Elastomeric Respirators for COVID-19 and the Next Respiratory Virus Pandemic: Essential Design Elements

Respiratory viruses may spread by fomite (touching a contaminated surface and then transferring the virus to the respiratory tract), droplet, or airborne routes (table 1). Determining the relative importance of these routes for an individual virus is difficult and may be controversial; for SARS-CoV-2, the fomite route of transmission is thought to be less important.^3,4  Many experts now believe that there is a continuous spectrum of different sized respiratory particles produced by persons infected with respiratory viruses, and that the traditional distinction between droplet and airborne transmission is arbitrary and obsolete.^5–7  During the current COVID-19 pandemic, there has been controversy, as in previous respiratory virus pandemics, concerning whether respiratory protection should protect against only droplet or droplet and airborne particles.⁸  However, there is now substantial evidence that airborne transmission of SARS-CoV-2 transmission is likely^5,9–12 ; inhalation of respiratory particles may be the predominant route of transmission. Difficulty in culturing putative airborne pathogens from air samples because of various technical obstacles does not rule out airborne transmission.^5,12  Therefore, it seems prudent to protect healthcare workers (and the public) accordingly, by using respiratory protection whenever possible. The two major categories of respirators used for respiratory protection in health care include negative and positive pressure mode respirators.

The wearer of a negative pressure mode respirator draws air through a filter and into a facepiece during inhalation. There are two main types of negative pressure respirators: disposable filtering facepiece respirators¹³  (such as the N95 filtering facepiece respirators, familiar to many healthcare workers), and elastomeric respirators,¹⁴  which are reusable (table 1; fig. 1). The effectiveness of respiratory protection depends upon fit, filtering efficiency, and resistance to breathing (table 1).

Fit defines the extent to which there is an occlusive seal of the respirator with the face, preventing leakage and forcing all inhaled and exhaled air to pass through the filtering material (or in some cases through an exhalation valve). In contrast, barrier face coverings¹⁵  (“cloth masks”) and surgical masks¹⁶  are not designed to make a tight seal to the face (table 1). Only when there is an occlusive seal with the face, as with a respirator, does almost all of the air entering the facepiece pass through the filtering material, providing the optimal opportunity to remove particles from the air.^17,18  In the United States, the Occupational Safety and Health Administration (Washington, D.C.) Respiratory Protection Standard¹⁹  requires that employees must be medically cleared to wear a respirator and that employers must measure the fit of respirators for employees upon first selection and annually thereafter using standardized tests that may be qualitative or quantitative.¹⁹  Numerous studies have shown that the fit of a filtering facepiece respirator varies considerably depending upon the particular design and materials, and the facial characteristics of an individual user; not all respirators fit the full range of human faces.²⁰  The face must be clean-shaven in order to obtain an effective fit. A recent American Society for Testing and Materials (West Conshohocken, Pennsylvania) standard for “respirator fit capability” may be used to characterize how well a particular respirator will fit a variety of facial characteristics.²¹  Moreover, filtering facepiece respirators are disposable, and the quality of the fit may deteriorate with repeated use.²² 

Filtering facepiece respirators typically utilize a layer of melt-blown polypropylene (or other synthetic material) fabric as the filter. The randomly oriented fibers of the melt-blown fabric filter particles by a combination of impaction, diffusion, and interception.²³  In addition, an electrostatic charge is often added to enhance filtration, causing the material to function as a permanent magnet, or “electret.”²⁴ 

Elastomeric respirators are constructed of various solid plastic or rubber materials that form the mask body and the portion of the mask that seals to the face. Filtering is accomplished by one or more exchangeable filters, constructed of material similar to that of filtering facepiece respirators, that attach to the mask body. The filtering material may be protected within a canister or cartridge (figs. 1 and 2). In the United States, the National Institute for Occupational Safety and Health (Washington, D.C.), a branch of the Centers for Disease Control and Prevention (Atlanta, Georgia), certifies the filtering efficiency of respirators using standardized tests²⁵  (table 1).

