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.32” 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.32 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,39,40 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,39,40 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.[1] It is known that lower differential pressure across the filter can result in higher filtration efficiency.[2] 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”

In the text describing Table 1 data, it reads “The average differential pressure across all of the fabrics at a flow rate of 1.2 CFM was found to be 2.5 ± 0.4 Pa, indicating a low resistance and represent conditions for good breathability”, but it should read “The average differential pressure across all of the fabrics at the low flow rate was found to be 2.5 ± 0.4 Pa.”

Similarly, in the Materials and Methods section, at the end of the “Detection of Aerosol Particles” subsection, “Two different flow rates of 1.2 CFM (a face velocity of 0.1 m/s) and 3.2 CFM (a face velocity of 0.26 m/s) were used that corresponded to rates observed at rest to moderate activity, 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 result in measurements at net airflows that can be significantly (an order of magnitude or more) lower than these respiration rates.”

The Materials and Methods section has an error in the specification of the satin fabric. Line #6: change text from “satin (97% polyester and 3% spandex)” to “satin (100% polyester)”.

In the Materials and Methods section, under Differential Pressure, the text “The differential pressure (ΔP) across the test specimen was measured ∼7.5 cm away on either side of the material being tested using a micromanometer. The ΔP value is an estimate of the breathability of the fabric” should read “The differential pressure (ΔP) across the test specimen was measured ∼7.5 cm away on either side of the material being tested using a micromanometer.”

Reference 1 has an error. The correct reference is “Ma, T.; Shutler, N. How to Sew a Fabric Face Mask. The New York Times. March 31, 2020.”

In the Supporting Information, the title of Table S1 should be changed from “Filtration efficiencies of various test specimens at a velocity of 0.26 m/s (3.2 CFM) and the corresponding ΔP values. The filtration efficiencies are the weighted average of seven replicates. ΔP indicates the pressure difference across the sample and is a good indicator of the breathability through the sample when used as mask” to “Filtration efficiencies of various test specimens and the corresponding differential pressure (ΔP) values across the specimen. Prior to testing the airflow flow rate was adjusted to 3.2 CFM with no mask covering the tube opening. Mounting of cloth samples result in measurements at net airflows significantly lower than these respiration rates. The filtration efficiencies are the weighted average of seven replicates.”

Also in the Supporting Information, the following changes apply to Table S2:

There is an error in the part number for the Chiffon–90% polyester–10% Spandex sample. The correct part number is “Jo-Ann Stores (16376949)”.

The satin sample specification of “97% polyester and 3% spandex” is incorrect. It should read “100% polyester”.

The information for Spandex is removed since it is not discussed in the paper.

A revised Table is included if that is more convenient.

Table S2

Specific Information on the Various Fabrics Used. Table showing the composition, microstructure, approximate porosity, thread diameter, approximate thread pitch, and the source of the materials (where applicable). Pitch and thread diameter often vary depending upon the weave direction resulting in the variation noted