Cheap and simple mask fit testing

2022 February 01

  1. Introduction
  2. Disclaimer
  3. Masks
    1. What are fit testing and seal checking?
    2. How to perform a seal check
    3. Cloth and surgical masks
    4. Acquiring genuine N95s
    5. Donning and doffing
    6. Mask re-usability
    7. Do glasses provide protection?
  4. OSHA fit testing
  5. Quick-and-easy fit testing
    1. Preparing the testing solution
    2. Performing a fit test
    3. Results
  6. Other resources
  7. Thoughts on protecting against covid


Short version: Read the disclaimer, how to make the testing solution, how to perform the test, and my results.

US healthcare workers who rely on respirators, such as N95s, for their safety undergo annual fit testing to make sure that they are protected. Of course professional fit testing equipment is priced like all aspects of American healthcare… beyond the reach of the typical American. Here I detail the steps I took to imitate this procedure and how you can perform them as well.

Professional PPE usage is intended to provide a level of protection suitable for someone in sustained direct contact with highly-infectious covid patients. If you were to have a passing encounter with a contagious person in public, your exposure risk would be much lower than in a hospital setting. My goal is to gain most of the protection of professional PPE with a minimum fraction of the effort, in the hopes that this compromised level of protection is more than sufficient for my likely exposure level.

If you live in my area and would like masks or to borrow my testing solution, let me know!


If you need professional-level respiratory protection, disregard this article and follow the relevant regulations. When I write things like (e.g.) disposing of a mask after a single usage is wasteful, I am writing for those, like me, who are looking for modest protection from incidental covid exposure in a public setting. I am not an expert.


Masks are not perfect at blocking small particles. There are two ways they can fail: either through leakage around the sides, or through inadequate filtration of air that passes through the filter. Masks must compromise between these two failure modes, as additional layers of filtration material improve filtration but increase breathing resistance, and therefore increase the tendency for air to pass through any small gaps around the sides.

What are fit testing and seal checking?

The goal of fit testing is to identify a model of mask that gives a high quality fit for a specific person. The person is exposed to a strong-smelling/tasting chemical while wearing a mask; the odiferous chemical is aerosolized into tiny droplets for which the mask’s material has a high filtration efficiency, so detection of the chemical indicates that air is leaking around the mask. Note that a fit test does not test whether the filtration material is adequate: it only checks for leaks. A wide variety of particulate sizes and types would need to be used to validate the quality of the filtration media, but are irrelevant for detecting leakage.

Those in a professional setting will typically undergo fit testing annually through a procedure regulated by OSHA; see below. A variety of masks are tested until one that passes the test is found. Like other garb, a proper fit depends on the shape of the mask matching the person wearing it, so there is no one “best” mask for everyone.

Once an appropriate model of mask has been found, seal checking (or a “fit check”) is performed everytime that mask is donned or adjusted before entering the hazardous area. It only takes a few seconds and I do it everytime I go out: I find leaks the majority of the time.

How to perform a seal check

To perform a seal check, cover the filtration surface of the respirator with your hands (while wearing it), and either breathe in (negative pressure) or breathe out (positive pressure). You should feel increased breathing resistance, and you should not feel any air passing the side of the mask. The mask should also visibly deflate / inflate slightly. Adjust the mask and repeat if leaks are found.

See a video of a seal check.

The increased breathing resistance is more apparent in the negative pressure test, as breathing in is harder than breathing out. However breathing in tends to tighten the seal, concealing leaks, so leakage around the sides is easier to detect when breathing out. If you wear glasses, fogging of the glasses can be the most obvious sign of leakage around the nose on breathing out. I suggest using a mask with a foam nose insert and wearing it high up on the nose to reduce leakage there.

Don’t forget to shave: studies find that facial hair located at the mask’s seal increases the leakage by a factor of 20 to 1000 times. Even a small stubble compromises the seal.

