Efficacy of Double Layer of Tea Towel as Filter Medium

Tea towel filters do not confer the same protection as an N95 or N100 rated filter media. As such, you probably do not want to use a tea towel filter mask when intubating a patient with COVID-19.
But it would confer some protection that will improve your odds when you must venture out into public to add to the existing benefits of social distancing.

A single layer tea towel up against the mouth is difficult to breath through, but it can be made much more comfortable by opening the mouth wide when breathing. This increases the surface area that is available for filtration. This should also reduce the velocity of air approaching and passing through the filter during inspiration and expiration. Given that a lot of studies show increased penetration rates with increases in “face velocity,” this is probably a good thing. The downside is that if there is a hole or other defect in the filter, a wide open mouth is more likely to be breathing through a defective location than a less open mouth.

Tea towel penetration rates compared to surgical masks and FFP2 (surgical N95) mask.

• The study reported findings in the form of a “Protection Factor.” I’m trying to make sense of the math on how it is derived, but a PF of 1 is no mask at all. The tea cloth mask had a PF of 2.2 to 2.5, depending on the activity of the user. The surgical mask had a PF of 4.1 to 5.3. The FFP2 mask had a PF of 66 to 113. The highest ratings for each are probably a better measure of filter media performance. Thus a surgical mask is twice as good (5.3 vs 2.5) as a tea towel. And an FFP2 mask is 45 times better (113 vs 2.5).
  • If you invert the PF numbers (“The inverse of the PF (1/PF) can be interpreted as a probability (that any particle succeeds in moving through the barrier the mask provides”):
    • Tea towel = 1/2.5 = 0.4 = 40% penetration rate = 60% filtration rate
    • Surgical mask = 1/5.3 = 0.189 = 18.9% penetration rate = 81.1% filtration rate
    • FFP2: 1/113 = 0.00885 = 0.1% penetration rate = 99.9% filtration rate
• van der Sande M, Teunis P, Sabel R. Professional and home-made face masks reduce exposure to respiratory infections among the general population. PLoS One. 2008;3(7):e2618. Published 2008 Jul 9. doi:10.1371/journal.pone.0002618
  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440799/

Doubling the layers of a tea towel filter bring filtration performance to the level of a surgical mask for larger particles (bacteria that are ten times larger than COVID-19). Single layer tea towel performance is lower than a surgical mask, but still pretty good, for smaller particles (bacteriophages that are one quarter the size of COVID-19). Some good caveats at the end of the paper about their test materials being “fresh” (i.e. clean) and unworn (i.e. new). A ragged old towel that is wearing through is probably not going to work as well. And cleaning and drying fabrics does tend to tighten up the fibres, which is why your jeans don’t fit as well after washing them. Also good to note is that dried out virus particles may obtain easier passage through a filter.

• Double layer of tea towel has a pressure drop of that is twice as high as a surgical mask. This resistance to breathing would make use uncomfortable and or impossible for practical use unless the surface area for filtration is increased.
• Bacterial penetration test: A single layer of tea towel blocked 83.24% of aerosolized bacteria (0.95 um – 1.25 um, which is ten times larger than the 0.1 um COVID-19) and a double layer of tea towel blocked 96.71%, which is comparable to a surgical mask at 96.35%. A few of the other fabrics were tested by doubling the layers and found no significant improvement, while the tea towel had a significant gain in blocking performance.
• Viral penetration test: A single layer of tea towel blocked 72.46% of aerosolized bacteriaphage (23 nm or 0.023 um, which is four times smaller than the 0.1 um COVID-19) which is good, but not as good as a surgical mask at 89.52% blockage. The study did not test viral penetration against double layers of fabric. Although different methods were used, these numbers are close to those obtained in the van der Sande paper mentioned above.
• Caveats: “The materials used in this published study were fresh and previously unworn. It is likely that materials conditioned with water vapor, to create a fabric similar to that which has been worn for a couple of hours, would show very different filtration efficiencies and pressure drops. In contrast, a study of breathing system filters found a greater breakthrough of bacteriophage MS2 on filters that had been preconditioned. Although the droplet sizes for both virus and bacteria were the same and affected the filter media in a similar manner, it was suggested that the viruses, after contact with the moisture on the filter, were released from their droplet containment, and driven onward by the flow of gas.”
• Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?. Disaster medicine and public health preparedness.
  Davies, Anna & Thompson, Katy-Anne & Giri, Karthika & Kafatos, George & Walker, James & Bennett, Allan. (2013). Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?. Disaster medicine and public health preparedness. 7. 413-418. 10.1017/dmp.2013.43.
  https://www.researchgate.net/publication/258525804_Testing_the_Efficacy_of_Homemade_Masks_Would_They_Protect_in_an_Influenza_Pandemic