4 ms·

I think this post's title is heavily editorialized, and is not reflected in the study's conclusion, which is quite confusing: With mitigation measures such

I think this post's title is heavily editorialized, and is not reflected in the study's conclusion, which is quite confusing: With mitigation measures such as surgical tape or an ECG pad secured inside, the FFRs can perform nearly as well as an FFR with no exhalation valve Note "nearly as well". This will be contradicted by the conclusion. Also it's quite logical that if you tape over the exhaust valve, it will behave like a non-valved mask? When compared to models without valves: the maximum particle penetration through the unmitigated FFRs evaluated in this study was 55%, which occurred at 55 lpm, while the maximum penetration for surgical masks was 17% for the filter media and 76% when considering fit 17% seems quite a bit better than 55%. And then they conclude with FFRs with exhalation valves can reduce 0.35-μm MMAD particle emissions more consistently than surgical masks, procedure masks... followed immediately by: With mitigations that cover the exhalation valve, 0.35- μm MMAD particle emissions can be limited to 5% if the filter media does not interfere with providing a good seal to the exhalation valve 5% is where the number in N95 comes from, that's not any better? the exhalation valves could be improved by establishing criteria for valve performance Why would it matter to improve the valve design, if it's being covered? The conclusion is that taping with micropore performs just as well as a non-valved mask, but blocking the valve completely with an ECG pad performs better than not having a valve at all. How? Are there other design differences in valved masks that account for the difference? If I attach an ECG pad to the center of a non-valved mask would it improve performance?

maxerickson 6y ago
Why would it matter to improve the valve design, if it's being covered? The best valves filtered particles very well without any mitigation. If there was a standard there might not be anything to talk about (they could all be better than other masks). I think the statement about improving them is sort of misplaced in the flow (it's related to the containing paragraph and previous 2 paragraphs).
- ricardobeat 6y ago
  > The best valves filtered particles very well without any mitigation That's not supported by the study: 17% max penetration for surgical masks vs 55% for valved masks without any 'mitigation'. They only perform similarly for inwards flow, which is expected when both have an N95 rating.
  - maxerickson 6y ago
    The first paragraph of the conclusions: Based on a sample size of 13 models, this study found that unmitigated FFRs with an exhalation valve that were tested in an outward position(with particles traveling in the direction of exhalation) have a wide range of penetration, emitting between <1% and 55%. Further testing could measure greater particle penetration. See where it says "between <1%"? And Figure 5 shows the variation between the models.
    - ricardobeat 6y ago
      The range doesn't tell the true story. In Figure 5, the majority of models are worse than surgical masks, above 20%, vs "Particle penetration through the surgical mask ranged from 2% to 17%". Only 3/13 performed better, and 1% is just the lower quantile for a single one of them.
      - maxerickson 6y ago
        Right, and the ones that perform the best? Presumably if the goal was to standardize on good valve performance, the performance would align with the best models rather than the majority.
      - incrudible 6y ago
        You are still conflating the mask cloth with the cloth mask. It is not plausible that a loosely worn cloth mask performs nearly as well as the cloth itself.
simpleguitar 6y ago
OP here. Your confusion comes from comparing filter media performance to simulated as-worn performance. A good surgical mask media will very much likely provide 17% penetration. Note that this is just the media. When you wear one, there are gaps everywhere. So that's why the paper says "penetration goes up to 76% when considering fit". As for the rest, the paper is saying that with a good tape/blocking the exhaust valve, the masks perform as well as ones without the valve. And they speculate that with a better designed exhaust valve, even better source control than as-is valve might be achievable, but there is no standards to measure it. PS: As for your last points, figure 4 answers it.
- ricardobeat 6y ago
  > the masks perform as well as ones without the valve > CDC now says valved N95 has better source control than cloth/surgical masks There's quite a difference between as well as and better than. I think that qualifies as editorializing, which is discouraged by HN guidelines. > penetration goes up to 76% when considering fit They quote a separate study for that. The numbers from their own testing are 2%-17% for surgical masks, and fit doesn't seem to be factored in for the FFR ones - all the testing was done completely sealing the mask edges. On top of that, most of the FFR models perform worse than the 17% baseline, as seen in Figure 5. My last point was: what can be the physical explanation for a covered valve performing better than not having a valve? The numbers are there for the specific model that achieved it, yes, but I'm surprised there is no hypothesis put forward. EDIT: I looked into the study where the 76% figure comes from: https://www.isrp.com/the-isrp-journal/journal-public-abstracts/1150-vol-26-no-3-2009-pp-54-70-rengasamy-open-access/file https://www.isrp.com/the-isrp-journal/journal-public-abstrac... - again ranges don't tell the truth, a single model tested went into the 76% range, most stayed around 20%. In addition to that, the study says "76% when considering fit [Rengasamy et al. 2009]" but the results in the 2009 article are also from a mask sealed around the edges, they didn't actually test for fit. Enough masks for a day now... these inconsistencies just trigger my curiosity.
  - mc32 6y ago
    FFRs have better elastic that hold fit much better than surgical masks which have weak elastic loops. (remember the pictures of nurses with red rings around their faces from wearing N95s 8hrs+ ? Thats due to tight fit.) Surgical masks are better than cloth or gaiters but N95s are better yet.

