the series of dry heat cycles. Fit testing was performed to determine if fit degraded over the series of dry heat cycles. Surrogate single-use respirators of each category were exposed to 1, 3, 10, and 20 cycles (80°C (176°F) for three hours (180 min)) and compared with unexposed controls.
The bacterial filtration efficiency (BFE) test is designed for testing filtration materials and devices labeled for protection against biological aerosols. This test is performed by aerosolizing a suspension of S. aureus and exposing a sample with constant flow rate and fixed air pressure. The testing performed utilized a challenge suspension of S. aureus at 1.7 to 3.0 × 103 CFUs with a mean particle size of 3.0 ± 0.3 μm. The aerosols were drawn through a six-stage, viable particle Andersen sampler for collection and quantification and compared with the upstream challenge. BFE testing was conducted by a third-party laboratory in compliance with ASTM F21019 and EN 14683.7
The differential pressure (DP) test is designed to measure air flow through the single-use respirator to demonstrate breathability. The testing performed measured the differential air pressure on both sides of the single-use respirator using a monometer at a constant flow rate. DP testing was conducted by a third-party laboratory in compliance with EN 14683.7
Cytotoxicity testing is designed to demonstrate that there are no residuals from the dry heat process and no adverse changes to the materials. The testing performed using a surface area extraction ratio (3 cm2/mL) in which full single-use respirators were extracted in 214 mL for category 1 and 289.7 mL for category 2 of Minimal Essential Media (MEM) with 5% bovine serum for 24 to 25 hours at 37 ± 1°C with agitation. The test extracts were diluted in cell monolayers to the following ratios: 1:2, 1:4, 1:8, and 1:16. The diluted extracts in cell monolayers were performed in triplicate and incubated at 37 ± 1°C with 5 ± 1% CO2 for 48 hours. The cell monolayers were examined microscopically and scored based on the degree of cellular destruction from three wells, with the average reported as the final cytotoxicity score. Cytotoxicity MEM test results with a score of 2 or lower were considered noncytotoxic. Testing was conducted by a third-party laboratory in compliance with ISO 10993-5.10
Band attachment testing was conducted by measuring tensile strength. Tensile strength testing is designed to evaluate the performance of the elastic band attachment to the single-use respirator. Testing was performed using Instron equipment (model no. 5544; Instron, Canton, MA), with a load cell of 20-pound capacity, to measure the force required to pull the elastic band from the single-use respirator. The surgical masks were cut into portions to allow for insertion of the band into one of the grippers and the single-use respirator into the other. The gauge length for each test was 2 inches with a rate of pull at 12.0 inches per minute. Results were reported in pounds of force required to separate the band from the single-use respirator. No standard procedures exist for this method to measure attachment of the band to the single-use respirator. As such, control single-use respirators were used to determine if a difference was seen between nonprocessed and processed single-use respirators.
A quantitative fit test (QNFT) is designed to measure the single-use respirator fit to the user. A particle generator (model no. 801168; TSI, Shoreview, MN) using sodium chloride in solution is used to generate a mist for measurement of particle counts both inside and outside of the single-use respirator. Because of the current COVID-19 pandemic, only one individual served as the test subject for this test, and particle counts were measured using a PortaCount Plus (model no. 8048; TSI). A series of activities were conducted during the test, including having the test subject breathe normally, take deep breaths, turn their head side to side, move their head up and down, read text out loud, smile, and bend forward. The surrogate single-use respirator categories, exposed 20 times at 80°C (176°F) for three hours (180 min), were compared with the corresponding control. Each test was given a fit factor score, and the overall fit factor was the sum of all tests. This test was performed in accordance with procedures from 29 CFR 1910.134.11
During the method suitability testing, it was determined that microorganism titers prepared using sterile water would die off during drying, which would lead to overstating the microbial inactivation. Therefore, microorganism titers were prepared using TSB and Middlebrook broth as the diluents. The change to TSB and Middlebrook broth also provided a microbial reduction challenge that may be representative of conditions observed with clinical use.
The extraction efficiency from each category and test microorganism was similar, typically greater than 60%, with the exception of category 1 with M. terrae demonstrating an extraction efficiency of 47%.
The results for the microbial inactivation for each surrogate single-use respirator and surgical mask category are summarized in Table 2. The initial inoculum of each microorganism ranged from 4.7 × 107 to 1.7 × 108 CFUs. Microbial log reductions were determined, and the applicable correction factor was applied to the final microbial counts. All test microorganism/category combinations consistently demonstrated 7 log10 reduction or greater following dry heat processing at 80°C (176°F) for two hours (120 min).
Single-Use Respirator Functionality
A series of five functionality tests were conducted on the two categories of surrogate single-use respirators (single or multiple cycles at 80°C [176°F] for three hours [180 min]) and controls (unexposed single-use respirators).
Single-use respirator BFE and DP results are summarized in Table 3. All test results showed no significant (P ≤ 0.05) changes in functionality compared with unexposed controls and passed all acceptance criteria. The BFE showed no measurable differences in bacterial penetration over multiple cycles in comparison with controls, maintaining greater than 99.9% efficiency. Similarly, the DP results showed no significant changes in air flow measurements compared with controls over repeated cycles.
