New information suggests that a large percentage of people with COVID-19 may be asymptomatic – meaning they have the disease and can spread it without even knowing it. It is more important than ever to wear a mask in public.

Of course, the problem is that masks are in such short supply, and the N95 masks that are available are needed by healthcare workers who can’t avoid contact with other people. In states such as New York, New Jersey, Maryland and others, where masks are now a requirement, the definition of what qualifies as a mask is pretty loose. In fact, most call it a “face-covering”, which seems to imply that nearly anything that covers the mouth and nose is acceptable.

But what if you want a little more protection than just a bandana?

People began making videos describing how to use cut-up HEPA vacuum bags as filter inserts. The result was that within days HEPA vacuum bags became nearly as rare as N95 masks and hand sanitizer. What’s a DIYer to do?

I was at our local hardware store for a supply run to make a few of the copper button pushers I had described in a previous blog entry. As I walked down the plumbing/HVAC aisle, I saw something that inspired my interest.

Air conditioners use filters. These filters are rated on a scale called MERV, from one to sixteen – sometimes higher – depending on what the filter will allow through. Some companies such as Filtrete, use their own rating system, but most of those also have an approximate MERV rating printed on them somewhere.

Masks aren’t rated on the MERV scale.  They have their own rating system which is actually two ratings, the letter and the number. The letter describes its ability to work in the presence of oils, which can lessen effectiveness. The N rating means that an N95 should NOT be used in the presence of oils. Here is a link to an FDA page all about protective gear, including some good info on N95 masks.

The 95 means that it is effective against 95% of particulates of a certain size. To test items, the particulate used is a smoke with average particle diameter of 0.3µm. So, an N95 mask is – when not exposed to aerosolized oils – effective against 95% of particles with a diameter of 0.3µm or greater.

Filters with a MERV rating of 14, which were the best I could easily find, are effective against 75% of 0.3µm particles – NOT as good as an N95 mask. Also, what makes an N95 mask better than, say, a standard surgical mask, is that they are designed to fit well to the face. So, keeping all this in mind, it must be said that using vacuum bags or air-filter material is NOT as good as using an N95 mask. But they are better than using only plain cloth, and every little bit helps.

Also, N95 masks are in such short supply that the average consumer should not be using them anyway.  Health care workers need them more.

Keep in mind that masks aren’t really designed to protect against a single stray virus microbe which, in the case of COVID-19 has an average diameter of 0.12µm. They are mainly meant to protect against accidental exposure to the very small droplets of potentially virus-laden moisture that we put out when we cough, sneeze or just breath normally, which are usually 0.5µm and up. Here is a link to a study about spray particle size, if you really want to go for a deep dive into the data. The main part is the chart, but the entire abstract is pretty interesting.

When the filter material is extracted from the filter frame, it turned out to be a surprisingly large amount of material – easily enough for several-dozen layered mask inserts.


Be VERY careful when taking the material out of the filter frame. The metal mesh is sharp and will cut you. It cut me. Use wire cutters and wear thick gloves.

Polyester filter material is best used as an insert in a cloth mask, not as the mask itself.

Filter materials are most effective when used in multiple layers.

Filter inserts should be disposed of after a single use.

Polyester filter material is not, as far as I can tell, washable.

No DIY mask is as good an actual N95 mask.

Leave the N95 masks for health care workers.

DO NOT USE FIBERGLASS FILTERS! Honestly, I don’t know why these are even available as air filters, other than that they are dirt cheap. Fiberglass filters are usually a blue or green, spun material and have a very low MERV rating anyway.
What you’re looking for are pleated, electrostatic filters. These are made of polyester. I put a link here and in my affiliate store to the best ones I could find on Amazon. Get the highest MERV rating you can find.

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Stay safe and…


One thought on “Another Weird Idea

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