Commercial fit-testing for masks needs quite fancy equipment like the Allegro Saccharin Fit Test Kit 2040 which combines undesireable features of being currently unavailable to order, expensive and having to take up room in my apartment after beign ordered. 

It seems to me that there should be a way to get fit-testing done without as fancy equipment as it's just about exposing oneself to the smell of one of the substances that are are suitable for fit-testing (that have the right molecular size). Has anybody here found a way to do reliable fit-testing for themselves without the commerical equipment?

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Mar 01, 2021


You do need fancy equipment, but not quite that fancy. The one thing you absolutely can't do it without is a nebulizer. This is a sort of modified spray bottle which converts liquid into aerosol. A regular spray bottle won't work, because the droplets it produces are too large. The nebulizer I have works by having a channel which pulls liquid up and suspends a drop of it in the path of bursts of air, which you produce by squeezing a rubber bulb. This is an expensive item because it's in short supply, but it's a simple plastic part that would be very cheap if it were made in bulk, and it doesn't seem impossible for someone to make a replacement home with hand tools and some expertise.

(A commercial fit-test kit will typically contain two nebulizers, when you only need one. The reason for this is because if you're following the formal procedure, you first use a low-concentration version of the aerosol to make sure the person has a working sense of taste, then a high-concentration version for the real test, and if you're testing a bunch of people in a row you don't want to have to clean out the nebulizer each time. For home use, you only need one.)

(The liquid you put in the nebulizer is just saccharin in water; you might as well use a bottle that's labelled as being for fit tests, but if you can't get one you can get saccharin powder at a grocery store and mix it with water yourself. Some test kits use denatonium aka Bittrex instead, supposedly because some people can't taste saccharin. A saccharin test is much more pleasant. I noticed that saccharin solution sometimes clogged the nebulizer, by leaving a crystalline residue when it dries out, and denatonium doesn't.)

The formal fit test procedure also uses a hood, which holds aerosol in place while you hold your head at various angles and read a passage aloud, in case gaps open only while you do that.

When I've administered fit tests to people, about half fail. Cheap KN95s usually fail by having big gaps around the edges, which can be fixed by taping them down with medical tape, if you really need to. Rubber interface P100s fail by not having the straps tight enough or by having incorrectly installed filters.

I have an instant pot that can usually do a good job of turning water into mist. If an atomizer doesn't work, how about simply creating steam with my instant pot?

I don't know the physics of what happens in an instant pot very well, but probably not; I would expect any heat-based method for producing mist is probably going to be leave the solutes behind. But this is easy to test; just saturate some water with saccharin, turn it on, and see if (without a mask) you can taste saccharin in the air.
I tested it and you are right, the mist produced had no effect. 
My answer links to a paper claiming that aroma diffusers can work well but humifiers, spray bottles, and spray bottles did less well. 

Do you have tips on how to not fail without having one of these test kits? Which N95s work best? Do rubber P100s tend to fit better?

N95s with a thick edge, like the 3M 8210Plus, seem more likely to fit than the tent-style KN95s. P100s with a rubber interface have had a pretty good (but not perfect) success rate, conditional on the straps being tight enough to bend the rubber a little. A Narwall mask can be checked for fit without using a fit test kit, by covering the input vent with your hand, breathing in and felling the pressure (but that kind of mask is incompatible with glasses).
A seal check (the procedure you mentioned) can't replace a fit test. For maximum protection, full-face respirators like the Narwall seem to require a quantitative fit test to be performed (rather than a qualitative fit test which is performed using a nebulizer). Unfortunately, the Narwall isn't designed for the quantitative test, and so, it can't be fit tested.
This might be better as it's own top level question. How about creating the question yourself?
My own personal experience with (non rubber/P100) respirators is that one with headbands (rather than ear loops) and a nose clip + nose foam is more likely to seal well. To minimize the risk of getting counterfeits, it's probably better to buy from a trusted retailer and prefer respirators for which the manufacturer offers some validation procedure (e.g. this one).


Mar 23, 2021


This paper from engineers at Cambridge University claims that a standard aroma diffuser and plastic bag is close to the performance of commercial equipment. That said, I'm not sure how much the total cost and prep time would compare to the nebulizer approach that jimrandomh suggests.

Paper Link



Qualitative fit testing is a popular method of ensuring the fit of sealing face masks such as N95 and FFP3 masks. Increased demand due to the coronavirus disease 2019 (COVID-19) pandemic has led to shortages in testing equipment and has forced many institutions to abandon fit testing. Three key materials are required for qualitative fit testing: the test solution, nebulizer, and testing hood. Accessible alternatives to the testing solution have been studied. This exploratory qualitative study evaluates alternatives to the nebulizer and hoods for performing qualitative fit testing.


Four devices were trialed to replace the test kit nebulizer. Two enclosures were tested for their ability to replace the test hood. Three researchers evaluated promising replacements under multiple mask fit conditions to assess functionality and accuracy.


The aroma diffuser and smaller enclosures allowed participants to perform qualitative fit tests quickly and with high accuracy.


