Re-use your N95… a lot!

So, I was already planning another mask post because of this cool research summary in Axios:

With the Omicron variant causing infections to surge to record levels, masking is more important than ever— and increasing evidence indicates the quality of mask makes a significant difference.

The big picture: Fitted particle-filtering masks like N95s are up to 75 times more effective at preventing infection with COVID-19 than surgical masks, according to a study published recently in the Proceedings of the National Academy of Sciences.

  • This echoes recent recommendations by public health experts about the value of N95 masks, reported by the Wall Street Journal.
  • It will take 25 hours for an infectious dose of COVID-19 to transmit between people wearing non-fit-tested N95 respirators but if they’re wearing a tightly sealed N95, they’ll have 2,500 hours of protection, per a study reported by the WSJ.

The details: In the recent study by researchers at Max Planck Institute in Germany, they investigated the risk when two individuals are speaking with each other at a close distance. Their study used the most conservative estimates for simulations, meaning that any scenarios with a risk below 1% in their study can be assumed safe in a real-world environment.

  • As shown in the data visualization above, only universal masking with well-fitted N95 or similar masks reduced the risk to below 1% after 20 minutes.
  • Surgical masks and situations where one person wore a poorly fitting mask or did not mask reduced the infection risk to between 4-10% after 20 minutes, which is still far above the acceptable risk level, they said.
  • This study did not investigate cloth masks, but other studies have shown that they can provide even less protection than surgical masks.
  • A separate experiment from the study found that social distancing barely helped, with the infection risk reaching 80% after just one minute, and 99.9% after 20 minutes.

What they’re saying: While the study was conducted before the presence of Omicron, these findings should hold up given the conservative estimates they used in their simulations, study authors Gholamhossein Bagheri and Eberhard Bodenschatz told Axios.

  • They recommend individuals buy an N95, KN95, or FFP2 mask that has a nose piece for adjustment.
  • The fit of the mask is the most important factor, Bodenschatz, said. “It turns out that leakage dominates over filtration.” Masks should have a tight fit around the face and nose to minimize any air leakage.

That’s honestly the most optimistic case I’ve seen for N95 efficacy among ordinary individuals, rather than officially fit-tested health care workers.  

And, I’ll emphasize here… fit, fit, fit.  I said to my son, yesterday, how can people invest in these high-quality masks and then not even make the effort to have a good fit?  And, then, I realized, I actually see this all the time.  Various 94/95 masks with clear and obvious gaps on the wearer.  That 95 means nothing if you’re just breathing through the gaps?  Also, I’ll proselytize again for the 3m N95 Aura because it’s the mask that’s definitely been the easiest to get a tight fit.  

Honestly, this study might well be a little optimistic on just how much protection to expect, but, even then, it’s still pretty damn encouraging.

That was going to be the extent of my mask post, but, not long after reading the above, I came across this, “How often can you safely reuse your KN95 or N95 mask?”  This is good!  People can absolutely reuse these masks many times and they need to know that so they won’t be scared off by the $2/mask price tag for an Aura.  It’s good to have articles pointing out– yes, reuse these high-quality masks!

That said, this article was so frustrating.  The “experts” were literally just pulling recommendations out of thin air:

“In the ideal world — or pre-pandemic — many masks were really viewed as single-use,” said Michael G. Knight, an assistant professor of medicine at George Washington University. “The reality is they do have a little bit more length in the amount of time we can use them.”

What’s crucial, Knight said, is making sure the mask has “maintained its integrity.” Think about how many times you’ve used it and for how long, he said.

“If I’m just putting a mask on to go to the grocery store for 45 minutes and I’m taking it off, that mask very well should be able to last me a couple of days,” he said.

But if you’re wearing a mask all day, such as during a long work shift where you may be sweating and talking all day to the point the mask becomes soiled, “then that may not be a mask that I can reuse.”


When you start seeing signs that the mask is soiled, “you’re getting to the point that that mask needs to be replaced,” he added…

Even with normal use, if you wear a mask for a few hours a day, “within four or five days, it’s going to be visibly soiled,” Knight said.

My goodness, yes, a good workout will soil your N95.  No fun!  But, that’s got to be such a tiny fraction of their usage.  Otherwise, since when does “If I’m wearing it for three hours” lead to lots of sweating that’s going to soil your mask.  Or, speaking from experience here… wearing a mask a few hours a day for a workweek does not leave it visibly soiled!

There’s literally nothing in this article about how long mask filters last and what happens to the filtration ability over time.  And, you know what?  The filtration ability is remarkably durable.  In this test, surgical masks kept their filtration ability strong for 40 days! (Mind you good surgical masks are made of the same filter material as N95’s, they just don’t give you a tight seal).  

Skeptical readers of our past mask longevity tests have worried about the masks getting gross. Don’t they grow bacteria?

Keer and Vivian made sure to let the masks dry out. They stored them on a desk or hanging on a knob.

At the end of the 40 days, the masks were not smelly or visibly dirty (other than a bit of makeup and lipstick). This fits with a peer-reviewed study of masks, which found that masks did not grow bacteria. The only bacteria on the masks was what was already on people’s faces.

Bottom Line: Surgical Masks Still Effective and Breathable After Prolonged Use

After 40 days (40 hours of use), surgical masks were still able to filter 95% of 0.3-micron particles. Thus, surgical masks last far longer than a few hours or days.

Of course, don’t just wear one mask.  Have a few; definitely let them air out between uses.  And, yeah, if it gets sweat-logged or the elastic gives out and you can’t get a good fit, throw it away (honestly, the fit will probably give out before filtration efficacy does)  But get some high-quality masks, make sure they have a good fit, and re-use them a lot. 

About Steve Greene
Professor of Political Science at NC State

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