Are N95 Masks Effective at Filtering Airborne Viruses?

Continuing the discussion from If you (individual or country) are still masking, why will you ever stop?:

I think you’re missing my point. It is presumably true that N95 masks are subjected to flow tests before being certified, and that they pass that test. While an appeal to authority is not irrefutable truth, the burden lies upon you to prove that the masks do not actually filter particulate matter. To convince me, you would need to show me an actual flow test, not simply note with a microscope that you can see holes the virus might slip through.

I’m all for skepticism, but I don’t have the knowledge in aerosol physics to come to any conclusion based on your microscope observations alone. For all I know, the filter could be thick enough and dense enough to be relatively opaque to aerosols that come across it.

As a counter-cite, here is a study from 1998 when the N95 standard was new - forgive me if I quote the wrong parts:

Qian Y, Willeke K, Grinshpun SA, Donnelly J, Coffey CC. Performance of N95 respirators: filtration efficiency for airborne microbial and inert particles. Am Ind Hyg Assoc J. 1998 Feb;59(2):128-32. doi:10.1080/15428119891010389. PMID: 9487666.

Excerpts from study (Click to show/hide)

In this study N95 respirator filters were tested with bacteria, NaCl, and polystyrene latex (PSL) particles, and procedures similar to those used previously were applied.(6–9) The N95 respirators were also tested at a flow rate lower than the certification flow rate of 85 L/min to examine their performance at a breathing rate that is typical for health care workers.


First, the size-fractionating aerosol generator was removed because no liquid particles were used in this study; all three types of solid test particles were generated by a six-nozzle Collison nebulizer (BGI Inc., Waltham, Mass.), which was supplied with clean compressed air at a pressure of 1.05 kg/cm2 (15 psi). Second, to measure particles in the most penetrating size range, a laser aerosol size spectrometer that measures in the 0.1 to 3.0 μm size range (LAS-X, Particle Measuring Systems, Inc., Boulder, Colo.) was operated in parallel with the previously used aerodynamic size spectrometer (Aerosizer, Amherst Process Instruments Inc., Hadley, Mass.). The smallest size at which the Aerosizer can properly measure aerosol concentrations is approximately 0.5 μm in aerodynamic diameter.(13,14) Both aerosol spectrometers measure number concentrations. For a polydisperse aerosol with a mean size at or near the most penetrating particle size, the respirator efficiency by count is equal to or less than the efficiency by mass. Thus, the minimum efficiency by mass of 95% is satisfied, if the count efficiency is 95% or higher.

The nebulized test particles were mixed with clean dilution air to attain an aerosol concentration of about 80 to 120 per cm3 in the test chamber. The aerosol was passed through a 10 mCi 85 Kr electrical charge neutralizer (TSI Inc., St. Paul, Minn.) before entering the test chamber. The N95 respirators, randomly selected from the boxes in which they were supplied, were sealed to a head form and tested by measuring the aerosol concentrations inside and outside the respirators with both particle-size spectrometers. Since particle deposition in the human respiratory tract depends on the aerodynamic particle size, all data are presented as a function of the aerodynamic equivalent diameter. While the Aerosizer data are recorded as a function of aerodynamic diameter, the LAS-X size spectrometer data are recorded as a function of optical equivalent diameter. The latter was converted to aerodynamic diameter through consideration of the particle density (2.2 g/cm3 for NaCl). The optical equivalent diameter of the LAS-X size spectrometer is based on the instrument’s calibration with PSL particles. The size and index of reflection differences between the NaCl and PSL particles have been neglected in the size conversions.

Each data set was repeated five times. The results of the measurements are presented by their means and standard deviations. All tests were conducted at a temperature of 25 6 3°C and a relative humidity of 20 6 2%. All respirators were equilibrated at the test conditions for 24 hours or more before experiments were performed with them.


The minimum efficiencies at the most penetrating particle size are about 96% for the N95 respirator, 92% for the DFM respirator, 82% for the DM respirator, and 71% for the surgical mask.

[…] As seen, the electrically charged polymer fiber filters of Companies A and B have a minimum efficiency of about 96.2% at the most penetrating particle size when tested at 85 L/min, and the charged fiber filters of Company C have a minimum efficiency of 95% (i.e., the filter materials of all three companies pass the certification requirement).

