Reusable Respirators for Healthcare Use
Advancements in Elastomeric Respirator Technology Address Source Control
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U.S. healthcare systems are beginning to incorporate elastomeric half-mask respirators (EHMRs) into their respiratory protection programs because they can be cleaned, disinfected, and reused, as described in a consensus study report published in 2019 by the National Academies of Sciences, Engineering, and Medicine.
During times when demand for respiratory protection is high and supply is limited, EHMRs can be used to address shortages of filtering facepiece respirators (FFRs), such as those with an N95 protection level, which are intended to be single-use and disposable. However, EHMRs traditionally have exhalation valves—a design feature that has raised concerns when source control is desired because it opens to allow exhaled air to escape through an unfiltered valve as well as the filter material. Source control is the use of protective measures to prevent transmission of disease or contaminants. When a respirator has an exhalation valve, breath that is exhaled through this valve when the wearer is breathing, talking, coughing, or sneezing could contaminate the healthcare workers’ environment.
Most exhalation valves are made up of a natural rubber, silicone, or neoprene membrane that sits between a support structure and a plastic cover. During inhalation, the membrane closes against the support structure and blocks the valve hole, thereby forcing the inhaled air through the filter material. During exhalation, if there is adequate air pressure, the air lifts the membrane and opens a passageway through which unfiltered exhaled breath can escape. This helps to reduce overall breathing resistance and high CO2 concentration, thereby increasing the comfort for the user, especially over an extended period of time. However, if bacteria or viruses are present in the exhaled breath, they may contaminate the surroundings. NIOSH, the institute within CDC responsible for testing and approving respirators for occupational use, has been working with manufacturers to address this issue. NIOSH recently issued several new approvals for EHMR models for occupational use; in these cases, the manufacturers redesigned existing products to function as both respiratory protection and source control (search results from the NIOSH Certified Equipment List show the approved models). These EHMR models include those without exhalation valves, where exhaled air is filtered by the particulate filters in both directions. One manufacturer has an approved exhalation valve filter accessory that is installed over the exhalation valve port to fit the company’s series of NIOSH-approved EHMRs. In this case, exhaled breath is filtered when passing through the exhalation valve filter accessory. NIOSH’s Certified Equipment List includes a feature to search for current NIOSH approvals of EHMRs without exhalation valves: users can select the “Quick Searches” tab, then click “NV EHMRs” under “Respirators of Current Interest” to find non-valved, NIOSH-approved EHMRs. NIOSH is also engaged in research to further understand the critical factors that affect the performance of respirators with exhalation valves as source control and is developing new technology based on this knowledge. One goal of this research is to produce functional prototypes for use in NIOSH-sponsored hospital demonstration projects. Together with technical data packages, these prototypes will be available to respiratory protection manufacturers to leverage for the development of respirators that they may then submit to obtain the necessary approvals. NEW SOURCE CONTROL TECHNOLOGIES Although not traditionally designed or evaluated for use as source control, respirators help to provide source control against disease transmission by blocking respiratory secretions from the nose or mouth of an infected individual. If not abated, respiratory secretions create airborne particles called aerosols. To study how disease is transmitted through aerosols, NIOSH considers three aspects of airborne particles—size, composition, and shape. (Studies published in the Journal of Infection, Aerosol Science and Technology, and The International Journal of Health Planning and Management further examine these aspects of airborne particles and disease transmission. Find links to these studies under “Resources” below.) Particle size is important because larger particles fall to the ground more quickly, while smaller particles can remain suspended in the air for days or weeks. Larger particles are also capable of greater inertial energy, meaning they can travel longer distances and exert greater force when impacting upon surfaces such as a person’s skin or clothing. Size is also important when considering the filtration efficiency of an air filter, such as the new accessory filters being incorporated into EHMR designs for source control. Many masks effectively filter larger particles, but respirators are composed of special materials that attract smaller particles, which would easily pass through openings in masks made of woven material such as a cotton T-shirt. Particle composition refers to the contents of the particle. Some may contain biological organisms. Human