Correction to Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks

Note that all mentions in the paper and Supporting Information of flow rates of 1.2 CFM and 3.2 CFM and face velocities throughout the text, tables, and figures should be explicitly interpreted as flow rates measured when there was no cloth mounted, i.e., under unrestricted flow conditions. With the cloth on, they are only indicative as representing relatively higher and lower flow conditions for the same cloth. The actual flow rates will be significantly lower. In the first section of the paper, the text “Tests were carried out at two different airflows: 1.2 and 3.2 CFM, representative of respiration rates at rest (∼35 L/min) and during moderate exertion (∼90 L/min), respectively. ” should be changed to the following: “Tests were carried out by initially setting two different airflows whose values were 1.2 and 3.2 CFM when there was no cloth sample mounted at the end of the tube, representative of unhindered respiration rates at rest (∼35 L/ min) and during moderate exertion (∼90 L/min), respectively. Mounting of cloth samples results in measurements at net airflows that can be significantly (an order of magnitude or more) lower than these respiration rates. ” In the Results and Discussion section, in the discussion of Figure 4a, the previous incorrect description, “These cloth hybrids are slightly inferior to the N95 mask above 300 nm, but superior for particles smaller than 300 nm. The N95 respirators are designed and engineered to capture more than 95% of the particles that are above 300 nm, and therefore, their underperformance in filtering particles below 300 nm is not surprising. ” should be changed to the following: “The N95 respirators are designed and engineered to capture more than 95% of particles at 300 nm, at 343/245 Pa (inhalation/ exhalation) pressure drops and 85 L/min flow. Our studies, focused on cloth masks, are carried out at reduced pressure drops (2−13 Pa) and significantly lower flow rates where diffusional flow is expected to control transport across the fabrics. Considering this, and additionally noting the large error bars for the N95 measurements in the <300 nm range (as discussed in the paper), conclusions and comparisons (with cloth fabrics) from our data regarding the N95 and surgical mask performance should not be drawn. Our pressure drops may be more appropriate to what unfitted cloth masks will likely experience under real-life conditions due to leakage around the edges. Pressure drop measurements for manikin fitted N95s and surgical masks have varied from 20 to 40 Pa and from 1 to 18 Pa. It is known that lower differential pressure across the filter can result in higher filtration efficiency. While our cloth efficiencies measured are high, they are at significantly lower airflows. The strategy for cloth mask design would therefore be to increase the effective mask surface area significantly without increasing the seal perimeter in order to increase airflow, while retaining a low differential pressure, and a high filtration efficiency. ” We request the following paragraph to be added at the end of the Conclusions section: “Finally note that our measurements are carried out at low differential pressure values (2.5−13 Pa) across the fabrics. This has also resulted in the measurements being carried out at flow rates significantly lower (order of magnitude or more) than typical resting respiratory rates. We believe the focus on cloth masks should center around lower pressure differentials that can be sustained practically in unfitted cloth masks. One approach would then be to depart from traditional designs to significantly increase the area of the cloth to increase net airflow while not overly exerting the face seal. ” The Table 1 title should be changed from “Filtration Efficiencies of Various Test Specimens at a Flow Rate of 1.2 CFM and the Corresponding Differential Pressure (ΔP) across the Specimen” to “Filtration Efficiencies of Various Test Specimens and the Corresponding Differential Pressure (ΔP) across the Specimen (prior to testing, the airflow flow fate was adjusted to 1.2 CFM with no mask covering the tube opening; mounting of cloth samples result in measurements at net airflows significantly lower than these respiration rates”