Cloth and surgical masks

Neither fit testing nor seal checking serves much function for cloth or surgical masks, or KN95s. (Though if you try, let me know!)

Double-masking to improve filtration is generally unhelpful as it can increase leakage instead. However, a close-fitting cloth mask worn over a surgical mask can be beneficial if it improves the fit of the latter. Never wear an N95 over another mask: this combines the fit of a surgical mask with the breathing resistance of a respirator.

A better alternative to double-masking is a mask fitter / mask brace. A medical face mask rated at ASTM-2 or ASTM-3 with a correctly sized mask fitter might be compared to the protection of an N95.

Cloth masks are often found to be substantially inferior to surgical masks.

Acquiring genuine N95s

I have purchased 3M Aura N95s online from Northern Safety Industrial and Home Depot. In both cases they cost on the order of $2 per mask. Another source is Industrial Safety Products. I believe that mask manufacturers, particularly 3M whose masks are very popular in healthcare, prioritize selling to medical distributors, so any masks sold to the general public are after medical demands have been satisfied. (Note that 3M masks have become scarce again since covid omicron hit the news.)

Only purchase N95s and KN95s from an established seller… and certainly not off of Amazon.

The majority of KN95s in the US are counterfeit!

For a mask to qualify as an N95 it must be verified by NIOSH, a part of the CDC, to meet specific regulations of performance. (“KN95” is a standard regulated by China, and inferior to N95 due to the use of ear loops; “FFP2” is the European equivalent, with “FFP1” inferior and “FFP3” superior.) Refer to this CDC page for guidance on recognizing counterfeit masks and how to look up your mask’s NIOSH approval.

I believe the majority of “counterfeit” N95 masks in the US simply lie about being NIOSH approved; this can easily be detected by the above. Detecting forgeries, which imitate genuinely approved N95 masks, is harder.

All images are click to zoom.

Donning and doffing

Above I linked a video of a seal check, which included demonstration of donning and doffing. CDC procedures are to only handle masks by the straps when donning and doffing. I doubt this matters much; of course you should wash your hands after handling the mask regardless.

A little etymological diversion… “don” and “doff” are contractions of “do on” and “do off”. They had become obsolete by the 17th century when Sir Walter Scott repopularized them, though the words “dup/dub” (open) and “dout” (put out) went extinct.

Mask re-usability

Disposable masks are meant to be used once. However this is excessive and wasteful of limited resources.

Unless you are visibly soiling the respirator surface, the limiting factor on the re-use of your mask is the strength of the elastic band. As it weakens with use, particularly during donning and doffing, the mask is held less tightly to the face and leaks develop. This is also why masks with ear loops can’t seal as effectively as those whose straps go behind the head.

Certain masks allow for the tightness of the elastic head bands to be adjusted by the wearer: this greatly extends the life and performance of the mask by allowing the fit to be tightened as the elasticity weakens with re-use.

Quality of N95 fit declines measurably by around 5 to 30 uses, though still far superior to cloth or surgical masks.

Masks can be sterilized between uses with UV light; however leaving them in a paper bag at room temperature for several days is a simpler and more reliable technique. I rotate through several masks and don’t reuse them more than once every 3 days. I write the date of its first use on each mask’s bag and then infrequently dispose of the oldest mask.

Water and other cleaning fluids should only be applied to cloth masks. Brief exposure to water (e.g. in light rain) is not a concern. Alcohol-based cleaners will destroy the filtration material!

Source (pdf). Mask failures were found to significantly increase after the second day of mask re-use. The subjects were hospital workers who had performed OSHA-regulated fit testing and were trained to do a user seal check with every mask donning. The length of each shift or the number of donnings / doffings performed was not recorded. Workers consistently overestimated whether their mask would pass the fit test (even though seal checks would have revealed any obvious problems). I would not read much into the exact numbers found by the study, but the rise in mask failures with reuse is significant.

Do glasses provide protection?

