Air
Filtration
Also known as air filtration
Removing particles from air by passing it through a medium that captures them. A recirculating process: it cleans air already inside the house, and by itself neither introduces outdoor air nor removes gases.
In more detail
Filters do not work like sieves, which is why they capture particles far smaller than the gaps between their fibers. Four mechanisms operate at once. Large particles are caught by straining and by impaction, when inertia carries them into a fiber instead of around it. Mid-sized particles are intercepted when their path brings them within one radius of a fiber. The smallest move erratically by Brownian diffusion and collide with fibers as a result. Efficiency is lowest between the impaction and diffusion regimes, which is where the standard test sizes sit.
Two locations, two different jobs. Central filtration in the air handler treats every room the ducts reach, and its ceiling is set by how much filter area the equipment can accommodate without losing airflow. A portable unit treats one room, is rated by clean air delivery rate rather than media efficiency, and works only while it is running at a speed the occupants tolerate.
Gases are a different problem. Particle filters do not remove carbon dioxide, radon, carbon monoxide, or volatile organic compounds; those require sorbent media such as activated carbon, in quantities most consumer machines do not carry, and with a service life that ends silently. A filter's rating also describes the medium, not the machine: air that slips around a poorly sealed frame leaves unfiltered no matter what the media would have done.
Why it matters
Filtration is the cheapest meaningful air-quality intervention in an existing house and the one most likely to be specified backwards. What reaches the room is airflow multiplied by efficiency multiplied by runtime, and the rating on the box only addresses the middle term. A high-efficiency filter in a system that now moves less air, or a purifier sized for the room but run on its quietest setting, both deliver less clean air than a modest filter with airflow and hours behind it.
The common misunderstanding
That the rating on the filter is the outcome in the room. Efficiency is one of three factors, and the other two — how much air actually moves through the medium and how many hours it runs — are properties of the system and the household rather than the filter. A MERV 13 in a unit that runs twenty minutes an hour is doing less than a MERV 8 in one that runs continuously.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.