LinkedIn·Wednesday, 12 August 2026·14d ago

Filters aren't sieves in the sense that they don't work by having holes smaller than the particles they need to catch. The actual capture…

AIRSCAN

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Filters aren't sieves in the sense that they don't work by having holes smaller than the particles they need to catch. The actual capture mechanisms are more interesting than that. Large particles (PM10 and above) have too much mass to change direction quickly. Air flowing through a filter has to curve around each fibre, and heavy particles can't make the turn. Their momentum carries them straight into the fibre, where they stick. That's impaction. Very small particles (deep into the ultrafine range, well below PM2.5) get caught because they're so light that surrounding air molecules constantly bump them around. That random, jittery movement - Brownian motion - sends them drifting sideways off the airstream and into a fibre before they can pass through. The problem zone sits around 0.3 µm (about 8x smaller than PM2.5 threshold). At that size, particles are too light for impaction to catch them, and too heavy for Brownian motion to knock them off course. Neither mechanism works well. This is the particle size most likely to slip through a filter. That 0.3 µm size is the one used to test HEPA filters. When a HEPA filter is rated at 99.97% efficiency, that number comes from testing it against 0.3 µm particles specifically - the size it's worst at catching. If you plot capture efficiency against particle size, you get a U-shaped curve. The filter performs well on the left side of the curve (ultrafine particles), performs well on the right side (large particles), and dips in the middle (around 0.3 µm). #Airscan #KnowYourAirSeries #IndoorAirQuality #AirPollution