A study based on data from a Chinese hospital from January to March 2020 found that wearing glasses reduced the risk of being hospitalized with covid by a factor of about 10. This study has been sometimes cited to suggest that glasses provide some protection against infection. However, as one might guess from the ridiculous conclusion they reached, the study is riddled with fundamental methodological flaws. Besides, anyone who has worn glasses in rain or wind is well aware they do not provide any protection against air-borne droplets reaching the eye.

I am not aware of any firm evidence that covid can, or cannot, be transmitted via the eyes. If you wish to protect your eyes you should wear goggles or a full-face respirator. I don’t think this is necessary but I did drop $5 on getting cheap goggles in case they might be useful in the future.

OSHA fit testing

OSHA publishes official regulations on how fit testing should be performed which employers are required to follow for employees who will be exposed to respiratory hazards. All fit testing procedures that I have reviewed are derived from these procedures. If reading “regulationese” is not your desired pastime, 3M has a one page quick reference to the qualitative test.

Quantitative fit testing involves puncturing the mask being tested with a device for measuring particles while the user is wearing it; it does not rely on the wearer’s senses to test the fit of the mask. This test yields a filtration efficiency, allowing to quantify how much better one mask is than another. We will not be considering quantitative fit tests further.

Qualitative fit testing yields only a “pass” or “fail” depending on whether the wearer detected any of the test substance while wearing the mask.

I summarize the official OSHA procedure as follows:

  1. A dilute solution of 0.83 grams sodium saccharine in 100 mL distilled water is prepared.

  2. A concentrated solution of 83 grams sodium saccharine in 100 mL distilled water is prepared.

  3. During the whole test the subject wears an enclosed hood into which the test solutions will be released.

  4. With no mask, the dilute solution is introduced into the hood to verify the subject can taste it, and at what concentration.

  5. With the mask being tested, the concentrated solution is introduced into the hood. The wearer performs several acts, including moving the head, grimacing, exercising, and talking, and should include the sort of motions that will be done in the hazardous environment.

  6. The mask’s fit passes if the concentrated solution is not detected at all.

Bitrex, a nasty bitter substance which OSHA describes as a “taste aversion agent”, can be substituted for the sodium saccharine solution. This increases the sensitivity of the test with the downside that you have to taste Bitrex.

The OSHA procedure recommends sticking the tip of your tongue out which I found to be unhelpful.

Why use the oddly specific concentration of 83 g / 100 mL? When I attempted to make that concentration, I found that it was just a little bit higher than the saturation concentration of sodium saccharine. Note that saturation is strongly dependent on temperature, and OSHA specifies warm water. Therefore my guess is that they simply chose the saturation point, to make the strongest-tasting solution possible. However, when I tried to look up the saturation point of sodium saccharine, I got wildly conflicting information: most sources simply said “greater than 10 g / 100 mL”, and others gave values which were well below what I observed.

The instructions I give in the next section use half this concentration to reduce annoyances like precipitation on temperature change.

Quick-and-easy fit testing

The process I describe is based on the official OSHA procedure described above; I removed the portions that are annoying (e.g., using distilled water), difficult (making a hood), or expensive (buying medical-grade equipment). This simplified procedure retains only the core concept of applying strong-tasting particulates to the outside of a mask and testing if they can be detected.

Preparing the testing solution

You require:

To prepare the testing solution:

  1. Weigh out about 8 grams of sodium saccharin.

  2. Mix into 20 mL of room-temperature tap water; it will take some effort to dissolve fully.

Alternatively, if you do not have any measuring equipment:

  1. Collect enough water to fill about one third of the aerosolizer’s reservoir (10 mL out of 30 mL).

  2. Slowly add in sodium saccharin, mixing it as you go, until there are solid particles that do not dissolve even with continuous mixing for more than a minute. This will be about 8 grams – nearly as much sodium saccharin as water.

  3. Dilute by mixing 1 to 1 with tap water. Mix thoroughly and pour liquid into the reservoir, disposing of any excess or remaining precipitate.

