Fact check: No, N95 filters are not too large to stop COVID-19 particles

Eric Litke

The claim: "N95 masks block few, if any" COVID-19 particles due to their size

As many states and communities ease restrictions related to the COVID-19 pandemic, the debate over mask usage has intensified.

Businesses, churches and governments have implemented all manner of policies — some requiring masks, some leaving it up to each person, some even banning masks. And that has spurred many armchair epidemiologists to weigh in, including a Facebook page with nearly 1 million followers.

A June 4 post from Why don’t you try this? went a step beyond the homemade mask debate to claim that even the N95 masks used by health care workers are pointless in the face of COVID-19.

“COVID 19 virus particle size is 125 nanometers (0.125 microns); the range is 0.06 microns to .14 microns,” the post said. “The N95 mask filters down to 0.3 microns. So, N95 masks block few, if any, virions (virus particles).”

In other words, the post asserts the virus is smaller than the filter on the N95 mask, so the N95 mask doesn’t work.

Experts say this claim flies in the face of numerous studies and reflects a failure to grasp fundamental principles of how viruses behave and how face masks work.

As masks became harder to get, hospitals began looking for ways to re-use them. Dan Cates demonstrates how used N95 masks will be placed onto plastic racks to be sterilized by a robot utilizing ultraviolet light at Regions Hospital in St. Paul, Minn.

Here’s what we found.

Virus particles don’t exist alone

The science of mask functionality gets really small, really fast. The unit of measurement here is microns — 1/1000th of a millimeter.

The size-based argument against N95 laid out in this claim assumes mask filtering works something like water flowing through a net — particles in the water smaller than the net opening pass through, while larger items don’t.

But the physics involved don’t work like that at all.

The COVID-19 particle is indeed around 0.1 microns in size, but it is always bonded to something larger.

“There is never a naked virus floating in the air or released by people,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech who specializes in airborne transmission of viruses.

N95 mask on table in front of medical worker

The virus attaches to water droplets or aerosols (i.e. really small droplets) that are generated by breathing, talking, coughing, etc. These consist of water, mucus protein and other biological material and are all larger than 1 micron.

“Breathing and talking generate particles around 1 micron in size, which will be collected by N95 respirator filters with very high efficiency,” said Lisa Brosseau, a retired professor of environmental and occupational health sciences who spent her career researching respiratory protection.

Health care precautions for COVID-19 are built around stopping the droplets, since “there’s not a lot of evidence for aerosol spread of COVID-19,” said Patrick Remington, a former CDC epidemiologist and director of the Preventive Medicine Residency Program at the University of Wisconsin-Madison.

Fact check: What's true and what's false about coronavirus?

Size matters, but not how you think

But that’s not the only logical flaw in this claim.

The N95 filter indeed is physically around the 0.3 micron size. But that doesn’t mean it can only stop particles larger than that. The masks are actually best for particles either larger or smaller than that 0.3 micron threshold.

“N95 have the worst filtration efficiency for particles around 0.3,” Marr said. “If you’re smaller than that those are actually collected even better. It’s counterintuitive because masks do not work like sieving out larger particles. It’s not like pasta in a colander, and small ones don’t get through.”

N95 masks actually have that name because they are 95% efficient at stopping particles in their least efficient particle size range — in this case those around 0.3 microns.

Why do they work better for smaller ones? There are a number of factors at play, but here are two main ones noted by experts:

The first is something called “Brownian motion,” the name given to a physical phenomenon in which particles smaller than 0.3 microns move in an erratic, zig-zagging kind of motion. This motion greatly increases the chance they will be snared by the mask fibers.

Secondly, the N95 mask itself uses electrostatic absorption, meaning particles are drawn to the fiber and trapped, instead of just passing through.

“Although these particles are smaller than the pores, they can be pulled over by the charged fibers and get stuck,” said Professor Jiaxing Huang, a materials scientist at Northwestern University working to develop a new type of medical face mask. “When the charges are dissipated during usage or storage, the capability of stopping virus-sized particles diminishes. This is the main reason of not recommending the reuse of N95 masks.”

Our ruling: False

We rate this claim FALSE because it is not supported by our research. The COVID-19 virus itself is indeed smaller than the N95 filter size, but the virus always travels attached to larger particles that are consistently snared by the filter. And even if the particles were smaller than the N95 filter size, the erratic motion of particles that size and the electrostatic attraction generated by the mask means they would be consistently caught as well.

Our fact-check sources:

  • Interview with Patrick Remington, former CDC epidemiologist and director of the Preventive Medicine Residency Program at the University of Wisconsin-Madison, June 9, 2020
  • Interview with Linsey Marr, professor of civil and environmental engineering at Virginia Tech, June 9, 2020
  • Email exchange with Lisa Brosseau, retired professor of environmental and occupational health sciences, June 9, 2020
  • Email exchange with Jiaxing Huang, materials scientist at Northwestern University, June 9, 2020
  • Email exchange with Bill Hanage, associate professor of epidemiology at Harvard University’s School of Public Health, June 9, 2020
  • Center for Infectious Disease Research and Policy, COMMENTARY: Masks-for-all for COVID-19 not based on sound data, April 1, 2020
  • Email exchange with Nancy Leung, post-doctoral researcher in infectious disease epidemiology at the University of Hong Kong, June 9, 2020
  • Email exchange with Yang Wang, assistant professor of civil, architectural and environmental engineering at the Missouri University of Science and Technology, June 9, 2020

Contact Eric Litke at (414) 225-5061 or elitke@jrn.com. Follow him on Twitter at @ericlitke.

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Our fact check work is supported in part by a grant from Facebook.