A stupid-simple face-mask design based on pleated furnace filters
Marcus Leech
Canadian Centre for Experimental Radio Astronomy
http://www.ccera.ca
The emergence of the COVID-19 global pandemic, caused by the novel corona virus, SARS-CoV-2 has caused many to consider wearing some type of face mask to reduce risk of exposure to aerosols and droplets when having to leave the home for essential reasons like grocery shopping, etc.
Unfortunately, there has been overwhelming pressure on the supply chains for suitable masks, and those pressures have tended to deprive medical personnel of much-needed medical grade (N95 or better) masks. Hoarding behaviors by ordinary non-medical persons, early in the pandemic, have caused widespread shortages of N95 medical masks.
The situation is more than a little distressing.
For best results, the mask should completely seal-off the mouth and nose from any air that hasn't been through the filter. The filter medium should, ideally, filter out sub-micron particles with high efficiency. The selected material should also exhibit very small pressure drops across the medium, which translates into “minimum breathing effort”. Achieving both high filtering efficiency and very-low pressure drop is a significant challenge, and not one that is likely to be easily addressed by any DIY effort to produce a useful mask.
The reality is that a mask for an ordinary person who is already exercising prudent distancing practices are not very stringent—you wear a mask as part of your “toolkit” of practices and behaviors to reduce your risk of contracting the virus. Medical personnel are in a much higher risk situation, and therefore require masks made from the most-appropriate, designed-for-purpose materials.
The main requirement of a DIY mask is that it reduce risk by some measurable amount, and not that it be perfect. There is growing evidence that in parts of Asia where mask-wearing in public is extremely common, the illness rates have been measurably lower. Most such masks are not particularly high-tech, and are usually worn to help deal with smog rather than microbial assault. But they have a happy side-effect of reducing exposure to aerosols and particulates that can carry viruses and bacteria.
Many groups throughout the world have embarked upon community programs to produce masks that can be used both by health-care workers who have nothing else, but also ordinary people going about their daily business. This document describes one of many possible designs. It isn't unique or particularly special, and there are many approaches to the problem.
In parts of the world where forced-air heating systems are common, the air-handling sub-systems include a replaceable filter unit, which is usually some form of fibrous filter offering various levels of filtering capabilities, and simultaneously offering acceptable pressure-drop across the filter medium.
We will concentrate on filters with a 1” (25mm) thickness, and on filters that use a pleated, paper-like filter medium.
Nearly any hardware/home-wares store will carry pleated filters with MERV ratings from 8 to 11, and the trick is to find the “bargains” that give you the most number of face-masks per filter, given constraints on things like how much you have to butcher the resulting filter unit (and thus how much material must go to waste).
•A good office stapler—doesn't need to be heavy-duty, but something that can drive staples through all the pleated layers
•Scissors
•Garden shears (may not be necessary, depending on how the filter is constructed)
•25mm (1”) thick furnace filters
•Standard thin hair-elastics
•Staples for the stapler
Proceed by first taking apart the filter unit—the only thing we're interested in is the filter material itself. The housing frame is usually paperboard, and usually has a wire mesh on one or both sides of the filter. These need to be removed. Some filter designs make this easy, others, less so. The type that are harder will require you to use garden shears or heavy cutters of some sort (don't go ruining your sewing scissors for this) to remove the paperboard frame. Some designs glue the filter media to the frame, others leave it “floating”. The “floating” designs not only make it easier to disassemble, but also leave you with more undamaged filter material to work with after removing the frame and metal mesh.
Once you have the material in front of you, you'll need to figure out how to arrange things to maximize you mask yield. The mask width should be about 23-25cm, with the pleats arranged to run across your face. The mask height should be such that when the pleats are somewhat expanded, you have about 15-20cm of coverage from the top of your nose to below your mouth—ideally with some material able to curve around your chin. I've found that with a notional 15” x 20” filter, I'm able to produce 3 such masks, with not very careful butchering of the filter. Your mileage may vary.
Once you have your pieces cut out, bunch each piece, then staple the ends (this is where having a good office stapler comes in handy. It doesn't have to be RED, and it doesn't need to be stolen from your office mate).
Once the ends are stapled, staple one hair-tie to each end—optionally, you can use hot-melt-glue for this part without significant penalty.
You should end up with something like this:
In use, the pleats are stretched over your mouth/nose area, and the hair-ties are applied to stretch behind the ears.
In our lab where we made a few of these, we happen to have (pure coincidence) a UV sanitizing lamp assembly, so, we place them under the lamp for a few minutes prior to packaging 3 at a time into plastic bags.
1See: https://en.wikipedia.org/wiki/Minimum_efficiency_reporting_value