In addition to fit and filtration, resistance to breathing is an important property. Increased resistance to breathing results in increased negative and positive pressures inside the respirator during inhalation and exhalation, respectively, which could increase leakage around the respirator facepiece. Resistance to breathing also affects comfort and tolerability.²⁶  In the United States, the National Institute for Occupational Safety and Health requires that respirator filtering material be tested for the pressure gradient across the filtering material at 85 l/min constant airflow; the “inspiratory” pressure gradient must be less than 35 mm H₂O (343 Pa), and “expiratory” pressure gradient must be less than 25 mm H₂O (245 Pa). Methods for testing the pressure gradient across filtering material typically specify the cross-sectional area of the material to be tested, since the cross-sectional area affects the pressure gradient under conditions of constant volumetric flow rate. The National Institute for Occupational Safety and Health method for certifying filtering facepiece respirators specifies that the entire respirator is sealed onto a plate for testing; approved filtering facepiece respirators typically have inspiratory and expiratory pressure gradients in the approximate range of 5 to 15 mm H₂O at 85 l/min airflow.²⁷  Currently, there is not a standardized test for measuring the pressure gradient across a respirator during actual use. A study of a single model of a filtering facepiece respirator found peak pressure gradients of less than 20 mm H₂O during exercise.²⁸ 

Powered air purifying respirators employ a fan or pump that brings air through the filter and into the facepiece. Filtration is important, but fit is less so; a powered air purifying respirator with a loose-fitting hood offers greater protection to the wearer than an N95 filtering facepiece respirator because it provides more than enough filtered air to overcome any inward airflow between the hood and the wearer’s head.²⁹  Powered air purifying respirators with a loose-fitting hood generally have an Assigned Protection Factor (table 1) of 25, or 2.5 times the Assigned Protection Factor of 10 for a filtering facepiece or elastomeric respirator.¹⁹  Loose-fitting powered air purifying respirators have the advantage of not requiring fit testing, and contrary to negative pressure mode respirators, do not increase resistance to breathing. Their performance is not affected by the presence of facial hair as they do not make a tight seal to the face. It is important to understand that the exhaled air passes around the edges of a loose-fitting powered air purifying respirator hood without filtration; only the inhaled air is filtered.

Disadvantages of powered air purifying respirators include their greater complexity of use (i.e., fan motors and batteries), higher cost, and the possibility of impaired communication caused by fan noise. A potentially serious shortcoming of loose-fitting powered air purifying respirators is that the exhaled air is exhausted into the ambient air without filtration. This could hypothetically result in contamination of a sterile field or transmission of a respiratory pathogen by the wearer.³⁰  The unfiltered exhaust from a powered air purifying respirator is analogous to an unfiltered exhalation valve on a negative pressure mode respirator. We suggest that powered air purifying respirators should be available for healthcare workers who, because of facial features or facial hair, are unable to be fitted for a filtering facepiece or elastomeric respirator, or when an increased Assigned Protection Factor is desired. However, we also suggest that during a pandemic, negative pressure mode respirators (i.e., filtering facepiece respirators or elastomeric respirators) are likely to be a better solution for providing respiratory protection for large numbers of healthcare workers.

Tight-fitting half facepiece powered air purifying respirators are also available, although they have been used much more widely in industry than in health care (fig. 3). Tight-fitting powered air purifying respirators require fit testing and would not be suitable for use with facial hair that interferes with the seal of the mask body to the face. An Assigned Protection Factor of 50 is possible, five times that of a negative pressure elastomeric respirator and twice that of a typical loose-fitting powered air purifying respirator,¹⁹  making this an attractive option when higher levels of protection are desired.

Full face elastomeric respirators (sometimes referred to as “gas masks”) that cover the eyes, nose, and mouth within a single mask body are another option to consider. Such respirators are used routinely by the military, police, and fire fighters and for industrial applications in which infectious agents, chemical toxins, or radioactive particles can irritate or damage the eyes or gain entry to the body through the eyes, as well as the respiratory system. While eye protection in the form of glasses or goggles has been advised for protection from SARS-CoV-2,³¹  the evidence for respiratory viruses causing infection from contact with the conjunctiva is not strong.^32–37  In general, full face masks are heavier, are less comfortable in warm environments, and may have greater interference with the field of vision in comparison to half masks (which do not cover the eyes). Nevertheless, a study of acceptability to healthcare workers of military-style elastomeric full face respirators in comparison to filtering facepiece respirators (used with eyeglasses or goggles) found a high level of acceptance for the full face respirators when worn for brief periods (up to 40 min).³⁸ 