respiratory secretions also contain liquid droplets, which can quickly evaporate in dry air, creating tiny aerosols that can remain airborne for several days. Particle shape determines how particles move in the air and how they penetrate or stick to surfaces. Human secretions can be elastic. When they are strung together with mucus, smaller droplets can be pulled through the air like beads on a string. By understanding aerosol size, composition, and shape, NIOSH can evaluate some aspects of the protection afforded by new technologies. However, to understand disease transmission, NIOSH must also consider three additional components—source, transport, and reception (see the 2011 paper “Preventing Airborne Disease Transmission: Review of Methods for Ventilation Design in Health Care Facilities” in Advances in Preventive Medicine). A source of infection can be from respiratory droplets. Depending on the stage of illness, an infected individual may not be contagious and shed only a few viruses or may be highly contagious and shed millions of viruses. Aerosol transport determines four things: 1. how long the particles stay suspended in the air 2. how wind currents affect the particles’ momentum 3. the impaction force of particles onto surfaces 4. how moving particles change by growing (accumulation) or shrinking (evaporation), with their concentration being reduced by mixing with clean air (dispersion) In the case of micro-organisms, transport can include exposure to air and light that can change their ability to reproduce. Reception of particles through breathing is not the same for all particle sizes. Large particles deposit in the nose, while smaller particles travel deep into the lung where they can enter the bloodstream and even pass the blood-brain barrier (see papers in Brain Edema XIV and Proceedings of the American Thoracic Society, listed under “Resources” below). By considering the above factors, new technologies that filter exhaled breath reduce exhaled aerosols from inside the EHMR at the source, resulting in a reduction in particles before potentially contaminated air is released into the surroundings. For EHMRs using filter material that has an N95 protection level, at least 95 percent of potentially contaminated particles are removed. EXHALED BREATH FILTER ADAPTORS Prior to EHMR manufacturers redesigning their products to offer source control capabilities, NIOSH initiated research to design exhaled breath filter adaptors (EBFAs) for select commercially available EHMRs that were of interest to healthcare workers. Researchers are using EHMR models from Honeywell International Inc., 3M Company, and MSA Safety Inc. based on the prevalent use of these manufacturers’ respirators in healthcare. Currently, all manufacturers except one have opted to eliminate the exhalation valve port and force the exhaled breath back through the respirator filters as a solution for source control. NIOSH is interested in addressing the comfort level of respirators in terms of CO2 concentration, moisture buildup, and heat levels within the ori-nasal cavity, in addition to source control in these respirators. Therefore, the adaptors will be designed to allow the exhaled breath to exit the respirator and be filtered for source control within the adaptor. NIOSH researchers conducted initial studies to understand the effects of blocking the exhalation valves and directing the exhaled air back through the main (inhalation) filters as well as covering the exhalation valves externally with surgical masks. Researchers’ test results demonstrated the feasibility of modifying EHMRs by blocking the exhalation valve and redirecting the exhaled breath though the main filters while remaining below the NIOSH exhalation resistance performance requirement. However, modifying the respirators to filter the exhaled breath caused increases in both the exhalation resistance and inhaled CO2 levels.
Figure 1. Exhaled breath filter adaptors (EBFAs) for Honeywell RU8500 respirator (left) and 3M 7500 respirator (right).
The next phase of the study is to design and construct custom EBFAs for each respirator model currently used in NIOSH-sponsored hospital demonstration studies. The manufacturers of these respirators except one (3M) do not offer an EBFA for their specific model used in these studies. The NIOSH-developed adaptors will be fitted to the same respirator models and used in new hospital demonstration studies. The adaptor developed by 3M will also be used in its approved configuration in these studies. Researchers will create and prototype four unique adaptor designs, which will include replaceable filter elements. Each EBFA will be shaped to appear as though it is an integral part of the respirator, matching the contours of the host respirator where possible. It is important to verify that the respirators fitted with one of the EBFA prototypes remain below the NIOSH exhalation resistance performance requirement. Researchers expect that using an accessory filter will reduce CO2 levels within the respirator more than when the exhalation valves are simply blocked, which should increase user comfort. The first iterations of the EBFA design for the Honeywell North RU85001M/RU85004M and 3M 7500 (#7000002162) respirator models are shown in the photo and illustration in Figure 1.
