Air filter foam.

Air filter foam is polyurethane in an open-cell structure that has then been reticulated: the thin membranes between cells removed, leaving only a lattice of strands. That last step is what separates filter foam from upholstery foam. Both are open-cell; only the reticulated one lets air through without a fight.

For air duties the chemistry is polyether rather than polyester. Polyether resists humidity well, and air filters live in moving air of unpredictable moisture content — exactly the condition under which polyester foam breaks down over time. See polyester against polyether for the full comparison.

Standard supply is black, and that is functional rather than cosmetic: carbon pigment gives real resistance to ultraviolet light, which is the main thing that embrittles polyurethane. For any filter that sees daylight, black materially outlasts a pale foam.

• Pore grade, in PPI. 20 to 80. Coarse flows and holds dirt; fine captures and clogs. See choosing a PPI.
• Thickness, from 5 mm. The most under-used variable — a thicker section at the same grade holds proportionally more dirt without adding much resistance.
• Profile. Cut to your frame, not trimmed on site. See die-cutting to your drawing.

If you are unsure of the grade, do not guess — describe what the equipment is, what it draws in and how often anyone will service the filter. That points to a specification faster than a catalogue does.

• HVAC and air handling — intake and pre-filter stages, condenser guards, evaporative coolers.
• Compressors and blowers — intake elements where pressure drop is an energy cost.
• Panels and cabinets — fan and louvre filters keeping dust off boards and terminals.
• Automotive and motorcycle — airbox and pod filter elements, single or dual stage.
• Acoustic grilles — dust protection for drivers without dulling output.
• Spray booths — intake filtration where airborne dust would land in wet paint.