Note: This is an updated version as of 3/5/21. I updated the bit about thread diameter realizing that I didn’t have the full picture. The edits are interspersed throughout the post where thread diameter is concerned. I learned a wee bit more and I present it to you here. (Thanks, J.)

I’m finding conflicting evidence on the efficacy of face masks in reducing transmission of COVID-19. This is somewhat distressing. But I’m still pro-mask, which I will get to in my takeaway at the end. In the meantime, allow me to invite you down the rabbit hole I dug for myself starting seven hours ago. Come in, come in. There’s evidence I’d like to show you and maybe a nugget or two of chocolate.

So here we have it: the face mask debate in the era of COVID-19. We’re still debating this apparently. A prolonged pandemic makes for a prolonged debate, I suppose.  

One the one hand, some researchers say masks are effective. For example, McIntyre & Chughtai say that community use of face masks is protective if used early, with hand hygiene, and if compliant (2020). That’s a lot of ifs, which says, to me, that it’s even more important that we adhere to mask-wearing guidelines and keep our hands squeaky clean. Howard et al. support these findings, saying that the available evidence suggests that near-universal adoption of nonmedical face masks when out in public, in conjunction with complementary public health measures (i.e., wash your hands, peeps!) can reduce community spread if the measures are sustained (i.e., not given up) (2020). Furthermore, a meta-analysis of observational studies on risk of viral infection, including SARS-CoV-2 infection, found that masks, compared with no masks, was protective in both healthcare and non-healthcare settings (Sommerstein et al., 2020). There were countless other articles I read through that supported the use of masks for reducing SARS-CoV-2 transmission. You’ll excuse me for not laying out all the evidence here as I would be here all day, and the next, and the next…

But then there is one researcher who strongly suggested against using face masks. In his paper, Vaunshelboim claims that medical and non-medical face masks are ineffective in blocking SARS-CoV-2 because the virus is a lot smaller (1000 smaller) than the thread diameter of face masks (2020). In short, he’s saying that the virus can easily pass through any face mask. He then goes on to describe the physiological and psychological effects, as well as the long-term health consequences, of wearing face masks, which, as you may have guessed from his position that face masks are not helpful, were all negative. Vaunshelboim concludes that the evidence challenges the safety and efficacy of wearing face masks as a protective intervention for COVID-19.

Feeling slightly defensive and peeved by Vaunshelboim’s conclusions (I’m an avid mask wearer), I zeroed in on the virus diameter versus thread diameter part. I wondered, does my Outdoor Research (OR) face mask have a large thread diameter? Have I been unknowingly leaving a thick trail of my breath particles thinking I was being all safe?

Then I learned that thread diameter is only one part of the puzzle when selecting a material for a face mask. Thread pitch and the manufacturing process also impact filtration efficiency as, together with thread diameter, they affect pore size (i.e., the gaps between the threads) (N95Decon, 2020). So, I’m not a face mask scientist, but Vaunshelboim’s point about average thread diameter making masks ineffective at filtering SARS-CoV-2 seems like a moot point to me.

Curious about the differences in fabric types, I found this chart of common mask materials and some of their properties:

I somewhat frantically ran to get my mask. I looked for the tag on the mask—the one that details the materials. Of course I cut it out—who wants that rubbing their face all the time? Luckily, OR’s website has the specs. The mask is 100% polyester, which I assume is 100% woven polyester (?).

That worried me a bit. Porosity doesn’t look great. But aha! I do have a reusable filter that lines the inside of my mask. So I looked that up on OR’s website and saw that it is made of nonwoven polypropylene. Sadly, despite the rabbit hole I dug myself, I could not find the deets of this material, and, furthermore, learned that there are different kinds of polypropylene. Unsurprisingly, OR does not list this level of detail in their specs. Maybe they don’t know? What I did learn is that unwoven is better for catching things (like a virus) and that unwoven polypropylene is commonly used in medical masks. So that must make it better? But Vainshelboim said medical masks weren’t efficient at filtration either. Did he consider polypropylene? Is that material implied in “medical mask”? Maybe he was stuck on his moot point (determined moot by moi, that is). I don’t know, and there are no numbers I could find to quantify the material. But it is now recommended to have polypropylene in your mask. So that must mean something, right?

Who would have thought a piece of cloth could incite such a battle among academics and researchers. But here we are. And here I am, as confused as ever.

***

Okay, I’m back from lunch and I’m riding a sugar high from vegan chocolate cake with strawberries. I told you there might be nuggets of chocolate. But this is an every-rabbit-for-themselves kind of rabbit hole.