Once the feasibility of the EBFAs for these respirators has been demonstrated, NIOSH researchers will seek to collaborate with the manufacturers by providing them with detailed test results and lessons learned related to their products. The manufacturers may use this information to adopt these designs and gain the necessary approvals for commercial production.
REUSABLE HEALTHCARE ELASTOMERIC RESPIRATORS The federal government has been exploring the use of elastomeric respirators in healthcare for more than a decade. These efforts began in earnest in 2009, when the Veterans Health Administration published the report “Better Respiratory Equipment Using Advanced Technologies for Healthcare Employees,” or “Project BREATHE” (PDF), which described 28 features important for respirators used in healthcare settings. This effort led to another partnership among NIOSH, the Department of Veterans Affairs, and manufacturers to develop respirators specifically designed with Project BREATHE features in mind. The hybrid respirator designed as part of this effort was not finalized as the developer did not identify a market for the product. Then, in 2017, the Biomedical Advanced Research and Development Authority awarded a contract to Applied Research Associates Inc. that advanced the development of reusable elastomeric respirators with testing and evaluation support from NIOSH. The technology was designed to address numerous needs identified within Project BREATHE and is expected to allow respirator facepieces and filter media to be cleaned and disinfected using standard hospital equipment. This effort is still underway.
In parallel, NIOSH initiated a research project to design a reusable healthcare elastomeric respirator (RHER) specifically for use by healthcare workers. One of the goals of this research is to mitigate the effects of disposable FFR shortages during a pandemic. In designing the RHER, NIOSH researchers created conceptual prototypes based on information in the Project BREATHE report—specifically, the features healthcare workers desire in a reusable respirator for efficient healthcare delivery. The RHER addresses the ergonomic needs of healthcare workers in addition to reusability. These conceptual RHERs consist of an ori-nasal mask body that incorporates exhalation valves, a filter attachment carrier, and a head harness assembly (see Figure 2). These features will allow the respirators to be donned quickly and be comfortable to wear for extended periods. The RHER is designed with a single filter element and maintains a profile similar to an FFR, unlike most of the available EHMRs that have two filter elements protruding from the respirator body. The RHER incorporates source control as part of its design, filtering the exhaled breath and diverting it backward from the sides of the respirator. Based on feedback from stakeholders, the RHER is designed to include the unique facial dimensions of the female population that comprises the majority of the healthcare profession.
Figure 2. Conceptual prototype reusable healthcare elastomeric respirator (RHER) with an ori-nasal mask body, filter attachment carrier, and a head harness assembly.
The current conceptual prototype design is being refined to be fully functional for evaluation in hospital demonstration studies with healthcare workers. These respirators will ideally be low profile and ergonomically designed to allow users a wide field of vision. The ori-nasal mask, attachments, and harness will be cleanable and reusable. The filter elements may be cleaned, disinfected, and reused or disposed of, depending on their condition after repeated usage.
The sealing surface of the respirator, which contacts the user’s face, will be contoured to fit the three NIOSH headform sizes (small, medium, and long/narrow) that are estimated to apply to about 85 percent of healthcare workers. The front of the RHER is equipped with a filter carrier that is printed out of a durable plastic material to test different filter media. These conceptual RHER prototypes were bench tested on NIOSH breathing machines to demonstrate the feasibility of meeting NIOSH’s performance requirements in the federal rule for approval of respiratory protective devices (42 Code of Federal Regulations Part 84), which describes standard respirator testing procedures.
Other important design features for the prototypes were identified based on stakeholder input regarding healthcare workers’ desired respirator features and an evaluation of the information collected during the Project BREATHE study. Additional key design features for the prototypes to be used in the demonstration studies include: • non-threatening appearance and color for the benefit of the patient • face contact surfaces made of silicone • filtration of exhaled breath that is also directed away from the patient • splash guards to protect against fluid ingress through filters • packable for storing in small spaces • easy dismantling and assembly without tools to aid with cleaning and disinfecting
During the design process, researchers created solid computer-aided design models and detailed drawings to meet materials and performance/design requirements. NIOSH also performed computational fluid dynamic (CFD) analysis to examine the fluid flow characteristics into, out of, and within the respirator cavity (see Figure 3).
