DIY virus mask
DIY materials for a mask don't have the potential to perform like a surgical mask or an N-95 mask, both of which are made with a polymer. Whereas a polymer acts mostly like a sieve, cotton and paper act mostly like mere filters. More about that lower down the page.
I tried saturating paper towels with an acrylic polymer, which does have some breathing capacity, hoping to simulate the sub-micron sieve characteristic of the N-95s and the surgical masks. (They are made with melt-blown micro & nano polymer fibers.) But achieving good enough breathability entailed thinning the resin so much that it lost its continuity and thus any sub-micron sieve characteristic it might otherwise have had.
But the following materials all make for pretty good DIY masks.
As for breathability, you'll probably want something between 1.25 and 2.75 from the chart below.
The lower end of that range would be for wearing your mask for just short trips to the store -- such as for a half hour or less. Don't hurt yourself by wearing a mask that restricts your breathing too much. Oxygen debt, even brief, can have serious consequences.. so I've come to recently appreciate..
As for effectiveness (which is closely related to breathability when the filter is cotton or paper rather than a polymer), I won't list the various results researchers have obtained. You've seen them all over the Internet. The numbers obtained vary a lot, and not much detail is given about how the testing was done -- such as whether the perimeter of the material was sealed against leakage, or was rather tested as typically worn. The difference is significant.
Also -- unless a researcher doing particle filtration testing on materials states clearly what the effectiveness per unit breathing effort is, for a given particle size, there is not much value in the information. Specifically, the comparisons lack uniformity, and thus meaning, without layering materials so as to make breathability equal among the materials being tested.
Below is a useful breathability comparison chart for various common materials. It is from my own testing, with perfectly sealed perimeters of the materials.
It's doubtful the paper towels you have at home are as thin as the ones I tested, in which case you'll likely want just one layer, which might score just over 1.0 on the scale below. Comes down to finding the right level of comfortable breathing.
To some degree, the same is true for any of the materials in the chart. No two brands or types of anything are exactly the same. So, after all the work.. the chart below is just to give one a sense of it.
This is a breathability chart, not an effectiveness chart:
Woven cotton fabric (or any woven fabric) will be worth very little as a filter if it is stretched. A well designed cotton mask has substantial hems (which become the straps as well). Even when tying the straps tight, as one should, the body of the mask won't stretch. See the first mask further down this page.
Provided the entire perimeter of a mask is sealed to ones face, doubling the layers of material does make a big difference in effectiveness for filtering out particles of any size.
Articles that state that it makes little difference to double up (or triple up etc) don't seem to make allowance for air escaping past the perimeter of a mask. They seem to be doing their testing with masks as typically worn, rather than with masks that have been sealed around the perimeter. Either that or they came up short in their attempt to seal the perimeter, which can be challenging. And lack of a good seal can easily be not detected.
Claims of only a 1 or 2 percent increase in effectiveness as a result of doubling a material cannot possibly be valid.
A typical calculation regarding using multiple layers of material looks like this :
Assume a single layer of material (with perfect seal around perimeter) has tested at being 55 percent effective (blocks 45 percent) for a particular size particle.
The following principle should hold roughly true in practice, provided the perimeter of the material has been perfectly sealed to ones face.
The product on the left is subtracted from 1 to obtain the effectiveness.
Surgical and N-95 masks have the advantage of being made with a polymer material. Clearly a large percentage of the gaps in the polymer material are smaller than virus-size, otherwise they would not fare better for effectiveness than cotton or paper fabrics per unit breathing effort.
Diatomic oxygen and nitrogen molecules are about 1/300 the size of the average covid-19 virus, which gives a polymer mask a lot of potential for good breathability while still blocking out more viruses than can a cotton or paper filter -- what with the cotton and paper filters having to rely exclusively on a simple ratio of "gap" to "solid area", since virtually all those gaps are larger than virus-size.
Unrestricted air flow yielded 7.06 horizontal pendulum distance in my testing of filters, while air flow through a perimeter-sealed surgical mask yielded 2.75 when subjected to the same air pressure.
A perimeter-sealed surgical mask will typically score over 90 percent effective against viruses, while a perimeter-sealed (and layered) cotton or paper filter with the same breathability score of 2.75 will typically score only about 60 percent effective.
Not surprisingly :
2.75 / 7.06 = 0.39.
Let's consider that to be the percentage of particles blocked by cotton or paper filters of that breathability level. It closely matches the experiments for particle filtering (blocking) regarding cotton or paper filters of that breathability level.
1.00 - 0.39 = 0.61.
In other words, I claim that if a filtering material must rely on a simple "gap" vs "solid area" effect, then the ratio in the calculation above is simply that mask's breathability, apparently as true in practice as in theory.
And its effectiveness, for any virus-size particle, would be 1.00 minus that ratio -- at least roughly speaking.
And again, I'll follow the calculation rule further up this page when it comes to stacking layers of material.
Two mask types are listed below.
Even when wearing a mask, maximize your distance from people. Normal exhalation by an infected person contains viruses, and fabric masks offer just moderate protection.
My sister Pam makes the finest cotton masks - very well engineered with 3D contour - which have an opening for inserts.
I stuff cotton balls along the sides of the nose:
A pleated mask:
Do an accordian fold on three or four full sheets (11" x 11") paper towels. The paper towels I used were the simple, budget type paper towels. The more expensive paper towels, with enhanced strength or whatever, will give different effects -- which will affect how many layers to use.
Looking forward to trying shop towels for this if I come across some.
The perimeter fit is better than I thought it would be. However, there will still be plenty of leakage around the perimeter. Stuff cotton balls along the sides of the nose and the sides near the ears.
Breathing masks that do not incorporate a valve are essentially symmetrical in their functioning, meaning that they protect the wearer to a similar degree that they protect others -- whether involving sneezing, coughing or regular breathing. That applies to surgical masks of course. And I know you've read otherwise elsewhere.
AEROSOL TRANSMISSION VIDEO (length 3:20)
(thanks to Luiz Vinholi for the video)
.mov version -- click inside video window
.wmv version .mp4 version
© Roger Luebeck