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.

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.