How Do N95 Masks Work?

N95 masks have become one of the most coveted pieces of personal protective equipment (PPE) during the COVID-19 pandemic; however, the mechanism by which these masks capture particles has very little to do with the gaps between mask fibers (ie, the N95 doesn’t work like a strainer).

Instead, N95s filter large and small microscopic particles using a layer of fibers arranged to maximize the chance of contacting these particles. Weak, short-range electrostatic forces (Van der Waals) help these particles “stick” to fibers. Depending on the size of the particle, some are more likely to travel in straight lines (their inertial path) and others through Brownian motion (scattered, haphazard motion due to redirection from encountering air molecules).

But what about medium-sized microscopic particles? Electrostatic attraction due to an electric field present in the N95 helps create dipole-dipole interactions to capture these particles. In fact, the “95” in N95 refers to the percentage of medium-sized aerosolized particles that the mask effectively captures. This electric field is why N95s cannot simply be washed with soapy water or bleach.

Remember, the most important determinant of an N95’s performance is its seal on your face! Make sure you get fit-tested, shave those beards, etc.

Drop me a comment below with questions! 🙂

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  1. Thank you, Rishi, you truly are an inspiration. Indeed, I have learned a lot from you, and this website is a godsend! Intermolecular forces, I should have thought about that. I wish I could shadow you for a day/night; it would probably increase my chances of getting accepted into CRNA school! God Bless!

  2. Does the ‘exhalation valve’ allow air to enter during inspiration or is it ‘one way’ allowing exhalation only?
    Is there an easy way to block an exhalation valve so an infected user will not spread virus through the valve?
    Thank you!

    1. The exhalation valve is a one-way valve, so as you said, it permits exhalation only (assuming no equipment fault and properly fitted mask). Most of us will wear a surgical mask on top of the exhalation valve; however, I’m not sure what the evidence behind spreading the virus FROM the mask operator to the outside world is. In reality, if someone actually knows they have COVID, they should be quarantining themselves in the first place.

    1. As long as it doesn’t have an exhalation valve. This would protect the person wearing the mask but not those around that individual.

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