The performance differences between the respirators of different companies have been interpreted as follows. In the design of respirators the efficiency level for the filtration material is set by the thickness of the filter material and by several filter characteristics, such as filter diameter and packing density. To keep the pressure drop low for maximum breathing comfort and the efficiency above the certification level, a compromise is selected among these parameters. As pointed out in the Experimental Materials and Methods section, the filtration materials of the tested respirators differed in thickness and packing density, and, apparently, also in fiber size and the degree of electrical charge embedded in the fibers. For particle sizes above 0.75 μm, the filtration efficiencies of all tested respirators are 99.5% or higher […]


In these experiments the mean aerodynamic diameters of the B. subtilis and B. megatherium bacteria were measured to be about 0.8 and 1.2 μm, respectively. For both bacteria, the filtration efficiencies of the N95 respirator are 99.5% or higher, similar to the data for NaCl and PSL particles. Therefore, the filtration efficiencies of the tested N95 respirators can be regarded as 99.5% or higher for M. tuberculosis. If TB bacteria are contained in droplets of sizes larger than 1 μm and there is a good face seal, the filtration efficiency is also 99.5% or higher.


  The gaps in even an N95 mask are literally hundreds of times bigger than the virus that you think it can stop.  Roughly fifty microns wide, compared to a virus that ranges from 0.05 to 0.2 of a micron.  The mask is mostly empty space.

  Extraordinary claims require extraordinary proof.  To see the structure of the N95 mask that I examined, to see how wide open it is compared to the size of a virus, and to claim that it will in any meaningful way impair the passage of particles as tiny as viruses are, certainly qualifies an an extraordinary claim.  I think it very fair to say that the burden of proof rightfully belongs to anyone that wants to claim that such a mask will effectively obstruct the passage of viruses.

  And keep in mind, once again, that the masks that most people have been wearing are not nearly as good as a proper N95.

Which burden I believe is met by the flow tests in the certification process as well as in the many studies replicating that process, one of which I linked to and quoted above.

You are free to disagree with me about where the burden of proof lies, but we will get nowhere if all you have is your observation of gaps with a microscope, an inexplicable dismissal of the authorities I cite, and an unwillingness or inability to cite or perform a flow test with contradictory results.

ETA: I would also be interested in someone explaining to me how this works in theory to support the effective filtering the above cite (and widespread expert opinion) attests to in practice. I understand that airborne viruses are usually attached to aerosols at 1 micron or larger, but Bob_Blaylock’s image shows gaps much larger than that in the filter.


  Do you have the ability to perform any such flow test?  I certainly don’t, and I very much doubt that you do either.  We’re talking about viruses, particles that are so tiny that you cannot see them in a light-based microscope.  My same microscope that cannot, even in its most extreme configuration, see a virus, can easily, in a much lower-power configuration, clearly show me the holes in a mask, that you’re trying to tell me a virus cannot fit through.

I don’t, but I give credit to the authorities when they say they do (precisely because I can’t/won’t do the tests myself), and in my opinion the burden is on you to disprove them. Whether that means you find a contradictory study (that withstands my scrutiny), or whether you do a flow test yourself, is none of my concern.


The first thing to realize is that an N95 mask is not a net, and does not work like a net. It has an electrostatic charge distribution on it that attracts small particles. You can’t see electric fields with your microscope, but that doesn’t mean they’re not there.

Pardon me for having the physics knowledge of a fourth grader. I know - from grade school - that if you rub a piece of amber on a rug you can attract a feather with the static electricity. Are you saying the N95 filter fibers are charged like amber to attract small particles? How strong is the attraction and how much does it decay with distance?


The short answer is “strong enough to catch a 50-nanometer-sized particle, at a distance of the radius of the gaps in the material”. The proper way to answer the question is to take a mask, present it with a bunch of particles of that size under controlled conditions, and measure how many make it through. Which the mask-makers have done to get them certified, and which @Bob_Blaylock has not.

It’s also important to realize, by the way, that it’s not just individual viruses that we want to stop. We also want to stop aerosolized droplets of saliva, because that’s mostly what carries the virus. And those are much bigger than virus particles, and ordinary cloth masks are pretty good at stopping those, at the source (though not as good as N95s).

