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How do you run a fit-test for a mask at home when you don't have fancy equipment?

by ChristianKl1 min read28th Feb 202130 comments


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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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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?

2jimrandomh2moI 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.
4ChristianKl1moI tested it and you are right, the mist produced had no effect.
1Owain_Evans25dMy 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?

7jimrandomh1moN95s 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 [https://narwallmask.com/] 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).
3ChristianKl1moThis might be better as it's own top level question. How about creating the question yourself?
1ofer1moMy 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 [https://safeguard.3m.com/Guest#/Validation] ).

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.

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:


Don't bother. If you're going to half-ass this then the better strategy is to make sure you leave.

Take your full face mask and pull the straps so tight it is uncomfortable. It should leave marks. Take your hand and clamp it over the inlet/s, then inhale. If air comes in, your straps aren't tight enough. If it fits properly and you're blocking where the air is supposed to come in then it should feel like you're about to suffocate. If it's easier for air to enter and exit via the inlets/outlets, then that's mostly what it is going to do. Don't have a beard, and if you're especially serious, don't have any hair on your head either. 

Assuming this is just about covid paranoia, the rule is the same as with chemical weapons, smoke inhalation, etc. - the mask is to stop you dying on the spot, it's not there to let you set up camp. Leave the area immediately and seek safety. The priority isn't to have a space suit level of environmental protection, it's about having something good enough that you can successful run away from the problem in. 

Besides, with the kind of paranoia on this forum I'd think it would be easier to simply side step the entire fitment problem by getting yourself a regulator/hood/oxygen mask/etc. and an oxygen tank. You don't have to give a damn about leaks in a positive pressure system, you just need to keep an eye on your tank gauge. If people with crap lungs can wheel oxygen tanks around and get on with their day then it's a mostly solved problem.