The next phase of the project is to test different filter media combinations and design and build the filter element for the future RHER prototype. The complete RHER prototype assembly must be resilient enough for human subject testing, including cleaning and disinfecting. With the limited production tooling planned, NIOSH can produce up to 1,000 RHER prototypes for use in future NIOSH human subject studies in hospitals.
Figure 3. Reusable healthcare elastomeric respirator (RHER) computational fluid dynamic analysis.The graphics show the airflow path of the exhaled breath on a horizontal plane in line with the mouth as indicated in the bottom graphic. The upper graphics show how the breath is diverted and exits the respirator rearwards. The arrows within the airflow in the top right graphic indicate velocity and direction in the form of velocity vectors.
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UPCOMING RESEARCH In July 2021, the Strategic National Stockpile (SNS) began to purchase NIOSH-approved EHMRs without exhalation valves, and those with filtered exhalation valves and their corresponding particulate filters, for deployment to interested organizations. In advance of the SNS purchase, NIOSH coordinated the development of its research plan with the SNS effort to better understand the challenges and implementation of EHMR use. NIOSH posted a Federal Register notice to identify organizations that planned to request EHMRs from the SNS in order to learn more about their intended implementation of these respirators and to determine their interest in participating in a NIOSH study.
About 100 organizations responded that collectively requested approximately 150,000 EHMRs from the SNS. Fifty of these organizations expressed an interest in participating in NIOSH’s EHMR demonstration project, including hospitals, healthcare clinics, university health systems, dental facilities, long-term care facilities, fire departments, emergency medical service departments, and police departments. On average, organizations desired 1,800 EHMRs for their employees, though the number of desired respirators varied widely across organizations: from eight EHMRs to 30,000. In healthcare settings, organizations desired a range of 10 to 34,000 EHMRs, with an average of approximately 4,000 per organization.
As part of the NIOSH study, employees in a variety of job roles—including doctors, nurses, dentists, dental hygienists, firefighters, emergency medical services personnel, and police officers—will use SNS-distributed EHMRs during applicable job tasks as their source of respiratory protection. Over a period of several months, organizations and employees will provide valuable feedback to NIOSH about the benefits, barriers, and integration of EHMRs in the workplace. The feedback provided to NIOSH will be used to develop and enhance EHMR implementation guidelines and best practice documents for both healthcare and first responder settings. Feedback gathered during the study will also inform possible updates to CDC’s guidance on EHMR use and maintenance for workers who encounter more hazardous scenarios, such as firefighters.
NIOSH hopes its implementation guidelines will help support sustainable EHMR use during routine operations. Additionally, manufacturers may be able to use these findings to make comfort, usability, and fit improvements for respirators. Research activities will start in late 2021 and progress through 2022.
ROHAN FERNANDO, MS, is a senior research engineer with the NIOSH National Personal Protective Technology Laboratory (NPPTL).
LEE PORTNOFF, MS, is a research biologist with the NIOSH NPPTL.
EMILY J. HAAS, PhD, is a research health scientist with the NIOSH NPPTL.
Acknowledgement: The authors thank Jackie Cichowicz for her assistance with this article.
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Advances in Preventive Medicine: “Preventing Airborne Disease Transmission: Review of Methods for Ventilation Design in Health Care Facilities” (2011).
Aerosol Science and Technology: “What Aerosol Physics Tells Us About Airborne Pathogen Transmission” (2020).
Brain Edema XIV: “Influence of Nanoparticles on Blood-Brain Barrier Permeability and Brain Edema Formation in Rats” (2009).
Department of Veterans Affairs Veterans Health Administration: “Better Respiratory Equipment Using Advanced Technologies for Healthcare Employees (Project BREATHE)” (PDF, 2009).
The International Journal of Health Planning and Management: “An Overview of the Effect of Bioaerosol Size in Coronavirus Disease 2019 Transmission” (2020).
Journal of Infection: “The Role of Particle Size in Aerosolized Pathogen Transmission: A Review” (January 2011).
National Academies of Sciences, Engineering, and Medicine: “Reusable Elastomeric Respirators in Health Care: Considerations for Routine and Surge Use” (2019).
Proceedings of the American Thoracic Society: “Deposition of Inhaled Particles in the Human Respiratory Tract and Consequences for Regional Targeting in Respiratory Drug Delivery” (2004).