We Put Double Masking Through a Leakage Test

This week the big trend in pandemic talk is "double masking." This is the practice of putting one mask on top of another to do a better job of filtering out virus aerosols.  Amanda Gorman did it at the inauguration, and we shouldn't underestimate the role of fashion here.

Here is a chart based on Google Trends over the past 20 days, showing how the phrase has been picking up:

The line really began to tick-up on the 25th, when Dr. Anthony Fauci endorsed the idea of double-masking by saying 

If you have a physical covering with one layer, you put another layer on, it just makes common sense that it likely would be more effective, and that's the reason why you see people either double masking or doing a version of an N95.

Experts agree that a very important aspect of mask effectiveness is how well it fits. Masks that don't fit properly allow leakage of unfiltered air when inhaling and exhaling. 

A leaky mask won't work well regardless of what material it's made of. So, Corona-zona Labs Senior Research Engineer Bryan and I did an experiment to find out how much doubling-up addresses problems with leakage. Our research questions were

1. does adding a cloth mash on top of a melt-blown medical procedure mask test help reduce leakage? And, 
2. does a second mask reduce leakage as well as a mask fitter?

Test Rig

Temporarily lacking fancy aerosol generating and measuring equipment here at The Labs, we decided on the test rig shown here:

That's a rigid foam head of the kind used to hold wigs. We bored a hole from the mouth to the back just big enough to accept the nozzle of a Samsonite LS-889 Dual Voltage Folding Travel Hair Dryer.  We measured hot air escaping the masks using a FLIR ONE Gen 3 infrared camera. We painted the head black to increase the thermal mass of the surface (foam has low thermal mass).

Here is a panel showing the setup in visible light and infrared, as well the test in infrared (taken after about 30 seconds of running the dryer on low heat) for each of three conditions.

Procedure Mask Alone

Here is an enlarged version of the Test-IR photo for the procedure mask alone, annotated with a few green ovals:

The colors go from blue (coolest) through purple, red, orange, and yellow through white (hottest) to indicate how hot the surface is. 

In this image we see the typical trouble areas for procedure masks.  There is a large leak under the chin, and another near the ear, which exists on the other side too. 

The biggest area of leakage is on either side of the nose, even though the nose wire was properly formed.  Those areas are just about as hot as the area right in front of the mouth.  So, this is a major source of leakage.

Procedure Mask + Cloth Mask

Here is the enlarged Test-IR image of a procedure mask with a cloth mask worn over it:

Here we can see that the bright areas under the chin and on the sides have been practically eliminated, so the combo has helped with that leakage. But there is still a large problem around the nose, with little to no improvement over the procedure mask alone.

Procedure Mask + "Fix the Mask" Fitter

For the third test we combined the procedure mask with a "mask fitter" called Fix the Mask. An earlier post summarized a study showing that such appliances can drastically cut down on leaks and improve the filtration efficiency of procedure masks.  Fix the Mask was my favorite in a recent review. 

Here is the Test-IR image of the procedure mask plus Fix the Mask:

The difference from the double mask is striking. There is no apparent leakage under the chin or on the sides.  Unlike the other two tests, the fabric of the procedure mask is uniformly bright, indicating that most of the hot air is passing through the mask fabric. There is still a little leakage on each side of the nose, but it is nothing compared to the previous two images. 

Conclusions

Our first research question asked: Does adding a cloth mash on top of a melt-blown medical procedure test help reduce leakage? The answer is yes. The procedure mask plus cloth mask combo does a good job of reducing or eliminating the leakage under the chin and on the sides. However, it still allows for considerable leakage on either side of the nose. It is hard to detect any real improvement over the the procedure mask by itself in this area. 

Our second research question asked: Does a second mask reduce leakage as well as a mask fitter? The answer is a definite no. Fix the Mask eliminated bottom and side leakage entirely, and almost eliminated leakage around the nose too. 

Limitations

There are some limitations to our tests. First, they apply only to leakage. Double masking probably has benefits other than just reducing leakage, like improving filtration. 

Second, we have no idea how the pressure from our hair dryer compares to the pressure created human breathing. The pressure seems to me higher than a normal exhalation, perhaps something more like that one would use to blow out candles on a birthday cake. 

Senior Research Engineer Bryan sees it differently:

I don't agree that the hair dryer obviously has a higher output pressure than a human exhalation.  Humans are pretty good pumps.  I think the airflow is ok as long as it doesn't deform the mask and open vents that aren't there as a result of the mask not fitting.

Regardless, the same setup was used for all three tests, suggesting that the comparisons are valid even if the pressure is not a good analogue for human breathing.

Third, for obvious reasons we could only study exhalation leakage with our test rig.  It may be that the difference between the double-mask and fitter conditions would be less for inhalation because the vacuum would tend to pull the doubled mask closer to the face.

Finally, the foam head used in our test setup is the equivalent of a small hat size (23.5 in circumference). It may be that the nose-seal produced by the fitter would have been better with a more normal-sized fake head.

Header image by Sumanley xulx from Pixabay. Other images by Corona-zona.