How do filters remove particles from the
air? It’s tempting to think of air filters as microscopically small
versions of kitchen 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 bundles of fine fibers.
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 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.
Ao at the end of the day, it’s not the smallest airborne particles that are
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.