Measuring the fitted filtration efficiency of cloth masks, medical masks and respirators, 2025, Tomkins et al.

[ahimsa]

Small study, but I thought it was interesting how well N95 respirators worked without any fit testing. I remember at the beginning of the pandemic there was a lot of concern about how well these type of masks would work for the average person without any fit testing.

Measuring the fitted filtration efficiency of cloth masks, medical masks and respirators

Amanda A. Tomkins, Gurleen Dulai, Ranmeet Dulai, Sarah Rassenberg, Darren Lawless, Scott Laengert, Rebecca S. Rudman, Shiblul Hasan, Charles-Francois de Lannoy, Ken G. Drouillard, Catherine M. Clase

Published: April 21, 2025

Abstract

Importance
Masks reduce transmission of SARS-CoV2 and other respiratory pathogens. Comparative studies of the fitted filtration efficiency of different types of masks are scarce.

Objective
To describe the fitted filtration efficiency against small aerosols (0.02–1 µm) of medical and non-medical masks and respirators when worn, and how this is affected by user modifications (hacks) and by overmasking with a cloth mask.

Design
We tested a 2-layer woven-cotton cloth mask of a consensus design, ASTM-certified level 1 and level 3 masks, a non-certified mask, KF94s, KN95s, an N95 and a CaN99.

Setting
Closed rooms with ambient particles supplemented by salt particles.

Participants
12 total participants; 21–55 years, 68% female, 77% white, NIOSH 1–10.

Main Outcome and Measure
Using standard methods and a PortaCount 8038, we counted 0.02–1 µm particles inside and outside masks and respirators, expressing results as the percentage filtered by each mask. We also studied level 1 and level 3 masks with earguards, scrub caps, the knot-and-tuck method, and the effects of braces or overmasking with a cloth mask.

Results
Filtration efficiency for the cloth mask was 47–55%, for level 1 masks 52–60%, for level 3 masks 60–77%. A non-certified KN95 look-alike, two KF94s, and three KN95s filtered 57–77%, and the N95 and CaN99 97–98% without fit testing. External braces and overmasking with a well-fitting cloth mask increased filtration, but earguards, scrub caps, and the knot-and-tuck method did not.

Spoiler: Image comparing different mask types

 

[Sean]

the knot-and-tuck method did not.

Useful to know.

 

[AliceLily]

This proves that what I am doing has been working very well for me the last 3-4 years. I wear a surgical mask with a cotton mask over top. I haven't had a cold at all which is unheard of for me.

I caught Covid last year for the first time but I am sure it was from my sick neighbour and I was unmasked around her at the time.

I catch trains and buses with people who sound full of colds or Covid and I have found this double masking excellent so far.

The masks are put straight into a laundry bag after each use and I use the surgical masks up to 20 times before throwing out and starting with 10-15 new ones.

 

[AliceLily]

I always forget to mention that I sanitize my hands a lot when out too. So both measures have given me good protection.

 

[BrightCandle]

I have been using FFP3 masks since March 2020, I had a packet from before the pandemic for working with MDF, they were at the time about £1.50 for 3! I wont be using anything less. Its pretty clear they work well, I have fit tested the ones I use now so I know they work as specified.

This pandemic could have gone a completely different way if instead of assuming the public couldn't handle to learn a 5 minute video on how to fit FFP2/3 masks they had spent all that focus on giving people that skill and ramping production. Instead they still deny airborne transmission of disease against all the evidence.

 

[Sean]

This pandemic could have gone a completely different way if instead of assuming the public couldn't handle to learn a 5 minute video on how to fit FFP2/3 masks they had spent all that focus on giving people that skill and ramping production. Instead they still deny airborne transmission of disease against all the evidence.

Quite extraordinary and disturbing, isn't it. So much suffering and death could have been prevented by a simple, cheap, safe, proven protection.

Yet here we are. Somehow it has become thoroughly politicised and demonised, probably beyond repair any time soon.

 

[Mij]

The instructions on my Breathe Teq N95 mask advise to discard them after 8hrs of use.

 

[Sasha]

I wish I knew more about when to discard them. I use 3M N95's, and they're expensive, but I only use them for about five minutes at a time. I don't know whether it's total time in use or time since opening that counts, or whether, if you leave them for ages, they reset to be good as new.

