It seems counterintuitive that 0.3 microns would be harder to capture than 0.01 microns—that’s 30 times smaller. But the root of the problem is our intuition to think of HEPA filters like a net. If a particle is smaller than the holes in the net, it gets through. And it’s basically how HEPA filters work for particles bigger than 0.3 microns. These particles either can’t fit through or their inertia causes them to hit the filter fibers. But when we get to really small particles – like particles under 0.3 microns, things start getting weird. Particles that small have so little mass that they actually get bounced around like a pinball when they hit gas molecules. These tiny particles are small enough to fit through HEPA filters if they flew straight. But because they fly in zigzag patterns, they end up hitting the fibers and getting stuck. Here’s why that 0.3 micron number comes up all the time. The weirdness of Brownian motion works its magic under 0.3 microns. The more easily understandable filtering works its magic above 0.3 microns. But where those two processes overlap is the weak spot. So it is important to see how efficient the HEPA filter is at catching the 0.3 microns.
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