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 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! 🙂
The attractive electrostatic force which captures small particles in a N95 filter is not the Van der Waals force. The Van der Waals force is the force of attraction between two charge-neutral particles with no static multipolar electric field, e.g., between two helium atoms. It is the force that arises due to an instantaneous, fluctuating electric dipole moment that spontaneously arises in one particle, and which induces a dipole electric polarization in a nearby particle. The direction of the induced dipole is always such that the force is attractive, is very weak, and is of exceedingly short range. This is the force, for example, that causes helium to become liquid below 4.2K, at atmospheric pressure.
A N95 filter has a layer of permanently charged fibers. The attractive electrostatic force which captures small particles in a N95 filer is the force between a permanent charge and the electric dipole this permanent charge induces in a nearby, uncharged, but electrically polarizable, particle, such as a water vapor aerosol particle. The same as for the Van der Waals force, the direction of the induced dipole is always such that the force is attractive. While shorter range than the force between two permanent charges, the force between a permanent charge and an induced dipole is both stronger (at a given range) and is longer range than the fluctuating dipole-induced dipole Van der Waals force of attraction.
Thanks for sharing your insight!
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!
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?
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.
I sew my own masks at home, as do many others, for repeated washing and use.
It’s easy, now to find the necessary fabric for top layer, fusible interfacing, and bottom layer that lies against the face, as well as elastic to wrap around head or ears.
My question is whether an electrostatic material is easily available, say in fabric or other stores.
Also whether it is washable, or needs to be replaced, or the mask discarded after one day’s use.
The articles I’ve seen so far on the need to use electrostatic N95 masks, so far, don’t seem to want to address such questions of availability and re-usability.
I honestly have no idea Robert. While I’ve heard of many creating their own masks for personal use, donation, or sale, I don’t know if there are any commercially available electrostatic materials for such a use. Although, I wouldn’t consider them a substitute for something like an N95 mask without rigorous testing.
Hi, do N-95 masks also filter the air that is coming out?
As long as it doesn’t have an exhalation valve. This would protect the person wearing the mask but not those around that individual.