Filter Inserts Impact Cloth Mask Performance against Nano- to Micro-Sized Particles

The United States Centers for Disease Control and Prevention and World Health Organization recognize that wearing cloth face coverings can slow the transmission of respiratory diseases via source control. Adding a partial layer of material with a high filtration efficiency (FE, e.g., polypropylene sheets that meet the HEPA standard) as an insert can potentially provide additional personal protection; however, data on the necessary areal coverage are sparse. The relationship between insert area ratio (IAR) relative to fabric area, FE, differential pressure (.P, a surrogate for breathability), and quality factor (QF, a ratio including FE and.P) utilizing two fabrics (rayon and 100% cotton lightweight flannel) and three insert materials (HEPA vacuum bag, sterilization wrap and paper coffee filter) was investigated. The effect of inserts on particle flows mimicking human exhalation is semiquantitatively and qualitatively examined using flow visualization techniques. The following was found: (1) The relationship between FE,.P, and QF is complex, and a trade-off exists between personal protection from filtration during inhalation and source control from leakage during exhalation; (2) FE and.P of the composite covering increase with IAR, and the rate is dependent upon insert type; (3) improvements (decrements) in the QF of the composite assemblage require inserts with a higher (lower) QF than the fabric and larger differences yield greater gains (losses); (4) the increased.P from an insert results in increased leakage during exhalation; (5) to minimize leaks,.P must be as low as possible; and (6) small relative areas not covered by an insert (i.e., IAR slightly smaller than 1) strongly deteriorate the benefits of an insert similar to small leaks in a covering.

Automated summary

The study found that adding materials with high filtration efficiency to face masks can provide additional personal protection, but there is a trade-off balancing increased filtration efficiency and breathability. The filtration efficiency and breath resistance of the composite covering increase with the increase of insert area ratio, and the quality factor of the insert interacts with the quality factor of the fabric.