How a filter catches particles smaller than the gaps in it

This is written to be used rather than admired. Each section below is a decision about particulate filtration, and each one has a default.

A filter is not a sieve

The gaps between fibres in a filter are far larger than the particles it is rated to capture, sometimes by a wide margin. If capture depended on gaps being smaller than particles, resistance to airflow would be impossibly high. Instead the filter is a tangled mat of fibres, and particles are removed as air weaves its way between them.

The mechanisms that do the removing depend on particle size, air speed and fibre diameter rather than on any hole size. This is why a filter's rating describes a percentage captured at a specified size rather than a pore dimension.

Four different ways to catch something

Large particles cannot follow the air as it curves around a fibre, so their momentum carries them into it and they stick. Slightly smaller ones follow the air but pass close enough that they touch a fibre and are held there by surface forces. Very small particles are knocked about by air molecules and wander randomly, which makes them collide with fibres despite the flow.

Many filter media are also given an electrostatic charge, which attracts particles that would otherwise pass through untouched. Every particle encountering the filter is subject to all four, with one dominating depending on how big it is.

The awkward size in the middle

Large particles are caught easily by momentum, and very small ones are caught easily by their random wandering. Between those regimes sits a size that is too small for momentum and too large to wander much, and it is hardest to capture. Filter ratings are defined at that most penetrating size precisely because it represents the worst case.

This is the reason a filter rated at that size performs even better on both larger and smaller particles. It also means a claim about capturing the smallest particles is less impressive than it sounds, since those are relatively easy.

Resistance is the price of capture

Every fibre a filter adds captures more and also obstructs the airflow that carries particles to it in the first place. A restrictive filter fitted to a fan that cannot overcome the resistance simply moves less air, cleaning the room more slowly. Pleating increases the surface area within a given frame, which lowers the air speed through the medium and reduces resistance.

That is why high efficiency filters are deeply pleated rather than flat, and why crushing the pleats ruins them.

A loaded filter both captures better and resists more, which is why efficiency and airflow move in opposite directions with age.

The charge fades even when the filter looks clean

Electrostatically charged media rely on the charge to capture particles their fibre spacing would otherwise miss. The charge dissipates over time, particularly in humid conditions or once fibres are coated with captured material. Efficiency therefore falls while the filter still looks serviceable and still passes air freely, which is a misleading combination.

Mechanically, washing such a filter usually removes the charge permanently, even if the medium survives the washing physically. Media that capture mechanically without a charge age more predictably, gradually restricting rather than quietly weakening.

Moving the air is half the job

A filter only cleans air that passes through it, so the volume moved per hour matters as much as the capture rate. A unit tucked into a corner or against furniture recirculates the same pocket of air rather than the room's air.

At the protocol level, doors, open windows and ventilation continuously introduce unfiltered air, which sets a floor on what any unit can achieve. Particles also settle onto surfaces, so a room can appear improved because dust fell rather than because it was captured. Removing the source is always more effective than filtering it, since a filter competes with a supply that never stops.

The takeaway

It is not the size of the gaps; it is what happens to a particle as it tries to get through them.

The constraint is almost always physical, and marketing rarely mentions which one.