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  • Writer's pictureDr Val

To Filter or Not To Filter?

If 2020 taught us anything its that there is a lot of science out there-and a LOT of “junk science”, as well! There were ads everywhere for new products to filter, purify, clean, etc the air you breathe in. BUT-how do they actually work? And what’s the difference?

Airflow 101

Air is tricky-you can’t see it, but it behaves almost like a liquid in the way it moves. That’s because it, too, is made up of all sorts of particles. Some are as small as atoms (like the oxygen that keeps you alive), others are larger-complex gases, viral particles, pollen, dust, dander, pollutants, smoke…the list is infinite! Air, like water, moves in a laminar (sheet) motion until something disturbs it. The faster the movement, the more that it tends to keep in a straight line. Think of a deep, fast moving river-now think of encountering rapids on that river. Air moves the same way.

Laminar/smooth flow on the left is interrupted by an ostacle-rocks-and becomes turbulent, or rough, on the right

That means when air encounters an obstacle, like a filter, it tries to move around it instead of through it. Air that moves around a filter doesn’t get cleaned. Air that moves through a filter does, but the extent of that cleanliness depends on a lot of different factors.


Mechanical filters can work in different ways. The simplest have fibers woven tightly to present a physical barricade to particles or debris, removing anything that is larger than the holes in the material. More advanced filters use an electrostatic charge, or even special materials such as activated charcoal to remove specific particle types. Chemical, electrical, radio and biological filters also exist, but for the sake of focus, we’ll save them for another article.

Filter material is rated by the percentage of particles of a certain size that the material will remove, on what is called a MERV (minimum efficiency reporting value) scale. The higher the rating, the more small particles the filters will remove. HEPA filters are at the highest end of the MERV scale.

Layering filter material can provide a cumulative effect. For example, using a filter with larger holes “upstream” allows coarser particles to be removed, which can make a finer filter more likely to catch small particles without clogging. The downside? Layers of material and very fine filters are heavy and also increase resistance to air flow, meaning you have to work harder to move air through them.

How Do We Apply This?

Revisiting our flow dynamics-the faster something is moving (higher velocity) the more likely it is to continue to move in that same direction. Breathing in moves air slowly compared to most exhalations. So: when you're breathing in, you're more likely to be moving air more slowly, which means it tends towards turbulent flow. Turbulent flow is more likely to sneak in via places we don't anticipate.

Masks have large holes, and large gaps. They’re great for filtering large particles (respiratory droplets, or sneezes….ew) on their way out of your nose or mouth. These are moving at high velocity, so they are likely to hit the fabric or material directly in front of them (due to laminar flow)

Smaller holes, such as those in a filter, prevent smaller particles from getting through. A tight seal-preventing air from flowing through the gaps-makes sure that all of the air you are moving goes through that filter. Air moving through the filter is air cleaned by the filter.

Focusing the filtration and seal on only the are where air is actively moving-the edges of the nostrils-results in a lightweight filter, and using a nasal dilator in combination offsets the increased resistance by helping air flow more freely. The result is comfortable and helps you Breathe Easily, Breathe Safely.

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