Aroma diffusers show significant promise in their ability to allow individuals to quickly, easily, and inexpensively perform qualitative fit testing. Our findings indicate that aroma diffusers and homemade testing hoods may allow for qualitative fit testing when conventional apparatus is unavailable. Additional research is needed to evaluate the safety and reliability of these devices.

This didn't work for me. I could taste the saccharin when wearing a surgical mask, but I couldn't taste it (but should have) when wearing two surgical masks. I suspect that the diffuser isn't creating the correct sizes of particles and/or is creating too many (the mask did become somewhat damp) and this may cause the smaller particles to combine into sizes that are too big or heavy to penetrate smaller gaps.


Mar 02, 2021


Trying to detect leaks by smelling stuff is useless, since smells aren't affected by particulate filters.

This is how to do a fit test correctly without using fancy equipment:

That was actually the procedure for doing a seal check, not a fit test.

This is how to do a fit test (assuming you have a test kit):

The "paranoia" can justified for several reasons.

  • You live with high-risk people.
  • You want to avoid long covid.
  • You want to wait for better vaccines.
  • You want to avoid vaccines altogether, since you're going to be wearing a face covering even after you get vaccinated.
  • You're embarrassed by the fact that you should have but didn't prepare for a pandemic (you're a "rationalist" after all and knew about this xrisk stuff!) and don't intend to make that mistake again. You think that much worse pandemics could happen in your lifetime, so you might as well get used to wearing the right gear today. Practice makes perfect.

Lugging around oxygen tanks is not as practical or necessary.

6Stuart Anderson3y
The science isn't in on long covid, and the stuff that's already known about it isn't reassuring. And even if some of us aren't technically in a high-risk group, we may have had very bad experiences with the flu, and we don't want to have an even worse experience with covid. That would be true only if those control protocols are good enough, but we have to assume that they're not due to the arrival of the second wave (or third wave in the US) and more contagious variants. It's too early to say whether this virus will stick around or not. Even if that'll be the case, we'd want to wait for the weak, cold-equivalent variants to appear before ditching our respirators. Even if they're not high-risk, why would they want to endure something that could make them suffer more than even a bad flu and possibly put them at risk of long covid? No, we can be preppers without being survivalists. If we need to become survivalists, we've failed and survival becomes much harder even if we're also survivalists. If we could have easily prepped for an obvious threat (a pandemic) but didn't, that means we're just not very good preppers. The only reason that someone didn't get a respirator is that they didn't prep, not that their prep wouldn't have worked. At the start of this pandemic, only a fool would have worn a mask if they had access to a respirator, because no one knew what the real risk was. Sure, it's better to become a survivalist than a statistic, but it's even better to prep in order avoid becoming a survivalist. Even if wearing a respirator wasn't strictly necessary for this pandemic, "cosplay" is important in case a worse pandemic arrives. This is less effective than just acquiring and (correctly) using a reusable respirator.
6Stuart Anderson3y
That seems to be the black-or-white fallacy. Given a certain level of risk there are some interventions that are cost effective and others that aren't.
1Stuart Anderson3y
I'm still not exactly sure why you seem to be against wearing reusable respirators when we can't run into our bunkers. It's almost-no-risk, high-reward, especially for a many of us that think there's a decent chance that we could live forever and that much worse pandemics could arrive in the not-too-distant future.
1Stuart Anderson3y
"Are you being gassed to death? Well, don't use a gas mask because it might provide a false sense of security." "Reusable respirators won't work because I said so." In another comment: "Seat beats? Forget it. Avoid driving and stay home forever. I just wear seat belts because it's the law." I wanted to know if you had any reasonable objection to wearing reusable respirators during pandemics (i.e., a strong rationale for why they're inadequate) but comments like these indicate that you don't.
1Stuart Anderson3y
Yes, and the reasoning you used is clearly the black and white fallacy. You are free to make reasoning errors and I'm free to point them out.  Reducing choices to two is again a textbook example of the black and white fallacy. Whether or not you are afraid of dying, living with CFS or something similar because you got long covid isn't fun.
1Stuart Anderson3y
Between running and staying there's walking a bit. In the pandemic context that means reducing your exposure but not moving it to zero. There's no reason to treat either of the extreme of no contact at all or normal contact as the only two choices.
1Stuart Anderson3y
Car accidents are a serious problem. It's possible to reduce that risk to zero by not driving any cars but that's very costly. Wearing a seatbelt on the otherhand isn't costly and therefore worth the effort to reduce risk. There's no reason to see either extreme of driving cars with maximum risk and not driving at all as the only possible choices.
1Stuart Anderson3y
It turns out that you can't live with non-zero risk. Both completely isolating yourself (the sepsis death of a rationalist but also other risks) and leaving your flat comes with risks.  Without engaging in serious consideration you don't know the true risk anyway. I don't think anyone here advocates taking actions regarding isolating or not without serious consideration and potential payoffs.
1Stuart Anderson3y