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How do you decide when to change N95/FFP-2 masks?

by ChristianKl1 min read10th Sep 202114 comments

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Covid-19
Personal Blog

I lack a gears model of how N95/FFP-2 masks works that tells me when it makes sense to change to a new mask. The same goes into how to interact with the mask to maximize it's Does someone have a gears model and a got idea when it makes sense to change masks to a new one?

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1 Answers

General Discussion:

Broadly speaking, there are five issues to worry about when reusing masks:

  1. Virus particles contaminate the mask's surface, and may spread to you while handling the mask.
    • Mask filter surfaces (where the air flows through) may have spent hours collecting COVID particles from the air, since you've been continuously pulling contaminated air through the mask. The filter surfaces may have hundreds or thousands of times the contamination seen in solid surfaces.
    • Reaerosolization of filtered particles (where particles trapped by the mask re-enter the air) is possible, but likely releases negligible amounts of virus (source, source) compared to the ~5% an N95 mask fails to stop.
    • There are a number of approaches for decontaminating masks. For coronavirus specifically, the simplest approach is to just let the masks sit. The time necessary to inactivate COVID virons depends on the temperature and humidity. Options include: (source)
      • 4 days at 21-23 °C, 40% humidity (However, this source indicates virons may be present after 6 days)
      • 1 hour at 70 °C, any% humidity (using, e.g., an oven)
      • Boiling water for 5 min (may lose ~8% filtration efficacy, but also cleans mask of dirt)
    • UV-C radiation can also decontaminate masks. However, this process is potentially unreliable because the UV intensity needed varies with mask material, masks with unusual geometry may shadow portions of the mask from treatment, and dirt or other soilage may block radiation (source). Make sure to use >= 1 J/cm^2 for >=1 minute (source). Don't use > 10 J/cm^2 to avoid damaging mask structure.
    • Chemical agents such as ethanol and bleach may reduce mask filtration (source).
  2. Loss of mask structure prevents a good fit to your face.
    • Generally, it's hard to properly fit an N95. Among 74 anesthesiologists, 63% of women and 29% of men failed fit testing, even with a fresh respirator (source). Overall failure rates were 43% after 4 days, 50% after 10 days and 55% after 15 days. Additionally, people were very bad at estimating the quality of their fits, with 73% of those who failed the test thinking they had a good fit.
  3. Loss of electrostatic charge worsens filtration efficacy.
    • N95 masks don't lose much efficacy if they're just stored, even for years at a time (source). However, they do eventually lose efficacy if they're actually used. With 8 hours per day of use, N95 masks retain ~95% efficacy after 3 days, ~92% efficacy after 5 days, and drop to ~80% efficacy after 14 days (source). Note: this refers to just the material's filtration efficacy, and does not take into account any further reduction due to worsened fit quality.
    • Mask electrostatic charge degrades more quickly in humid environments (source). Thus, a mask with an exhalation valve will likely last longer. An N95 respirator with exhalation valve is likely as effective at source control (preventing spread from you to others) as a cloth or surgical mask (source), but many establishments (such as airlines) do not allow masks with exhalation valves.
    • If you want to get fancy, this paper describes a procedure for recharging a mask's electrostatic potential. However, that won't help with the loss of structure issue.
  4. Accumulation of filtered particulate makes the mask harder to breathe through and makes inhaled air more likely to pass around the mask rather than through it.
    • I don't think this is usually an issue because loss of structure/efficacy will force you to change masks more quickly than the masks get clogged. However, if you're in a dusty/smokey location, it could be a problem. I suggest changing out a mask as soon as you notice it getting more difficult to breathe. You can also wear a surgical mask over the N95 to protect it from larger contaminants.
  5. Accumulation of sweat/dirt/etc makes the mask disgusting to wear.
    • I suppose this is up to personal preference.

Final Recommendation:

I'd suggest replacing an N95 mask at least once every 5 days, and preferably once every 3 days. I'd suggest 1 hour at 70 °C for decontamination. Additionally:

  • I'd recommend using a mask with an exhalation valve, if you can.
  • I'd recommend storing masks in a low-humidity environment while not using them.
  • It's best to handle potentially contaminated masks by the straps, especially during removal. If you absolutely must touch the mask itself, avoid touching filter surfaces or interior, and instead touch the edge of the mask somewhere that's away from your mouth and eyes.
  • Virus can still transfer to your hands, even with proper handling (source). You should wash your hands after handling a potentially contaminated mask.
  • You should never stack potentially contaminated masks. I.e., don't allow the filter surface of one mask to be in contact with the interior of another.
  • Wearing a cloth/surgical mask over the N95 will help protect it from splashes and large contaminants. However, this may accelerate the loss of electrostatic charge by increasing the humidity within the mask. Do this if you think your N95 may get spoiled otherwise.
  • If decontamination is a chore, one option would be to use a rotating set of masks, wearing one a day sequentially until you're worn them all once, then decontaminate the entire set using an oven.

