How long has one (K)N95 generally lasted you?

Wasn’t sure if this was better in the chitchat thread, but I thought this might be worthy of general attention, since the folks here seem to be more careful than average.

I take the bus to work x3 a week, where I generally don’t wear it at my desk (although maybe I should). I have some cheap ones I’ve been using 1/week (mainly because they’re cheap), but wondering if the more expensive ones I just got would be the same. I’ve read that these things don’t have an “expiration date” per se, but the more you use it, the more likely it is that it’ll get wet or not fit well or whatever. The package of the expensive ones says “single use,” but I’m not certain that’s meant to be literal, since the instructions are silent on this topic.

So how long do y’all generally use any one mask? Does it matter if it was $1 or $5 (assuming they’re both sold as an X95)?

“Single use” is because it is intended to be used in a clinical setting where it would normally be disposed between patients. the effectiveness of the non-woven melt-blown polypropylene filter material is based on its dielectric charge being able to attract small charged particles and stick them to the fibers; in the case of viruses and bacteria, it holds onto they desiccate and break apart.

I’ve used masks for multiple weeks with intermittent wear and dispose of them when they start to smell or become discolored but if I wore a mask for multiple hours a day I’d only trust it for a week or less and would alternate between a couple to let them thoroughly dry out between uses.


Is there any evidence that they work any better than ordinary cloth, any time past a single day (if that)? I’d expect that even taking them out of the package would start the timer for when they’re not effective any more.

No N95 masks that I have used come in any kind of hermetically-sealed packaging, and in fact most formed N95 masks just come in a loose (unsecured) plastic back inside of a printed cardboard box. Humidity levels below the dew point do not degrade masks within their expiration period so there is no reason for special packaging.

How long the melt-blown polypropylene retains its effectiveness with use is a complicated and difficult to quantify metric, in part because some people have very humid breath and other people release less moisture, depending on ambient temperature, relative humidity, exertion levels, et cetera. The polypropylene itself is naturally hygroscopic (does not absorb moisture) and doesn’t break down due to ambient temperature or components in breath; the conditions that will cause it to break down are exposure to solvents, ultraviolet or intense thermal radiation, and contamination which itself will absorb water and reduce the effectiveness of the mask in capturing small ionized particles. The fabric does not literally stop virions and small bacteria by geometry of the fabric; if the gaps were too small for these to pass it would be nearly impossible to draw a full breath. Instead, it electrostatically attracts small particles that are almost always charged and then holds onto them until they discharge or desiccate (dry out), which kills bacteria and deactivates viruses.

Although visual staining is probably not the best gauge of when a mask loses its effectiveness it is the easiest one to apply; if it has picked up a persistent odor that doesn’t go away in a few hours of drying in air that likely means it has enough contaminants that its effectiveness has declined although by how much is guesswork without some kind of occupational study or better yet culture surveys of bacteria and viruses, although this would obviously require at least a BSL2 lab even for innocuous pathogens.

There has been some work on nanoscale fabrics (i.e. carbon nanotube or carbon black embedded materials) but normal cotton, linen, and other natural woven fiber fabrics are next to worthless for filtering virions and small bacteria. You can easily tell this by fitting the material to your face and experimenting on whether you can smell perfume or cigarette smoke; if it can block perfume it is probably adequate against most pathogenic bacteria, and if it blocks cigarette smoke it should be good against virions. I have personally run some experiments with various woven fabrics using inhaled fluorescein and can verify that they do essentially nothing to prevent aerosolized particles from being expelled and presumably very little to nothing on the receiving end.


I’ve heard for months that if you’re not using a 95 mask of some kind, you might as well have just duct taped a piece of tissue over your nose. If I could get the same effectiveness out of a cloth mask, that’d be a game changer for my wallet.

I’ve got no data to back this up, but having worn KN95s all day every day at work from April 2020 until 5 weeks ago this was my technique:

I have 7 of them, each labeled for a specific day of the week. After wearing, it gets set aside where it can air out on all sides until next week.

Ref @Stranger_On_A_Train, each mask is replaced when it gets stained, the elastic detaches, the metal nose clamp detachs, or I notice it smells at all. One of which failures typically happens after at most 10 weeks, so I get up to 10 well-spaced 12-hour wearings from each mask.

My assumption is that 6 days off is plenty to 99.999% disinfect a used mask. The unknownable question is whether meanwhile the filtration has declined due to loss of static charge or whatever else.

Aaron Collins has done extensive testing of masks throughout the pandemic, and has published a gazillion you tube videos where he tests masks and answers questions. (He’s easy to Google)

He says that water and artificial sweat have been tested and don’t damage the effectiveness of masks (although you want it to be dry when you wear it.) He also says that alcohol and some other solvents damages the electrostatic properties – so much so that he strongly recommends against labeling your mask with a sharpie, as you are effectively punching a hole in its ability to filter.

Friends have access to some KF94 masks that actually say on the packaging that they are rated for 40 hours of office wear.

I think the idea of cycling masks, and discarding them when they develop a problem is likely fine.

Problems are: the elastic gets loose, the mask becomes visibly dirty, smells bad, or stops allowing air to pass freely (they get harder the breathe through if you use the same one long enough).

Separate from whether they’re still filtering well, mine have reduced stiffness that results in poor fit after about a day of wear. If I’m not getting a good seal, I grab a new one. Faces vary so YMMV.

That’s interesting – I find the opposite. When I first put on a brand-new mask, it’s a little too stiff at the edges, and doesn’t seal properly. But after I’ve worn it for a little (and perhaps after the edges become damp) it softens up and fits more snugly over my face. I can feel the leakage at first, and then feel that it’s stopped leaking.

This is one reason I usually put on my mask before getting in the car to go somewhere where I plan to use it. It takes a little while to “settle in”.

But that’s after about 5 minutes of use, not a day.