The purpose of diluting sodium saccharin is to discourage precipitation out of solution. The exact concentration is not important.

Once you have filled and re-attached the reservoir to the aerosolizer, and charged it, you are ready to perform a test. The button on the aerosolizer toggles whether it is on or off; check that it works. Some aerosolizers operate by dispensing once each time it is triggered, which may allow for greater control over the amount dispensed.

If its performance declines, check it is charged and check for precipitate accumulating on the outlet. When left unattended for a long time I find a significant build up of sodium saccharin clogging the device. It is easy to clean with a damp cloth or by flushing with tap water.

Performing a fit test

Test indoors in a location you can ventilate. See a video demonstration, though I recommend breathing naturally, unlike the person in the video.

Wearing your mask, point the aerosolizer at your face from a few inches away and turn it on for a few seconds. I suggest starting a little farther and then moving close and going around the whole perimeter of the mask, especially around the nose and under the chin. You don’t have to worry about getting the mist in your eyes.

While spraying the aerosol, breathe shallowly in and out through your mouth. I and others have found the solution produces no odor, so do not bother trying to smell it. Rather, it yields a very mild sweet flavor in the back of the tongue or throat, which can take a few seconds to be detectable. The solution is very strong and you will detect even small amounts getting past the mask. Take note of whether you can detect it, and under what circumstances.

If 10 or so seconds of spraying the solution directly at the seals of the masks produces no sensation, then the result is a complete success.

In any case, after removing the mask there should be a very stark difference as even lingering aerosol in the air will be easily detectable.

While the official OSHA procedure is a binary success or failure, it is easy enough to find gradations in how strong the taste is or how much spray is needed to be detectable. Assessing the quality of the mask fit based on what you detect is left to your judgement; I describe my own experiences below to provide context.

Do not breathe in the aerosol directly without a close-fitting respirator! In initial testing I tried to get a tiny whiff from a foot away without my mask but inhaled too strongly, and ended up coughing vigorously for the next two hours, with a sickly sweet flavor to every cough. If you avoid direct exposure it’s not too bad: others found the lingering aerosol to be unpleasant but tolerable without a mask.

If you are paranoid you can use a more diluted solution but just don’t spray it in your face and you’ll be fine.


I have used this process with about 15 masks across 10 people; the masks included a variety of N95 models, and one P100 mask.

Note that the concentration of solution described above is very strong. This is valuable as it increases the discrimination of the test – but do not be disappointed if your mask is not perfect. The results of this procedure cannot be directly compared to that of the official OSHA procedure, as the method of application of the aerosol is different, and subject to how the user operates the aerosolizer.

Some of the masks I helped fit test. At right is the popular 3M Aura 9210+ that fits me very well. Below is a 3M 9502+ KN95 mask with ear loops, and left is the 3M 8210 N95 mask, which is stiffer than the Aura series. Which of these masks fit best varied from person to person. The aerosolizer we used (“Nano Mini Disinfecting Sprayer” sold by Emerald Prairie Health) is at top.

Every N95 I helped test experienced some degree of leakage. Wearing a carefully adjusted 3M Aura mask, I could only detect a faint hint of sweetness in my throat after several seconds of spraying the aerosol directly at the mask’s seals; compare this to my experience above of exposure without a mask. On this basis I feel very confident in the protection that this mask provides me.

Some of the masks tested had different outcomes on different people, and vice versa: fortunately everyone who tried this procedure with me appeared to be satisfied with at least one mask they tried, up to their standard of precaution.

Some people, like me, felt re-assured in the benefit of their mask after experiencing the sharp contrast between comfortably breathing with aerosol spraying in one’s face and the unpleasantness of passing through that room without a mask well after testing had stopped.

