Photograph of  a low MERV filter How Air Filters Work

  • HOW AIR FILTERS WORK - CONTENTS: How air filters work to remove particles from the air stream: impaction, interception, diffusion filtering methods. What airborne particle sizes are an IAQ concern? - particle size vs. air filter characteristics vs. indoor air quality. Air filter maintenance.
  • POST a QUESTION or READ FAQs about HVAC filters used to clean indoor air: how filters work to remove airborne particulates and contaminants

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How do HVAC air filters work:

This article explains and explain just how air filters for HVAC systems actually work to trap and remove particles from indoor air. This website answers almost any question you might ask about air filters for heating or air conditioning systems. The page top photograph is of a low-MERV HVAC filter in an air handler.

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How do air filters actually remove particles from the airstream?

Photograph of a dirty HVAC air filter

Here we explain the three methods by which HVAC air filters work to capture and remove airborne particle contaminants from building air: impaction filtering, interception filtering, and diffusion filtering.

In these articles we are referring to filters installed on central air conditioning or central heating systems that move air through air handlers and duct systems. Standalone "air cleaners" are generally ineffective in buildings.

In articles at this website we explain how an air conditioning service technician will diagnose certain common air conditioning system failures or defects. We include photographs to assist readers in recognizing cooling system defects.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. Readers should also see

This photograph (above-left) shows a soiled surface of a conventional air filter up close. The fibers are clearly visible but not the openings through which air has to pass.

If we look at a high efficiency, high MERV or HEPA air filter under a microscope we'll see a mat of randomly crisscrossed fibers of filter material (fiberglass, polypropylene, paper, or other materials).

The space between the filter fibers will be larger than the smallest particle size which the filter is asserted to remove. So how do these filters stop the small particles? Let's look at three air filtration mechanisms in order of decreasing particle size:

  1. Impaction filtering: First, some airborne particles smack directly into a filter fiber, either because they happen to be on that unlucky (for them) trajectory or more likely because the particle is so large that it can't pass through the opening, or part of the particle smacks into the filter fiber.

    Air filter scientists call this method "impaction". Impaction captures larger particles which in a HEPA system are those larger than about 0.4u.

  2. Interception filtering: Second, some airborne particles moving in the airstream past the filter happen to be close enough to a filter fiber that they stick to it.

    How close? the particle has to be closer to the filter fiber surface than one radius or diameter of the particle itself. In other words, a 2u particle which is only 1u away from a fiber will probably stick to the fiber.

    Air filter scientists call this method "interception". Interception captures particles mostly in the 0.3u size to 0.1u size.

  3. Diffusion filtering: Third, very small particles, say below 1u in size, collide with larger gas molecules swirling around in the air turbulence caused by air moving by the filter fiber.

    This collision, called "diffusion" in filter science, causes the particle to zig zag out of the airstream passing through the filter opening and to stick to the filter surface.

What airborne particle sizes are an IAQ concern?

Photograph of a dirty HVAC air filter

What mold, house dust dust, allergen fragment, mite fecal, cat dander, or other airborne particle sizes are a concern for indoor air quality?

In the photograph shown here the large black Stachybotrys chartarum mold spores can be seen against our eyepiece micrometer which, after calibration, shows that these particular spores were about 7u x 15u in size.

The brownish tubular structures are fungal hyphae. Another, smaller fungal spore is in the background. What's not addressed by some of the science in the air filtration and IAQ field is just what particle sizes are a worry.

In general, larger particles, say 30u or 50u or long fibers, say 200u, are so big that they tend to be filtered in the nose of a human breathing that air. (1u here means 1 micron in size).

A more complete discussion about the size and behavior of problematic indoor air particles which form an indoor air quality concern can be read at PARTICLE SIZES & IAQ.


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