NPPTL Filtering Facepiece Respirators with an Exhalation Valve: Measurements of Filtration Efficiency to Evaluate Their Potential for Source Control

Introduction

Filtering facepiece respirators (FFRs) are used extensively by healthcare personnel (HCP) during a pandemic. FFRs are primarily reserved for those personnel who have a greater risk and longer duration of exposure compared with other workers and the general public. Some FFR models contain an exhalation valve, which is a device that closes to allow inhaled breath to be pulled through the filter media and opens to allow exhaled breath to be expelled from the respirator through the exhalation valve as well as the filter media. These FFR models provide the wearer with a level of protection like that of an FFR without an exhalation valve, and they are thought to increase the wearer’s comfort at high work rates and be suitable for longer periods of use. However, respiratory secretions expelled by wearers may exit along with air through the exhalation valve. A concern with FFRs with an exhalation valve is that individuals may spread disease if unfiltered, virus-laden aerosols pass through the valve.

During the COVID-19 pandemic, guidance from the Centers for Disease Control and Prevention (CDC) did not recommend using an FFR with an exhalation valve for source control (i.e., to filter respiratory secretions to prevent disease transmission to others) and advised that if only this option isavailable and source control was needed, then the valve should be covered with a surgical mask, procedure mask, or a cloth face covering that does not interfere with the respirator fit. The CDC requested research to provide improved science-based recommendations on the use of exhalation valves.

This study had three aims: (1) to measure the filtration efficiency provided by FFRs with an exhalation valve under conditions of inward airflow (i.e., in the direction of inhalation) and outward airflow (i.e., in the direction of exhalation); (2) to evaluate how particle penetration in FFRs with an exhalation valve compares to particle penetration in surgical masks, procedure masks, cloth face coverings, and fabric from cotton t-shirts; and (3) to determine the filtration efficiency of three modifications to the exhalation valve in FFRs with the goal of mitigating the emission of unfiltered particles. To accomplish these three aims, thirteen FFR models were each tested in two positions: inward position, which is used by the NIOSH Respirator Approval Program when testing N-type respirators, and outward position, which was used experimentally to channel airflow in the direction of exhalation. For the inward position, three mitigation strategies were used:
(1)covering the valve on the interior of the FFR with commonly available surgical tape,
(2)covering the valve on the interior of the FFR with an electrocardiogram (ECG) pad; and
(3)stretching a surgical mask over the exterior of the FFR.

The purpose of these three strategies was to measure the varying filtration efficiencies to determine their contribution toward source control. Both positions and all mitigation strategies were tested at three airflow rates: 25, 55, and 85 lpm (liters per minute). In addition to the FFR evaluations, researchers evaluated a selection of surgical masks, procedure masks, cloth face coverings, and fabric from cotton t-shirts using the outward position and flowrates described previously.