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How Filters Trap Airborne Bacteria and Viruses?

A genuine HEPA filter works, but not in isolation.

What are Particulate Matters (PM)? The Covid coronavirus is slightly bigger than most other viruses but still smaller than bacteria, mold spores, pollens, and hair by comparison. Smaller PM due to its lightness is airborne.


Understand the four (4) mechanisms of how PM is trapped on a filter, the diagram below shows the diffusion, interception, straining and impaction.

  • Inside a filter is a piece of plastic or fiberglass. Airflow carries the dust particle and makes a head-on (directly hitting) collision with the fiber
    of the filter and stuck there, we call that Inertial Impaction.

  • Straining is like a strainer. The air passes through the gap but the dust gets stuck between the two pieces of the fiber, and now as you imagine that if a piece of dust gets stuck there means more dust will accumulate even if the following particle is smaller. So, the dirtier your filter makes the better filter but that also blocks airflow.

  • Interception is when the particle grinds this fiber, and the frictions cause the particles to adhere to the fiber.

  • To understand diffusion, you must know that a very small particle has a smaller mass and almost no gravitational impact, hence it floats around erratically, bouncing with other particles and as it is tossed by the airflow it is incidentally attached to the fiber.


ASHRAE’s bare minimum recommendation is MERV 13 filters that filter 0.1-0.3 microns of particle size, on all air handling systems if you want to start reopening your businesses in the United States. Although a higher rating is good, the pressure drops are sacrificed. HEPA filter filters <0.3 microns. There are many types of filters available in the market, but the common ones are pleated, cartridge, bag, HEPA, and composite type. 


Using original replacement parts is important because the equipment manufacturer suggested filters were tested and approved according to applications and design. All reputable Air Handler Units (AHU), HVAC Systems, Air Purification Systems, and Fresh Air Systems have lab test reports from the country of origin that are accessible. Pay attention to certain parameters that are important to determine its efficiency in terms of lifespan, energy saving and performance. 

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