The GVS SPR457 mask, rated at P100, which passed the fit test convincingly by its owner. They also tested a Moldex 4200 airwave. The latter is very comfortable to breathe through on account of its low breathing resistance, but it does not have an adjustable nose piece, and was a poor fit for the people who tested it.

The P100-rated mask proved to be the only mask totally impervious to the testing solution – the person wearing it could not detect the solution at all.

If you decide you are only safe with a respirator rated above an N95, then you should absolutely perform a fit testing: a P100 that does not fit you is worse than an N95 that does. There is no reason to invest in an expensive, uncomfortable mask but not invest in checking if it fits you.

Other resources

Source. Several masks were tested with various degrees of fit: they found that ill-fitting N95s exceeded the performance of surgical / cloth masks. Most of the failure of cloth masks was due to leakage from poor fit, not due to the filtration being inadequate. Note the very small sample size.

Thoughts on protecting against covid

Wearing a mask or getting vaccinated has often been compared to wearing a seatbelt. Let us extend this analogy a little further, and talk about the multiple layers of protection in the Swiss cheese model of hazards.

I found this nice graphic of the swiss cheese model on wikipedia… alongside not one but two such graphics specialized to covid-19 hazards. Nothing like seeing your exact idea in graphical form to make you feel unoriginal. Though both of the artists seem very confused about the concept of “layers” so I stuck with the generic graphic.

I will somewhat arbitrarily group protection measures in three layers:

  1. Avoiding hazardous conditions.

  2. Avoiding hazard exposure risk when in hazardous conditions.

  3. Mitigating against severity of hazard when it happens.

For avoiding car crashes, this means:

  1. Not driving on a road with unsafe drivers.

  2. Driving safely and “defensive driving”.

  3. Wearing a seatbelt, as well as airbags and other safety equipment.

For mitigating covid, this means:

  1. Physical distancing from people with covid.

  2. Wearing a mask and having substantial ventilation or air filtration when around people with covid.

  3. Getting vaccinated, bolstering your immune system with sufficient sleep, and other health measures like losing weight.

You only need to actually take precautions from contagious people, just as you only need to avoid drivers who get into car accidents… but in practice that means taking precautions from everyone. I would like to emphasize that getting covid or spreading covid is not something that only happens to “other” people or to “bad” people. Wearing a mask around someone is not a moral condemnation of them any more than wearing a seatbelt when they drive. “We’ve been friends for 10 years, you don’t need to wear a mask around me” is asking for you to “trust” that they can’t catch covid… might as well “trust” that they won’t develop diabetes or cancer. Being a loyal friend does not make you immune to medical ailments!

So let us return to risk assessment, free of any distractions about morals. To catch covid, all three layers of protection must fail you. (Similarly for other endpoints of interest, such as being hospitalized due to or dying of covid.)

Early in the pandemic, without vaccinations or general access to quality masks, the only effective layer of protection was physical distancing (confusingly called “social distancing” in the media). This would be akin to having a 1950s car with no seatbelt or airbag in a snow storm: best just stay home.

Now this has changed. Vaccines, of course, are amazing, with perhaps a 10x or 20x reduction in death risk, and lesser protection against hospitalization and symptomatic illness. But vaccinations’ real value is in how easy and consistent they are: you can never forget to “put on” your vaccination, or worry about around whom you will choose to go unvaccinated. If you are willing to put in just a little more effort to get a higher level of protection, then you should invest in the other layers of protection.

My approach is to treat the second layer, masking, as my main line of defense against covid. It is not an impervious layer: vaccination is so easy that it’d be foolish to go without it as a backup, especially for situations where wearing a mask is infeasible. But I find investing in masks much preferable to investing in the first layer, which would entail avoiding people on the presumption that they may be contagious. Wearing a high quality mask whose fit I have verified gives me the confidence to not worry about who I interact with. (Don’t forget seal checks each time you wear your mask! I wouldn’t be surprised if doing seal checks reduces the leakage of my mask by a factor of 10.)

Of course, you can never be too careful about what might be in your mask.

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