 

[jnmaciuch]

I wish I knew more about when to discard them. I use 3M N95's, and they're expensive, but I only use them for about five minutes at a time. I don't know whether it's total time in use or time since opening that counts, or whether, if you leave them for ages, they reset to be good as new.

From what I have read, the primary issue with prolonged use is loss of strap elasticity and then warping of the shape of the mask.

Both are more likely to be issues with KN95s/N95s/FFPs as it’s the mask shape and strap tightness that contribute to the protective seal, forcing all air to pass through the mask itself and get filtered rather than finding ways to weasel in through gaps around the edges of the mask.

That’s why cloth masks are sometimes not recommended to wear on top of N95s—they actually distort the underlying fit.

For surgical masks, a cloth mask on top might help overcome those issues because it pulls the mask closer to your face, provided that it’s sufficiently snug around your nose bridge and there’s no gaps for air to get through. In that case, your main issue is the filtration efficiency of the materials themselves, which is much lower than N95 grade and will wear down from friction and moisture/face oils over time.

I actually have a hard time with typical N95s because I have a weird small and very angled nose bridge that makes it near impossible to get a good seal. The 3M aura masks are best for me since they have the foam nose bridge layer to fill in gaps.

I throw them out based on the elasticity of the blue bands (it can be hard to tell but I gauge by comparing to the elasticity of a brand new one) and make sure to never store them in a bag where they’re going to get squished or misshapen before the bands give out.

[edit: pulling the masks on and off frequently actually affects the elasticity quite a bit, so overall use time might not be the best metric]

 

[Sasha]

From what I have read, the primary issue with prolonged use is loss of strap elasticity and then warping of the shape of the mask.

Thanks, that's really helpful - I've got the Aura ones too. People talk about bacteria but for such use times I'd be amazed if that was any kind of a problem.

 

[jnmaciuch]

Thanks, that's really helpful - I've got the Aura ones too. People talk about bacteria but for such use times I'd be amazed if that was any kind of a problem.

Yeah I think it’d only become an issue if you’re in a hospital ward and wearing it for hours and hours while sweating.

For your case, I think stretching out the bands from putting it on and taking it off frequently is going to be the biggest issue. Since the aura rubber bands are just stapled on, I’ve actually had decent luck stapling a different band that can be looped and tied off in the back to make the fit more snug even if the band is a little worn out.

Took some experimenting but ended up saving me money since I could extend the use longer than I would otherwise. You just need to be sure that your stapling and tying is as tight as the brand new straps and that you don’t accidentally introduce any new punctures on other parts of the mask.

 

[Utsikt]

I understand that if you want to prevent anything from entering, you have to swap them out often. But if you only care about airborne viruses, surely the filters are good enough to outlast the components that contribute to the fit?

 

[jnmaciuch]

I understand that if you want to prevent anything from entering, you have to swap them out often. But if you only care about airborne viruses, surely the filters are good enough to outlast the components that contribute to the fit?

Unfortunately the mask is only as good as the percentage of air that ends up going through the filter material, and air will always follow the path of least resistance. If there’s a gap around your nose bridge, then a lot more of your air is coming in through that gap completely unfiltered than through the mask material.

You’ll still be better off than completely unmasked—60% filtered air is better than 0%, plus there will often be other environmental factors that help out and most people’s bodies can adequately clear some low level of airborne particles.
And this article shows that the masks give very good protection without extensive fit testing (though they’re not using old and distorted masks, so fit issues are likely less pronounced to begin with).

But the more people around you expelling viral particles and the less air turnover in the room, the more you’re relying on the mask itself to filter things out. And that’s when fit is paramount, because some greater percentage of completely unfiltered air increases viral load past the threshold of what the immune system can adequately clear before infection takes root.

 

[Mij]

My masks also come with an expiry date which will be November 2025. I bought them 2 years ago.

 

[Sean]

I actually have a hard time with typical N95s because I have a weird small and very angled nose bridge that makes it near impossible to get a good seal.

Masks sometimes come in two versions, one shaped for Asian faces, and another for everybody else. Might be worth checking that out.

 

[jnmaciuch]

Masks sometimes come in two versions, one shaped for Asian faces, and another for everybody else. Might be worth checking that out.

Thanks, unfortunately it seems to be a universal problem across masks, even ones that seem to work well for my fellow Slavs. It’s all good though, nothing that the foam tape on a 3M aura doesn’t fix!