Fair, but now I’m curious as to whether static attraction (or whatever the correct term) is the reason why N95 masks work, when looking at a microscopic image it appears that they have such large gaps as to impair function. I’m imagining an experiment with two filters being subjected to a flow test, one N95 and one of identical material that hasn’t been electrostatically charged, the hypothesis being that the N95 will significantly outperform.


I find it extraordinary that someone would think that the efficacy of masks is thought to be extraordinary. The science behind the mask filtering is well-established hence the reason for their use in medical settings. It’s not the size of the virus but the size of the aerosol particles which are huge compared to the virus itself. Also, as Chronos already mentioned, the filtering mechanism of N95, KN95, and even surgical masks rely on electrostatic attractions that are highly effective.

Here’s a simple breakdown on the physics and chemistry behind masks.

While surgical masks would not help you if you were in an enclosed room with a covid patient, even they reduce the chances of catching covid in moderate-risk setting like a grocery store.

Of course, masks work extremely well in protecting others from your germs since they prevent the formation of small aerosols by simply increasing the humidity high behind the mask. Again, this is also firmly established science.

I started a thread a year and a half ago on this which was probably a year after the efficacy of masks against SARS-CoV-2 was pretty firmly resolved.

@Retzbu_Tox provided some great links in this post

And… I’ve found my study!

Emroj Hossain, Satyanu Bhadra, Harsh Jain, Soumen Das, Arnab Bhattacharya, Shankar Ghosh, and Dov Levine, “Recharging and rejuvenation of decontaminated N95 masks”, Physics of Fluids 32, 093304 (2020)

@Bob_Blaylock, you may be interested in reading this one due to their budget flow test apparatus, if you wanted to conduct your own test.


  That sounds like a convenient bit of solid digestive waste from a mal bovine, crafted to argue that a mask would be able to stop such a particle, no matter how huge the gaps are.

  The gaps in the N95 mask that I examined are literally a thousand times bigger than that 50-nanometer particle.  A THOUSAND times bigger!

  “…the radius of the gaps in the material”.  So no matter how big the gap, a particle a thousandth the size of that gap, just by dumb luck happening to pass through the very center of that gap, the greatest possible distance from any of the bordering material, will be stopped?


Thanks! That pretty much answered all my questions on theory.

ETA: Except these two,

Though I think… based on searches, the decay is an inverse square based on Coulomb’s law.


The size of the particle isn’t particularly relevant. What matters is the charge distribution. If the electric field is strong enough, it will attract nearby particles over a radius far larger than the particle size.

You can do even better with an actively maintained electric field. But even the passive charge that can be induced on spun plastic is sufficient at short length-scales. Electric fields are very powerful.

Yep. The strength of attraction follows Coulomb’s law. However, I think you’re misunderstanding the mechanics of the masks. They don’t have regular pores. Instead they’re spun fibers that create a highly-layered material. So even if they didn’t have electrostatic properties, they’d still have serious sieving action. My graduate work involved the sieving of macromolecules through a gel matrix. Particles much smaller than the pore sizes are sieved by the network.

And, to reiterate, even aerosol droplets are large. The smallest aerosol droplets are 50-100X larger than the virus. And, this can’t be emphasized enough, the humidity of the masks ensures that droplets remain very large. So large that even a decent cloth mask will protect others from the mask-wearer.

Faster than that, probably, since it’s probably dipolar at the relevant length scales.

OK, then how do you explain the experimentally-observed fact that they do, in fact, work? Again: People have actually done the experiment. The fact that someone who hasn’t done the experiment doesn’t understand how they work doesn’t change the observed fact that they do work. Heck, even if the experimenters themselves didn’t understand it, the experimental fact would still stand.

  How do you explain other studies that clearly seem to show that they do not?

  You can find credible-seeming reports of studies to support almost any conclusion that you want on the subject.

How have you not responded to the numerous posts that say that the size of the aerosol droplets (many times larger than the size of the virus) and the sieving action of the masks are what matters?

I don’t, because I haven’t seen any of those studies. Got any links?

I get that - if particles ‘stick’ to the fibers, via Van der Waals force, and the filter is relatively dense and thick, chances are slim that there will be a large pore from one side of the mask straight through to the other. It’s like if I took a black shirt and folded it over once or twice, then held it up to see how much light bleeds through.