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 Anderson1moParanoia stops being paranoia if it is true. We have the mortality statistics for covid, so you tell me why the science is flawed and I'll listen. If you live with high risk people then covid would be a non issue for you because you'd already have been dealing with infection control protocols. You won't be avoiding covid. It will (arguably has) become endemic, exactly as all its predecessors have. You will get a variant at some point. Vaccination for coronaviruses is largely irrelevant for those who are not high risk. I've had to have a pointless bureaucratic vaccine before, and I fully expect this one to be more of the same. Prepping comes in two flavours: real and cosplay. The main component of actual prepping is attitude. For example, we had the situation where you couldn't get a mask, so what did you do? You cannot prep for everything, and even if you do prep things go wrong. Prepping isn't having the latest shiny mask, prepping is being able to figure out how to deal with having no mask. I gave people a ghetto fitment method on a real mask, I advised to get the fuck out of the danger zone, and I gave another solution involving positive pressure that removes the issue of fitment entirely. What I personally believe to be of risk is irrelevant to the efficacy of the advice.
1Florin1moThe 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 Anderson1moWe know the mortality rate of covid. We have provisional statistics on longer term issues. Either those numbers are too high for you or they aren't. I'm in the latter camp. Many people are treating covid as the worst thing that can happen here, whilst happily ignoring all other costs. Infection control procedures for high risk individuals are about protecting them from the world. The world stays every bit as deadly as it always was. Nerfing the world is impossible. Covid is a permanent feature in exactly the same way SARS, MERS, swineflu and the Spanish Flu are. They never go away for two good reasons: 1. Coronaviruses mutate like crazy, and thus low mortality or low infection variants occur as a result of selective pressures. Evolution is real. 2. Covid infects multiple animal species, plenty of which are part of our food supply. You don't ever ditch your hospital treatments, but it may be prudent to accept your own mortality. You will get old and that 'harmless' cold will be utterly deadly to you, simply because you don't have the immunity to deal with it, with or without medical interventions. Death is part of life. A lot of people seem to have a problem with that fact. Your vaccination question is answered by the statement it responds to: if people aren't at high risk then they aren't at high risk. Vaccination tilts your chances, nothing more, nothing less. Risk is a part of life. A lot of people seem to have a problem with that fact too. Prepping is not about equipment. Not having a mask didn't matter to me, because I wasn't setting foot outside my front door without a very good reason whilst the risk remained unknown. Once it was known the only reason I've worn masks is for legal mandate, manners, and to hide my face. If I seriously believed I was at risk I wouldn't be wearing a mask, I'd be at home behind a locked door. A mask is not an excuse to behave recklessly. That attitude is something that pisses me off no end. If there is risk in a si
2ChristianKl1moThat 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 Anderson1moThere are two levels of decision making here: the state, and the individual. In the case of the state we are in agreement that the decisions should primarily be governed by actuarial and logistical principles. We may not agree about where the statistical splits occur but we agree on process. In the case of me as an individual I am willing to accept levels of death and lasting disability orders of magnitude greater than the worst covid outcome statistics we've been presented with. I would have preferred to simply continue with life and take it on the chin based on what I've seen. If this is a problem warranting the sort of impositions we've seen then we should have pulled the pin on that grenade up front and without hesitation. Martial law. Pest houses. Lethal enforcement of borders. Collection of corpses in garbage trucks to be taken to pits for incineration. And the like. Either this thing is so deadly that it will kill and maim enough of us to make a difference or it isn't. And that's something you can figure out with a spreadsheet. In black and white.
1Florin1moI'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 Anderson1moA false sense of security can kill you. If a half face respirator is good enough for you then I have no objection to that - it's your life. You don't need my consent or approval. I gave my opinion. That's all it is, an opinion. Don't you believe that people can think different things? Things that might be appropriate to their own situation rather than yours? Do you not believe that error exists? Are you against people being able to speak freely? You and I don't agree, so what's the problem with that? You have two choices in a crisis (assuming that choice isn't made on your behalf by act of God) run or stay. Otherwise known as bugging out or bugging in. That's what a crisis is: something so exceptional and dangerous that you have to act for your safety. If you can wander the streets freely in some token PPE you bought from a hardware store then it is not a crisis. If the crisis lasts forever then it isn't a crisis either, it's just the nature of what the world has become. If you feel you need a mask, it's your choice, but if I really needed a mask to protect me from a deadly disease then I'm not setting foot outside my front door. I'd probably tripwire my property so that I got the jump on any potentially infected. Windows would be boarded up, and doors would be wedged. If someone touching you or breathing on you can kill you then you actually act like it. If people touching you can kill you then you're either not going to put yourself in that position, or you are going to take every precaution possible. I think that starts with an understanding that if this were a real crisis you do not want to be interacting with anyone you'd meet on the street under those circumstances. Predators, idiots, and the desperate. You are welcome to your religious beliefs regarding an earthly immortal existence, but it's fairly obvious we have ideological differences in this domain. I'm not afraid of dying and I consider it inevitable (and desirable. Ideologically, because I beli
3Florin1mo"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 Anderson1moYou've made choices about what you believe is warranted here, so have I. What's the problem? I'm not stopping you from doing anything in your own life. You can't stop me doing what I want in mine. I don't see a problem here either. Whatever you want here, you're clearly not getting it. I don't see how I can help you.
2ChristianKl1moYes, 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 Anderson1moIn this regard, we do not agree. You are welcome to labour the point. It's your time. What are the options that are missing in this circumstance? Believe me, if there's suddenly something to add to bug in and bug out I'm all ears. If it is about you not getting covid then that's in your hands and that of the capricious universe. If you are already taking all precautions you consider reasonable then what's left for you to do but hope fate doesn't decide to fuck you here? I have a chronic illness. You can get by just fine with what fate hands you (at least until it kills you, or you kill yourself). People just need to toughen up. Everyone that came before had their shot in way worse circumstances and we managed to get by then. Where's the grit now, when we need it?
2ChristianKl1moBetween 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 Anderson1moYou have a vector and a velocity. If your vector is out of town you are leaving regardless of your velocity. If your vector isn't out of town then you're staying regardless of your velocity. As for there being no reason, what happened to the deadly disease with horrible long term side effects? If I thought I could end up with lungs that were fucked for the rest of my life just by touching the wrong thing or breathing the wrong air then I wouldn't leave the house by choice. I would consider that a reasonable precaution. If you can reduce your exposure to zero then why wouldn't you if you believe exposure is a serious problem? Wouldn't it be reckless to take unnecessary risks? Isn't that the entire point here?
7ChristianKl1moCar 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 Anderson1moIf the act and the risk are not divisible then if the act is taken risk will always be non-zero. If the true risk of disability or death were unacceptably high then I am of the opinion that one shouldn't take that risk without seriously consideration and potential payoffs. I don't think the above stance is in any way controversial or unreasonable.
2ChristianKl1moIt 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 Anderson1moAs long as choice and consequence are married that's good enough for me. You don't need to know the true risk of an act to know if you'd be okay with being maimed or dying from it. More accurate knowledge of risks increases the granularity of your decision making, but coarse decision making is often acceptable (especially if you err towards caution).