Why the smallest particles can still be caught
A filter does not need openings smaller than every particle it captures. Air follows a winding path through the filter, but particles have mass and motion of their own. As they move, they can drift away from the air stream and touch a fiber.
Several effects help. A particle traveling close to a fiber may brush against it as the air bends around the fiber; this is called interception. A heavier particle may be unable to turn as quickly as the air and strike the fiber head-on. The very smallest particles, buffeted by collisions with gas molecules, move irregularly and are more likely to wander into a fiber. Once a particle sticks, it is removed from the passing air.
Capture has to be balanced with airflow
Packing fibers more densely can give particles more chances to hit them, but it also makes it harder for air to pass through. That resistance can reduce airflow or make a fan work harder. Filter design therefore balances particle capture against the amount of air that can move through the material.
The filter material is only part of the system. Air that slips around an ill-fitting filter bypasses its fibers, carrying particles with it. In a room, effective filtration depends on moving air through the filter rather than around it—so a well-sealed filter can matter as much as the fine web inside.
