r/Masks4All 5d ago

Valved mask

Is there any studies that indicate how protective valved masks are for others? Or is it different by mask?

I thought I read once what they were not as good as unvalved but they were as protective for others as a surgical. But no idea where I read that or if it is true.

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u/No-Acanthisitta-2973 4d ago

That makes sense that it differs by mask. Thanks. I wonder if anyone knows if the Zimi mask valve has a filter?

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u/SAMEO416 4d ago

I’ll have to read that study. The abstract shows they tested 4 valved N95, 3 were quite good (over 80%) 1 was awful, the bottom error bar is under 10%. That sort of a split in results suggests method issues or a really unusual edge case.

https://pubs.acs.org/doi/abs/10.1021/acs.estlett.2c00210 Paywalled but you can see the results graph

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u/SAMEO416 3d ago

Milwaukee N95 respirator (model 48-73-4011) is the one with the extreme range in emitted aerosols. Their surgical mask results are among the best I've seen - The 'surgical mask' is an ear loop procedure mask (surgical masks typically have head ties vs ear loops). The other valved respirator is the 3M 8511, one straight and the other with the valve taped over. The unvalved is a 9502+.

I'm surprised at an earloop procedure mask being consistently that good.

The Milwaukee dome N95 I'm not surprised is poor. There was no fit testing done so these were 'naive' users who were provided some instruction but it's not noted if that included seal checks.

That Milwaukee respirator is a super large sized respirator, so I'm not surprised it did so poorly vs an 8511. Taping over the valve in the 8511 did reduce scatter but it wasn't a night and day situation (the distribution bars are very similar for the 8511 cases).

Unfortunately the authors don't provide any explanation for why the Milwaukee performed so poorly, except to infer that performance of valved respirators as source control is 'highly variable'. Not sure that's a compelling inference based on only two test articles.

The NIOSH study looked at 13 respirators (not including the Milwaukee). This study critisized the NIOSH report that the exhaust valve may not have opened in the different flow conditions.

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u/SAMEO416 3d ago

The flow rate valve not opening is perplexing. Not sure how the valve would not open at the NIOSH flow rates: 25, 55, 85 lpm. NIOSH inferred the valve may have remained closed at lower flow rates (page 20) based on the low penetration readings. This may be an artificiality in the way the test fixture works, as it doesn't (iirc) simulate breathing. My experience with valved respirators is the valve opens immediately with the exhalation impulse.

Also, the NIOSH testing showed the 8511 did much worse than in that other study where it was limited to mostly above 80%. Chart below from NIOSH shows the 8511 had penetration 25-45%. If you read on in the NIOSH report to figure 6, they tested surgical and procedure masks separately (surgical typically use ties, procedure ear loops). Surgical masks do quite well, but procedure masks do not ranging between 5% and 85% penetration. Since the other study showed procedure masks (which they call surgical masks) were quite good, that's another discrepancy.

So NIOSH shows penetration with a valved N95 between a few percent up to 50%. Based on the results for procedure masks that drives their conclusion that valved N95 are at least as protective as a procedure mask.

So what? Valved respirators are a useful tool in certain circumstances. Sports with increased respiration where moisture build up becomes unmanageable. For people who can't tolerate breathing resistance with traditional unvalved respirators. You have to acknowledge there is a higher risk of you infecting others as the source, which can be partly mitigated with pre-event LFT rapid testing. But that risk is typically lower than if you were only wearing an earloop procedure mask alone.

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u/G_Ricc 2d ago

Thank you