A strainer filters out particles larger than its openings, and not particles smaller than its openings. So with a mask you’d expect that after a certain point, small enough particles will sneak through. But this isn’t how N95 masks work: the particles they filter are generally much smaller than the gaps between fibers in the mask! What’s more, an N95 mask is actually really really good at filtering both the largest and smallest small particles -- it’s medium-sized small particles that are hardest for it to block. This isn’t at all like a strainer, because N95s are much cleverer than strainers. 

The overarching goal of an N95 mask is instead to get an airborne particle to touch a fiber in the mask. Regardless of how big an airborne particle is, once it touches a fiber, it stays stuck to it and doesn’t become airborne again. This isn’t anything special about the fibers, but about the size of the particles. At a microscopic scale everything is sticky, because the weakly attractive force between molecules IS strong enough to hold small things in place (Van der Waals force). So you shouldn’t think of N95 masks like a fine window screen that keeps insects of a certain size out; you should think of them more like a sticky spider web that can catch an insect of any size, as long as it touches a strand. And so N95 masks use a bunch of different clever physics and mechanical tricks to get particles to touch their fibers. 

First, many spiderwebs are better than one. Unlike strainers, where stacking many identical ones doesn’t improve the filtering at all, more layers of sticky fibers means more chances for particles to get stuck. And how likely particles are to hit or miss a fiber depends in large part on their size. Particles larger than a thousandth of a millimeter basically travel in straight lines, because of their inertia. And because there are so many layers of fibers, their straight line paths are essentially guaranteed to hit a fiber and stick(capture by inertial impaction). Airborne particles that are really really small are so light that collisions with air molecules literally bounce them around, so they move in a random zig-zag pattern known as Brownian motion. This zig-zagging also makes it super likely that a particle will bump into a fiber and get stuck(capture by diffusion). Particles of in-between sizes are the hardest to filter. That’s because they don’t travel in straight lines, and they also don’t bounce around randomly. Instead, they’re carried along with the air as it flows around fibers, meaning they’re likely to get carried past fibers and sneak through even a mask with many layers. But N95 masks have a final trick up their sleeve. They can attract particles of all sizes to them using an electric field. In the presence of an electric field even neutral particles develop an internal electrical imbalance which attracts them to the source of the field. This is why neutrally-charged Styrofoam sticks to an abused cat - I mean, a cat whose fur has been charged with static electricity. And how static electricity helps N95 mask fibers attract all particles. But unlike a cat’s fur, an N95 mask’s electric field isn’t just ordinary static electricity. Their fibers are like permanent magnets, but for electricity: electrets! Just like you can permanently magnetize a piece of iron by putting it in a strong enough magnetic field, you can ‘electretize’ a piece of plastic to give it a permanent electric field. By electretizing the fibers in an N95 mask, they gain a long-lasting ability to attract particles, which means they capture about 10 times as many particles as regular fibers. And this is, after all, the point of an N95 mask: filter out particles from the air. By taking advantage of the molecular scale stickiness of matter, using many layers of fibers that catch straight-moving large particles as well as zig-zagging small particles, and having an electric field that attracts all particles, you get a mask - not a strainer - that’s really good at trapping both small and large particles, and does a reasonably good job at filtering out middle sized particles. Precisely what fraction of those sneaky medium-sized particles get blocked gives you the number of the mask - if at least 95% of those particles are filtered out, then the mask is rated N95. Ok, so N95 masks can be very effective. But if you’re a healthcare worker wearing one of them, here are a few important things to look out for. 

The biggest influence on the performance of an N95 mask isn’t actually the mask - it's whether you wear it properly. If a mask isn’t fully sealed on your face, air (& particles you’re trying to filter) can just bypass the filter entirely. Dust, smoke, pollen, bacteria, and viruses all have different sizes, and so are filtered by N95 masks to different extents. However, germs for airborne illnesses don’t usually travel on their own – we breathe or cough them out in droplets which have a wide range of sizes. So the size of the virus or bacteria itself isn’t particularly relevant. N95 masks are intended to be disposable, but the demand from COVID-19 has led to a global shortage of N95 masks and the reality is that health workers have to reuse them - and thus decontaminate them. It’s important to be aware that certain kinds of decontamination (for example, using alcohol or liquids) can damage the electrostatic properties of a mask and destroy their filtering ability, even if the mask appears visually unaffected. N95decon is a volunteer team of scientists developing and sharing research-based decontamination methods so that masks can be reused during this crisis. 

Thanks for reading !!