 How do filters remove particles from the air? It's tempting to think of air filters as microscopically small versions of kitsch and sieves which block large particles from getting through while letting small ones past. As we'll see though, this picture is rather misleading. Many types of air filters are made from tangled bangles of fine fibres. Together, these create narrow, convoluted pathways for the air to pass through them. For the really large particles, these filters actually do behave a little bit like a kitchen sieve by physically blocking the particles from getting through, but this is pretty much where comparisons with sieves end. To understand why, imagine for a moment a car speeding along a winding road. The car fits easily on the road, but if it's going too fast it won't be able to navigate the bends and you just know that at some point it's going to crash. It's the same for small particles speeding through an air filter. As the air twists and turns, the particles can't keep up and ultimately they end up crashing out and being caught. Because of their inertia, which just like the car depends on how massive they are and how fast they're going, they can't stay the course. But what about the smallest, lightest particles, those that can easily slip along the winding path the air takes? Amazingly, these are also captured efficiently, but for a very different reason. Because these particles are so small, they have very little inertia and they dance around as they're bombarded by the air molecules around them. This rather unusual behavior is called Brownian motion after the first person to observe it and as they dance, they end up colliding with the filter and being captured. So at the end of the day, it's not the smallest airborne particles that are the hardest to filter out, but the medium-sized ones, those that are too small to be captured through inertia, but too large to be captured through Brownian motion. And this is why, when manufacturers design and test air filters, they don't use the largest particles or the smallest ones, but they use the middle-sized ones, those particles that are just the right size to slip through and make it to the other side.