It is important to note that while surgical masks are typically constructed from melt-blown fabric, and bear some superficial physical resemblance to filtering facepiece respirators, surgical masks are not respirators. Some surgical masks are cleared for marketing (in the United States) by the Food and Drug Administration (Silver Spring, Maryland) and meet American Society for Testing and Materials F2100 standards¹⁶  (table 1). Surgical masks are intended to block large droplets, splashes, sprays, or splatter from reaching the mouth or nose and to prevent the user’s droplets from exposing others. Surgical masks may capture some but not all of the particles contained in a wearer’s cough or sneeze.³⁹  Since surgical masks are not designed to make an occlusive seal with the face, the protection from inhalation or exhalation of infectious particles will be limited in comparison to a properly fitted respirator.^40–46  The American Conference of Governmental Industrial Hygienists (Cincinnati, Ohio) has published an informative graphic illustrating the comparative effectiveness of cloth face coverings, surgical masks, and respirators.⁴⁷ 

Filtering facepiece respirators such as N95 or the European Union equivalent FFP2 have been avidly sought during the COVID-19 pandemic; public health authorities initially advised that these respirators should only be used by healthcare workers, but recently some governments have recommended or even required their use by the public.⁴⁸  While filtering facepiece respirators provide effective respirator protection when properly fitted, they have significant shortcomings (table 2). A major shortcoming is the overwhelming number of respirators required during a respiratory pandemic, inevitably resulting in acute shortages, even when respirators are stockpiled in advance. During the COVID-19 pandemic, due to the shortage of respirators, there were numerous attempts to construct homemade respirators using three-dimensional printed materials, respiratory therapy equipment, anesthesia circuit filters, and other available components.^49–51  This situation revealed the critical shortage of commercially available respiratory protection. To the contrary, elastomeric respirators can be used for prolonged periods of time, and thus provide a durable solution during a pandemic.

Elastomeric respirators are widely used outside of health care for protection from aerosols and harmful vapors and gases. Their use in health care has been limited, but successful implementation has been reported.^52–54  In 2019, before the COVID-19 pandemic, a consensus report from the National Academy of Sciences, Engineering, and Medicine (Washington, D.C.)concluded that “reusable elastomeric respirators could be a viable option for use in surge situations” and that surge use would be enhanced if “reusable elastomeric respirators were a part of healthcare facilities’ day-to-day respiratory protection program.”⁵⁵  However, the report also acknowledged the shortcomings of existing elastomeric respirator products, and recommended “design of innovative reusable respirators and the implementation of robust respiratory protection programs…taking into account the distinctive characteristics of the healthcare workplace…” In light of the experience gained with respirators during the COVID-19 pandemic, this document now seems prescient.

There are several potential advantages of elastomeric respirators in comparison to filtering facepiece respirators (table 2).⁵⁶  Elastomeric respirators are ideally suited to pandemic surge use because they can be reused indefinitely, and filter cartridges or canisters can last for extended periods of time (see the Durability of Elastomeric Respirators section). Elastomeric respirators are capable of robust user seal checks because occlusion of the filter ports completely prevents breathing if the mask makes an effective seal to the face. In addition, elastomeric respirators tend to have better fit⁵⁷  than filtering facepiece respirators.

As already mentioned, the advantage of elastomeric respirators is that a set of filters can last for an extended period of time⁵⁸ ; however, the precise longevity is uncertain because the concentration and nature of particles to be filtered will have a significant effect on the lifetime of the filter. High-efficiency filters such as 95 (FFP2), 99 (FFP3), 100 series (N, R, or P), or high-efficiency particulate air filters can be used until they load with particles to the point that the resistance to breathing becomes excessive. Particles that are filtered become tightly bound to the filtering material by Van der Waals forces and do not come out of the filter in significant quantitites.⁵⁹  When filters are used in relatively dust-free environments such as hospitals, they would be expected to be highly durable, possibly allowing users to work through a pandemic with one or two sets of filters. Filters can be encased in plastic enclosures that can be handled without touching the filter material itself, allowing for external cleaning by, for example, disinfecting wipes (figs. 1 and 2). An ideal elastomeric respirator for a pandemic should be designed with filter ports able to accommodate filters from multiple manufacturers (to prevent supply chain disruptions). Since there is not a universal filter connection design, the use of filters from multiple sources would require the use of adapters specifically made for this purpose.³⁸  Sources of filters during a surge might include anesthesia and ventilator breathing circuit filters.⁶⁰  In the United States, the National Institute for Occupational Safety and Health would need to be involved in the design and certification of novel elastomeric respirators designed for use with a variety of filters.

