At the outset, publication by ACS is a significant imprimatur. I will also caveat that my experience is more in the deployment of filtration media than the materials science development side.
That being said, here are some thoughts:
- The general principle remains - you can choose breathability or you can chose good filtration. You can't have both. Fine particulate filtration requires energy.
- Table 1 references a pressure drop (ΔP) across the filter of 2.2 - 3.0 Pa. That is an unbelievably small number - so small as to likely be unmeasurable with their test setup. It's also indicates that essentially no mechanical filtration is occurring, which would conflict with information in the paper. For comparison, a fabric filter designed for 99% removal of dust <2.5 micrometers (2500 nanometers), would likely have a minimum pressure drop of about 1250 Pa, about 1000x higher. Which makes me think that the data in Table 1 are actually kPa. The reference indicates that units for data in Figure 2 were corrected post-submittal and publication. I wonder if that isn't also true for Table 1. Perhaps I'm wrong and those data are correct. But if I were doing peer review on the article, absolutely I would flag that and would recommend review and resubmittal prior to publication.
- If Table 1 is actually kPa, that's a noticeable pressure drop. 2.5 kPa is the same as 10-inches of water. So for reference, imagine having a soda (or soda pop, or pop, or Coke) in a Big Gulp size container that is close to empty. How hard do you have to suck on the straw? That's in the ball park of 10-inches of water, so that is how hard you would have to suck to pull air across a mask with 2.5 kPa pressure drop. But unlike a Big Gulp, where you can suck and breathe a bit for taking another sip, with a mask every breath would require that effort. And that is actually consistent with what I would expect for a filtration/particulate removal system that is achieving the performance suggested by their results.
- The study used a NaCl (sodium chloride, aka table salt) particle generator. NaCl is an inorganic salt, and is highly ionic. The COVID-19 virus is part of a family of viruses that are identified as "enveloped" viruses - which means that the exterior of the virus is a lipid (fat or oil). Lipids, by their nature, are the antithesis of ionic materials. In the kitchen, think about the difference between adding salt to a pot of water versus adding butter or cooking oil. The article mentions electrostatic attraction, but if a particle is not ionized, electrostatic forces don't come into play. So in that regard, I have reservations about the relevance of electrostatic attraction as removal mechanism for the COVID-19 virus. Again, if I were doing peer review on the article I would flag that and request that the relevance of electrostatic forces in the removal of the virus be documented or demonstrated.
- With my prior point in mind, non-ionic particles that acquire a water-soluble exterior will also develop an ionic charge. That trait is often used to allow such materials to be sorbed from a fluid stream. Many bodily fluids have the ability to attach to a lipid material and make the material transportable in the body (blood or lymph system). That's how our bodies move fat around. So when the virus is expelled from respiratory system, prior to evaporation of the surface water phase the virus could respond to electrostatic forces. Any layer that provides electrostatic removal would have a finite capacity . When that capacity is expended, electrostatic attraction is no longer relevant.
- The presence and importance of electrostatic attraction is assumed, without supporting data on how significant that might be. (It's possible that in a my quick overview of the article I overlooked those data.). The simple test would be to look at removal vs. head loss (while maintaining all other parameters constant) for presumed electrostatic and non-electrostatic materials. If electrostatic is a significant parameter, then the electrostatic materials would show higher removal efficiency for similar headloss. And the related question would be how well that differences sustains over time.
- The testing was done under ideal, lab conditions. Real-life conditions are never the laboratory. (A principle that was burned into me at 19-yrs old when a wonderful entrusted me to make my first significant engineering decision. But that's another story.)
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Long-winded I'm sure - but this would be my bottom-line.
1. The more sophisticated design you are talking certainly wouldn't be any worse, and could be better. If it makes you feel less anxious that is probably good. But without more data, you shouldn't think that you are equalling or exceeding the capabilities of a properly constructed, fitted, and used N95 mask.
2. If there is not significant resistance during inhalation (think of my 7-11 Big Gulp analogy), do not presume that you are getting significant removal of micron and sub-micron size particulates.