Cytotoxicity tests are summarized in Table 4. Both single-use respirator categories were determined to be noncytotoxic from the 1:2 dilutions and subsequent dilutions tested, as indicated by replicate MEM test results with scores of 2 or less. No significant change was observed between the control and the 20-times-processed single-use respirators.
Tensile strength testing (band attachment) and QNFT (fit to the user) results are summarized in Table 5. As the attachment test did not have a defined requirement for acceptability, a one-way analysis of variance was performed for the two surrogate single-use respirator categories with a significance level of P = 0.05. Data analysis from the 1×, 3×, 10×, and 20× cycles was compared against the unexposed control pull values. No significant change in the strength required to separate the band from the single-use respirator was observed. The QNFT also verified no significant changes in the fit of the single-use respirators after 20 cycles when compared with unexposed control.
This study confirmed that a dry heat process of 80°C (176°F) for two hours (120 min) would satisfy the requirements to appropriately decontaminate single-use respirators and surgical masks in an emergency use situation (e.g., supply shortage resulting from a pandemic). This dry heat process meets and exceeds the requirements for Tier 1 per FDA EUA guidance.5 Tier 1 requirements apply to single or multiple users demonstrating a 6 log or greater reduction of a Mycobacterium species (i.e., M. terrae). Additional testing with other microorganisms was completed as a verification of the broad-spectrum efficacy of the dry heat process, demonstrating a 6 log or greater reduction of four vegetative bacteria (i.e., two Gram-positive and two Gram-negative bacteria). The dry heat process was not only effective at microbial inactivation, but after repeated exposures up to 20 cycles at 80°C (176°F) for three hours (180 min), the process did not affect the single-use respirator efficacy, as demonstrated by functionality testing.
The type of microorganism of concern with COVID-19 is an enveloped virus, SARS-CoV-2. Enveloped viruses are particularly sensitive to inactivation.12,,13 The outer envelope, made in part from the lipid membrane of host cells from which they are generated, is required for these viruses to infect other cells; therefore, damage to these cells can inactivate them (rendering them unable to infect a new target cell). This is quite different from other types of nonenveloped virus (e.g., polio or parvo viruses) that are more difficult to kill.13 This difference makes enveloped viruses more sensitive to changes in drying, pH, disinfectants, and temperature.12 The sensitivity of viruses to heat processes has been well studied.12–14 Enveloped viruses, as examples, have been shown to be readily inactivated at temperatures in excess of 50°C. During clinical use, single-use respirators and surgical masks might be contaminated with other pathogens with higher resistance to inactivation than SARS-CoV-2. For this reason, the FDA EUA requirements specified a challenge for decontamination processes in this situation (e.g., emergency use) to include inactivating Mycobacterium, Gram-positive bacteria, and Gram-negative bacteria to represent clinical use.
Of important note, processing, which is defined as activity to prepare a new or used healthcare product for its intended use, is generally a multistep process. For example, used medical devices are subjected to an initial cleaning step followed by an antimicrobial process (e.g., disinfection and/or sterilization). In the case of single-use respirators and surgical masks, cleaning would not be appropriate, as the materials of construction (e.g., nonwoven) and the design (e.g., tortuous pathway) make the removal of clinical soil (e.g., oils, blood, mucus) impractical. Therefore, the dry heat process presented in this article should only be applied to visibly clean single-use respirators and surgical masks, with a focus on inactivating microorganisms remaining after use while preserving functionality. In addition, single-use respirators and surgical masks should be visually inspected to determine if their integrity has been compromised or if damage that would prevent reuse is present.
The dry heat process confirmed in this study can be readily achieved in commercially available dry heat ovens. The temperature of 80°C (176°F) typically is the minimum temperature programmable on most home use ovens and drying cabinets/ovens in healthcare facilities. Considerations for tracking, tracing, controlling, handling, storage, and redistribution for reuse of respirators can be found in the FDA EUA guidance.5
Pandemic preparedness plans should include provisions for PPE supplies, but when shortages occur, utilizing a processing strategy for key supplies, such as single-use respirators and surgical masks, may be necessary. Dry heat processing has been found to be an appropriate method for repeated (up to 20 times) single-use respirator and surgical mask decontamination. A dry heat process at 80°C (176°F) for two hours (120 min) has been confirmed to meet the requirements for microbial inactivation and functionality for visibly clean single-use respirators and surgical masks.
The authors thank Nelson Labs and the individuals involved in performing bacterial filtration efficiency, differential pressure, and cytotoxicity testing.
Joyce M. Hansen, BS, MBA, is vice president of microbiological quality & sterility assurance at Johnson & Johnson in Raritan, NJ. Email: firstname.lastname@example.org
Scott Weiss, BS, is director of industrial microbiology at Johnson & Johnson in Raritan, NJ. Email: email@example.comCorresponding author
Terra A. Kremer, BS, is a senior program manager in Microbiological Quality & Sterility Assurance at Johnson & Johnson in Raritan, NJ. Email: firstname.lastname@example.org
Myrelis Aguilar, BS, is a senior staff scientist at Johnson & Johnson in Raritan, NJ. Email: email@example.com
Gerald McDonnell, BSc, PhD, is a senior director in Microbiological Quality & Sterility Assurance at Johnson & Johnson in Raritan, NJ. Email: firstname.lastname@example.org