Quality Assurance During a Global Pandemic

An Evaluation of Improvised Filter Materials for Healthcare Workers

Abstract

Objective: 

The proliferation of improvised masks during the COVID-19 pandemic has raised questions regarding filter effectiveness and safety. We sought to compare the effectiveness of commonly used improvised filter materials against N95 industry standards.

Methods: 

Six different filter materials commonly used in the community were tested using both single- and multi-layer configurations with the TSI 8130 automated filter tester in accordance with National Institute for Occupational Safety and Health (NIOSH) standards for N95 respirators.

Results: 

Only three of the tested filter material configurations met N95 parameters with regard to filtration efficiency and pressure drop across the filter material—the: True-high-efficiency particulate air (HEPA) filter, four-layer MERV 13 and 14 HVAC filters.

Conclusions: 

Many proposed filter materials for improvised masks do not meet current industry standards and may pose safety and efficacy concerns. Care should be taken when selecting materials for this critical respirator component, particularly for health care workers or others at high risk for pathogen exposure.

BACKGROUND

The COVID-19 pandemic has resulted in a critical shortage of personal protective equipment (PPE) for the public, patients, and healthcare workers. Particularly alarming is the inadequate availability of the N95 respirator mask for front-line healthcare workers. In response to the existing shortages, potential for further depletion with ongoing patient care, and uncertain resupply capability, a homemade PPE movement has developed which has produced millions of masks and respirators for both the public and healthcare workers.

The predominate transmission route for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is respiratory droplet or contact transmission.¹ These involve larger particles, more than 5 μm, which travel only a short distance before falling and depositing on surfaces around an infected person. SARS-CoV-2 can also be transmitted through respiratory aerosols, which involve particles smaller than 5 μm. Healthcare workers in particular are at risk of this type of transmission due to exposure to aerosol generating procedures, such as intubation, positive pressure ventilation, and cardiopulmonary resuscitation (CPR). The World Health Organization, Centers for Disease Control, and European Centre for Disease Prevention and Control all recommend an increased level of protection, N95 (or similar European filtering facepiece 2 standard) respirators, for healthcare workers exposed via such procedures.^1–3

While homemade cloth masks can be used to prevent transmission through respiratory droplets, they are ineffective against smaller respiratory aerosols. For situations requiring an N95 respirator, several homemade respirator mask designs have been developed that use a variety of improvised filter materials ranging from cloth to HVAC filters.^4,5 While these designs are well-intentioned, major safety concerns remain and objective data regarding the efficacy of the improvised filter materials is absent. Given the lack of validation, we sought to analyze some commonly available filter materials against industry standards utilized for the N95 respirator.

METHODS

Six filter materials were compared in this study: (1) fiberglass-free minimum efficiency reporting value (MERV) 13 and 14 heating ventilation and air condition (HVAC) filters; (2) high-efficiency particulate air (HEPA)-type filter; (3) True-HEPA filter; (4) O&M Halyard H600 surgical sterilization wrap; (5) disposable shop towel; and (6) 0.22 μm laboratory grade polytetrafluoroethylene (PTFE) filter paper. Characteristics of these and the N95 are outlined in Table 1. Materials were tested in both single- and multiple-layer configurations. The TSI 8130 automated filter tester was utilized in accordance with the National Institute for Occupational Safety and Health (NIOSH) standards for N95 respirators, which measures penetration of a 0.3 μm sodium chloride aerosol at a flow rate of 85 L/min (Table 1).⁶ Primary outcomes include pressure drop (indicator of resistance/breathability) and aerosol penetration (protection).

TABLE 1: NIOSH N95 Certification Test Standards

RESULTS

With the addition of layers, performance improved for all materials. Of the materials tested the True HEPA filter, four-layer MERV 13 and 14 filters, and the lab grade filter paper showed comparable aerosol penetration (less than 5%) to the N95 (Table 2). Surgical sterilization wraps, PTFE based filters, and four-layer disposable shop towel filters exhibited high pressure drops (more than or equal to 25 mmH₂O). The resistance of the lab grade filter paper was excessively high which limited testing to flows of 45 L/min.

TABLE 2: Filter Material Performance

DISCUSSION

Several factors are vital to optimal and reliable performance of filters used in modern surgical masks and respirators. The type of material used as well as characteristics such as fiber diameter, porosity, and thickness, govern mask performance. The filtering mechanisms of straining, inertial impaction, interception, diffusion, and electrostatic attraction all contribute to a filtering facepiece respirator's ability to meet the stringent NIOSH requirements.⁷ Straining, inertial impaction, and interception are more important for filtering out larger droplets, while diffusion and electrostatic attraction become more important for smaller aerosols. Thus, adding more material to a filter may improve its ability to filter larger particles, it may not significantly increase its efficacy at filtering aerosols. Filtration ability also needs to be balanced against breathing resistance, which is measured as a pressure drop across the mask. High pressure drops lead to uncomfortable mask conditions and trapping of exhaled gases resulting in carbon dioxide rebreathing and development of hypercapnia. This makes prolonged use of high pressure drop masks both uncomfortable and dangerous since hypercapnia can decrease psychomotor performance.^8–10

Our results demonstrate that many proposed materials for improvised masks are not effective, offering far less filtration than recommended for protection against airborne pathogens in a health care setting. Equally concerning is that some of the tested materials demonstrated unacceptably high resistance. Of the materials tested, the True-HEPA filter and four-layer MERV 13 and 14 HVAC filters were the only configuration capable of meeting the NIOSH N95 standard with regard to both filtration efficacy and pressure drop. However, caution must be advised in repurposing these commercial products. Many HVAC filters contain proprietary materials that may contain fiberglass that may pose respiratory hazards, particularly when modified to fit homemade respirator masks. Regarding HEPA filters, not all “HEPA” grade filters are created equal. To meet True-HEPA standards, the filter must remove 99.97% or more of all particles that are 0.3 μm in diameter or larger. Many products labeled “HEPA- Type” and “HEPA Like” fail to meet this standard.¹¹

Although our study is not exhaustive in testing available materials, it raises design and testing considerations for this critical respirator component. We did not test for most penetrating particle size, so it is possible that the filter materials are less efficient at filtering particles smaller/larger than the 0.3 μm aerosol used in testing. It is also important to note that filter performance is only one of several considerations for effective mask design and implementation.¹² For example, the material used in some surgical masks is capable of blocking up to 96% of aerosols, yet the inability to develop a proper fit makes them inappropriate for use with an airborne pathogen.¹³ Further testing and regulation of specific mask designs is necessary prior to the widespread adoption of community created masks in health care settings.