Existing elastomeric respirators should undergo further development to improve their functionality. The properties of ideal elastomeric respirators are shown in table 3, and an artist’s conception of a future respirator is shown in fig. 4 (filters removed to show the respirator facepiece more clearly). Elastomeric respirators can make verbal communication difficult, in most cases more so than filtering facepiece respirators.^61,62  A few elastomeric respirators are equipped with a “speaking diaphragm” (fig. 1), which transmits sounds by a thin disc located in a circumscribed area of the mask body. For elastomeric respirators to succeed in health care, sound transmission must be improved substantially by the use of “speaking diaphragms” or other measures. Transparent mask materials may also assist with effective communication by making facial expressions and lip movement discernable.⁶³  Management of moisture is also a potential challenge since condensation of exhaled water vapor inside the mask can result in accumulation of moisture.

All but a few available elastomeric respirators have an unfiltered, valved exhalation port, which is undesirable in healthcare settings. Unfiltered exhaled air may contaminate a sterile field during a procedure, or result in transmission of a respiratory pathogen if the person wearing the respirator is infected. While these exhalation ports can be overcome by user modifications, such as by covering the exhalation port and removing the inhalation port valves, a respirator designed either without an exhalation port or with an exhalation port that can be switched off would be preferable. In some nonpandemic situations, unfiltered exhaled air may be acceptable—for example, if the person wearing the respirator is extremely unlikely to be infected with a specific transmissible respiratory pathogen (e.g., Mycobacterium tuberculosis in low prevalence areas). Some users have worn surgical masks over an elastomeric respirator with the idea of providing filtration for the exhaled air; however, this solution is unreliable due to the shortcomings of surgical masks described in the Surgical Masks section.

One of the potential challenges for elastomeric respirators is cleaning. Reusable respirators require periodic cleaning and decontamination. Elastomeric respirators have typically been designed for low-level disinfection such as cleaning with soap and water, disinfectant wipes, dishwasher disinfection at low temperature, or immersion in relatively weak chemicals such as diluted bleach.⁶⁴  While the SARS-CoV-2 virus is inactivated by low-level disinfection, many other pathogens are resistant to this method. The elastomeric respirator is in close proximity to the user’s mouth, nose, and airway, has contact with respiratory secretions, and has potential contact with a wide variety of pathogens commonly found in the healthcare environment, including multidrug-resistant organisms. Therefore, an argument could be made that elastomeric respirators are semicritical devices that should be subject to high-level disinfection.⁶⁵  Semicritical devices are those that come into contact with intact mucous membranes. High-level disinfection destroys all microorganisms except some bacterial spores. High-level disinfection could be particularly important if elastomeric respirators are processed by a hospital central processing unit and reissued to multiple users. High-level disinfection would require that elastomeric respirators be submersible (with the filter cartridges removed), and that the materials be resistant to heat or chemicals required for high-level disinfection. During surge conditions when other disinfection processes may be in short supply, heat-resistant respirator materials could be disinfected with boiling water.⁶⁶  The hard surfaces of reusable filter cartridges or canisters (fig. 2) could be cleaned with disinfectant wipes (low-level disinfection) between uses.

There are several examples of respirators that combine a plastic or rubber mask frame and sealing surface with melt-blown fabric filtering, rather than filter cartridges or canisters. Thus, these respirators have features of both traditional filtering facepiece respirators and elastomeric respirators. An example of such a hybrid respirator is shown in fig. 1.

The environmental impact of elastomeric respirators in comparison to disposable filtering facepiece respirators has not been well-studied. The environmental impact of cleaning reusable respirators should be considered. Respiratory protection must be effective and available for all people who need it. The experience of the COVID-19 pandemic has clearly shown that control of viral transmission must occur everywhere in the world in order to prevent the selection of more fit and dangerous variants of the virus. Respirators must be designed to effectively fit the wide variety of facial shapes and sizes found around the globe. Respirators must be produced in large enough numbers and at a low enough cost to ensure availability to low- and middle-income countries.

Elastomeric respirators have significant advantages during respiratory pandemics due to their durability and capacity for repeated use. Unlike filtering facepiece respirators, elastomeric respirators allow for robust user seal checks. However, to optimize their utility in healthcare settings, elastomeric respirators should be further enhanced with respect to ease of communication, moisture control, suitability for high-level disinfection, capacity to filter exhaled air, and adaptability to filters from multiple manufacturers. The authors believe that the development, implementation, and stockpiling of improved elastomeric respirators around the world should be an international public health priority.

Support was provided solely from institutional and/or departmental sources.

The authors declare no competing interests.