Anyway, as I sat down again, I had a thought. Vanshelboim says the viral particles are 1000 times smaller than the thread diameter of face masks, making the virus flow easily through the mask. But then I thought, wait a minute. It’s not like you cough, breathe, or sneeze virus directly. It doesn’t come out stark naked but rather in respiratory droplets. Right—duh. So, how big is a respiratory droplet that contains SARS-CoV-2? I went back to the science. According to Lee, the minimum size of a respiratory particle containing SARS-CoV-2 is 4.7 µm (2020). That’s still smaller than the mask thread diameter, at least of polyester, but we know that it’s the porosity that counts (i.e., smaller pores are better). So, saying the virus is soooo much smaller than the thread diameter (by > 1000x) doesn’t mean anything really. Wah-wah-wahhh.

But wait, there’s more!

The New York Times tells me, in a very cool simulation, I will say, that the smaller respiratory particles get bounced around by air molecules within the fibers of a mask (e.g. a cotton one), increasing their air time (weee!) in the fiber forest, and thus their chance of getting captured. And, furthermore, I learned polypropylene has an electrostatic charge that makes it good at capturing particles of all sizes. Remember physics, kids? May Chu, an epidemiologist, who co-authored a paper on the filtration efficacy of household materials used in face masks, likened it to static cling—that the cling traps incoming and outgoing respiratory droplets, which is what you want (Godoy, 2020).

***

Oof—that’s a lot of science. Now that I’m inching my way up to ground level (the rabbit hole was kinda fun, but God I need fresh air), here’s my takeaway—here, I’ll even italicize it for you (I see you, skimmers!): The efficacy of face masks in reducing SARS-CoV-2 is controversial, but I see much more evidence for pro mask to reduce transmission. And, furthermore, a mask is better than no mask, a proper fitting mask is better than a haphazardly worn one, and there’s a buzz around polypropylene (pun intended) for good reason.

I hereby claim that I remain pro mask.

(Where’s a gavel when you need one?)

References

Godoy, M. (2020, July 1). A User’s Guide to Masks: What’s Best at Protecting Others (And Yourself). WUSF Public Media. https://wusfnews.wusf.usf.edu/2020-07-01/a-users-guide-to-masks-whats-best-at-protecting-others-and-yourself

Howard, J., Huang, A. Li, Z., Tufekci, Z., Zdimal, V., van der Westhuizen, H-M., von Delft, A., Price, A. Fridman, L., Tang, L-H., Tang, V., Watson, G. L., Bax, C. E., Shaikh, R., Questier, F., Hernandez, D., Chu, L. F., Ramirez, C. M., & Rimoin, A. W. (2021). An evidence review of face masks against COVID-19. Proceedings of the National Academy of Sciences, 2021, 118(4). doi: 10.1073/pnas.2014564118

Konda, A., Prakash, A., Moss, G., Schmoldt, M., Grant, G., & Guha, S. (2020). Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. ACS Nano, 14(8), 10742-10743. doi: 10.1021/acsnano.0c04676

Lee B. U. (2020). Minimum Sizes of Respiratory Particles Carrying SARS-CoV-2 and the Possibility of Aerosol Generation. International journal of environmental research and public health, 17(19), 6960. https://doi.org/10.3390/ijerph17196960

MacIntyre, C. R., & Chughtai, A. A. (2020). A rapid systematic review of the efficacy of face masks and respirators against coronaviruses and other respiratory transmissible viruses for the community, healthcare workers and sick patients. International journal of nursing studies, 108, 103629. https://doi.org/10.1016/j.ijnurstu.2020.103629

N95DECON. (2020). N95DECON Research Document, Not Peer Reviewed, Version 1.0, 8/6/2020. https://static1.squarespace.com/static/5e8126f89327941b9453eeef/t/5f2c4463a5c9f75a38d2b26f/1596736614213/N95DECON_cloth_mask_breathability_filtrationtechnical_report_v1_200804.pdf

Sommerstein, R., Fux, C.A., Vuichard-Gysin, D. et al. Risk of SARS-CoV-2 transmission by aerosols, the rational use of masks, and protection of healthcare workers from COVID-19. Antimicrob Resist Infect Control 9(100) (2020). https://doi.org/10.1186/s13756-020-00763-0

Vainshelboim B. (2021). Facemasks in the COVID-19 era: A health hypothesis. Medical hypotheses, 146, 110411. https://doi.org/10.1016/j.mehy.2020.110411