maxerickson 6y ago

Why would it matter to improve the valve design, if it's being covered? The best valves filtered particles very well without any mitigation. If there was a standard there might not be anything to talk about (they could all be better than other masks). I think the statement about improving them is sort of misplaced in the flow (it's related to the containing paragraph and previous 2 paragraphs).

ricardobeat 6y ago
> The best valves filtered particles very well without any mitigation That's not supported by the study: 17% max penetration for surgical masks vs 55% for valved masks without any 'mitigation'. They only perform similarly for inwards flow, which is expected when both have an N95 rating.
- maxerickson 6y ago
  The first paragraph of the conclusions: Based on a sample size of 13 models, this study found that unmitigated FFRs with an exhalation valve that were tested in an outward position(with particles traveling in the direction of exhalation) have a wide range of penetration, emitting between <1% and 55%. Further testing could measure greater particle penetration. See where it says "between <1%"? And Figure 5 shows the variation between the models.
  - ricardobeat 6y ago
    The range doesn't tell the true story. In Figure 5, the majority of models are worse than surgical masks, above 20%, vs "Particle penetration through the surgical mask ranged from 2% to 17%". Only 3/13 performed better, and 1% is just the lower quantile for a single one of them.
    - maxerickson 6y ago
      Right, and the ones that perform the best? Presumably if the goal was to standardize on good valve performance, the performance would align with the best models rather than the majority.
    - incrudible 6y ago
      You are still conflating the mask cloth with the cloth mask. It is not plausible that a loosely worn cloth mask performs nearly as well as the cloth itself.

simpleguitar 6y ago

OP here. Your confusion comes from comparing filter media performance to simulated as-worn performance. A good surgical mask media will very much likely provide 17% penetration. Note that this is just the media. When you wear one, there are gaps everywhere. So that's why the paper says "penetration goes up to 76% when considering fit". As for the rest, the paper is saying that with a good tape/blocking the exhaust valve, the masks perform as well as ones without the valve. And they speculate that with a better designed exhaust valve, even better source control than as-is valve might be achievable, but there is no standards to measure it. PS: As for your last points, figure 4 answers it.

ricardobeat 6y ago
> the masks perform as well as ones without the valve > CDC now says valved N95 has better source control than cloth/surgical masks There's quite a difference between as well as and better than. I think that qualifies as editorializing, which is discouraged by HN guidelines. > penetration goes up to 76% when considering fit They quote a separate study for that. The numbers from their own testing are 2%-17% for surgical masks, and fit doesn't seem to be factored in for the FFR ones - all the testing was done completely sealing the mask edges. On top of that, most of the FFR models perform worse than the 17% baseline, as seen in Figure 5. My last point was: what can be the physical explanation for a covered valve performing better than not having a valve? The numbers are there for the specific model that achieved it, yes, but I'm surprised there is no hypothesis put forward. EDIT: I looked into the study where the 76% figure comes from: https://www.isrp.com/the-isrp-journal/journal-public-abstracts/1150-vol-26-no-3-2009-pp-54-70-rengasamy-open-access/file https://www.isrp.com/the-isrp-journal/journal-public-abstrac... - again ranges don't tell the truth, a single model tested went into the 76% range, most stayed around 20%. In addition to that, the study says "76% when considering fit [Rengasamy et al. 2009]" but the results in the 2009 article are also from a mask sealed around the edges, they didn't actually test for fit. Enough masks for a day now... these inconsistencies just trigger my curiosity.
- mc32 6y ago
  FFRs have better elastic that hold fit much better than surgical masks which have weak elastic loops. (remember the pictures of nurses with red rings around their faces from wearing N95s 8hrs+ ? Thats due to tight fit.) Surgical masks are better than cloth or gaiters but N95s are better yet.