Additionally, you may want to consider alternatives to N95s. Half-face elastomeric respirators are designed to be reusable, are far more protective than even a properly fitted N95, are much easier to fit properly, and I personally found them much more comfortable than expected. Additionally, they only require replacement filters when breathing becomes difficult, so they cost less in the long run.

Finally, at the highest tier of protection, you can buy powered air purifying respirators for $300 or make your own for $15-30. I don't have any experience with either option, so I can't comment much.

Finally, at the highest tier of protection, you can buy powered air purifying respirators for $300 or make your own for $15-30. I don't have any experience with either option, so I can't comment much.


The usual problem with (all?, most? of) the personal-use, non-professional PAPRs, including the $300 example you mentioned, is that they haven't been independently tested by anyone.

With the DIY PAPRs you mentioned, you at least know that the filters should work (since they're certified for hospital use) and when you build it, it's easier to ensure that the seals also work.

Virus particles contaminate the mask's surface, and may spread to you while handling the mask.

Is this a big deal, given that COVID doesn't really spread via fomites? Or is the concern: The virus particles will be so close to your mouth that you'll just inhale them off of the mask?

2ChristianKl11dAs far as I understand there's also something like a minimum effective dose for COVID-19, which I wouldn't expect to be reached that way.
1Quintin Pope11dThank you for the question. I'll add my response to my answer. Keep in mind that the filter surfaces (where the air flows through) of the mask may have spent hours collecting COVID particles from the atmosphere, since you've been continuously pulling contaminated air through the mask. The filter surfaces may have thousands of times the level of contamination typically seen in solid surfaces. It's best to handle potentially contaminated masks by the straps, especially when removing the mask. If you absolutely must touch the mask itself, avoid touching filter surfaces, and instead touch a portion of the mask's edge that's away from your mouth and eyes. However, virus can still potentially transfer to your hands, even with proper handling (source [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5705010/pdf/nihms917525.pdf]). Thus, you should wash your hands after handling a potentially contaminated mask. Reaerosolization of filtered particles is possible, but seems to only occur to a significant degree when the humidity is low and the particles in question are large and dry (source [https://multimedia.3m.com/mws/media/409903O/respiratory-protection-against-biohazards.pdf?mod=article_inline] , source [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5705010/pdf/nihms917525.pdf]). Virus particles are typically either small and dry or large and wet (when suspended in water droplets), so I don't think this is the primary concern. I guess avoid inhaling too close to the mask's outer surface if you're worried about this.
4ChristianKl11dHow do you get into a threat scenario where you would assume constant exposure to COVID particles in the atmosphere? In Germany, we have an app that measures whether or not I'm near people who then get tested positively with COVID-19. While only ~10% of infected people actually use it, that gives an idea of how much exposure there is. In all that time I traveled in public transportation I was never exposed to a person who then tested positively. This suggest to me that exposing myself to multiple people with COVID-19 in a single day is very unlikely and I don't spend more then 20 minutes exposed to the same person.
1Quintin Pope11dPeople rarely talk, laugh or scream on public transport, so the risk is much lower compared to somewhere like a bar or hospital. Also, I’m talking about relative contamination levels. Even if you’re only lightly exposed for 20 minutes, the concentration of virus on your mask is probably ~hundreds of times higher than the concentration on your clothes. Consider the volume of air you breathe in 20 min. 95% of the virus in that air is now on your mask. Compare that to the volume of virus that settles out of the air onto your clothes. Considering COVID can remain in air for hours, that amount is likely much smaller.

Re "Loss of electrostatic charge worsens filtration efficacy", this paper might also be relevant (e.g. figures 1 and 2; though I don't know how to interpret them).

With 4 hours per day of use, N95 masks retain ~95% efficacy after 3 days, ~92% efficacy after 5 days, and drop to ~80% efficacy after 14 days (source).

I think the paper you linked to reports on an experiments in which respirators were worn for a total of 8 hours per day, not 4.

1Quintin Pope10dYou're right. I'll fix that.

I use an antiviral N95 mask. 

https://n95maskco.com/products/respokare-n95-face-mask-30-pack

How well do you think this does against item (1)? And how would it affect your final recommendation?

1Quintin Pope5dI see no reason to doubt their claims about inactivating viruses on the mask. However, at ~$8 per mask, it would be cheaper to just use one normal n95 per day than to use one of these for 3 days. I expect the antiviral masks will also lose filter efficacy and fit quality with reuse. Also, I don’t think self inoculation is very likely if you’re careful about handling the mask and wash your hands after. So, it’s probably overall safer to use one n95 a day than to reuse an antiviral mask for 3 days.

I'd recommend using a mask with an exhalation valve, if you can.

Excellent write-up. Though, I think it's important to note that, if unfiltered, exhalation values can potentially spread disease.

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1Quintin Pope11dI address that in the general comments section. Exhalation valves do make n95s worse at source control compared with n95s without a valve. However an n95 with a valve is still about as good at protecting people around you as a cloth mask or surgical mask.