The University of North Carolina at Chapel Hill

Face Masks: Types and Tips

06.01.20


Three UNC-Chapel Hill professors who've worked with face masks break down the different types, how they work and what you should know.

Authors:

Devin K. Hubbard, Ph.D., Teaching Assistant Professor, UNC/NCSU Joint Department of Biomedical Engineering; Lead Design Engineer: FastTraCS

Phillip Clapp, Ph.D., Assistant Professor, UNC Center for Environmental Medicine, Asthma, and Lung Biology

Paul Dayton, Ph.D., William R. Kenan Distinguished Professor, Interim Chair, UNC/NCSU Department of Biomedical Engineering

devin-hubbard-300x300

Devin K. Hubbard, Ph.D.

Phillip_Clapp

Phillip Clapp, Ph.D.

paul-dayton

Paul Dayton, Ph.D.

A Note on Masks & Respirators:


Look, we aren’t mask experts—but we’ve learned a LOT working on them in the past 12 weeks. So, if you’re at home sewing masks, or thinking about these problems like we are, maybe some of this will help.

Types of passive (non-powered) masks

There are a few major categories of passive (non-powered) masks:

1. Respirators – Designed to fit tightly and seal around the face. Respirators include N95, and are designed to filter out particles and protect the wearer from particulates. When worn correctly, these masks have an overall filtration efficiency of 95% or better.

 

2. Tie-style surgical masks – Designed to cover the face and nose, these masks tie behind the head. In the US, the those cleared by the FDA are designed to protect OTHERS from the wearer and to protect the wearer from splashes. These masks are not designed to fit tightly. When worn correctly, these masks have an overall filtration efficiency between 30-70%

3. Procedure (Ear-loop) masks – Designed to cover the face and nose, these masks hook around the ears and are designed to protect OTHERS from the wearer, and to protect the wearer from splashes (if cleared by FDA). These masks are not designed to fit tightly. These masks generally have overall filtration rates below 40% when worn correctly.

Key Takeaways

4. Home Sewn Masks – Designed to cover the face and nose, these masks are sewn from various materials and are intended to protect others from the wearer. Overall filtration rates for these masks are widely variable.

Three things for respirators to work as intended

First things first: Masks are NOT respirators [1]. In order for a respirator to function as intended, there are three major things that have to happen:

1. The material must filter particles out.

Respirators like N95s are tested and certified (by NIOSH in the US) to see if they can capture particles that are generally 0.3 microns in diameter (approximately 200 times smaller than the width of a human hair)—why that size?

Well, it turns out that size is particularly difficult to capture [2]. N95 respirators generally get a significant percentage of their performance from an electrical charge that is put into the material—which is one reason you should NOT wash an N95 respirator with soap and water: it can remove the charge and make the respirator less effective. It’s also one of the reasons that even the best sewn masks/respirators fail a quantitative N95 fit test: a needle will poke a hole that is hundreds or thousands of times larger than 0.3 microns.

3D printing a respirator? Well, if you’re using a filament/FDM printer, there will be gaps large enough for those pesky particles to sneak through in between the layers of plastic.

2. It must fit correctly and tightly to the face.

In order for a respirator to achieve and pass an N95 fit test, it MUST fit correctly. In practice, everyone who works in an environment that requires them to wear an N95 should regularly undergo a fit test to ensure they are wearing the correct size mask.

In our experience, the most common places for a mask or respirator to leak are around the nose (particularly where the nose meets the cheeks), cheeks, and under the chin. This is the biggest reason that ear-loop and surgical tie masks are so inefficient—large gaps.

Want to put a flap over a hole in your mask for your straw so you can drink? Think again—that is analogous to drilling a basketball-sized hole in your tub and trying to fill it up with nothing more than a towel covering the hole. It turns out getting a proper fit is probably the most challenging part of getting a respirator to perform to the best of its capability…the difference between 95% and 70% could be a leak the size of a pinhole.

3. There must be enough material.

This one may be less intuitive. When you breathe in, your lungs generate a negative pressure to draw air into your body. If you cover your nose and mouth with material, it increases breathing resistance because it effectively reduces the total area through which air passes on its way in (which is why it’s harder to breathe with a respirator or mask on).

The effect is similar to having to breathe through a straw. The difference is that instead of a single passage for air to go through, a mask/respirator has millions of tiny holes between the material that allow air to pass through.

Why does this matter? Well, if you decrease the total area of the material (ex: a 3D printed respirator with a 2”x3” filter), the pressure drop across the mask will increase (smaller straw: have to breathe harder to fill lungs). When the pressure-drop increases, the particles can sneak their way through and around the mask into your lungs. This turns out to be one of the reasons that 3D-printed respirators with small filters are less effective than they could be.

Face masks vs. respirators

Face masks are a LOT different than respirators.

 

Because ear loop, tie surgical, and sewn masks are really designed to protect the wearer from splashes and others from the wearer’s respiratory droplets (by slowing down/diverting them), their designs and materials are different than respirators.

Putting a HEPA filter material in your sewn mask might seem like a great idea, except that it serves very little purpose. Particles still find their way through the pinholes and gaps around the chin, nose, and cheeks. Not to mention, this style of mask is not intended to protect the wearer from respiratory droplets.

Let’s talk a bit about how masks are different from respirators:

First of all, they are tested to different standards. In the US, various ASTM standards are used to evaluate and classify masks into three “levels” (for those curious, the standards include F1494, F1862, F2100, F2102, F2299, and 16 CFR Part 1610). Level 3 masks are the most breathable, and most effective at blocking splashes, and stopping bacteria & 0.1 micron particles from getting through (think if the wearer coughs). Notice these materials are NOT tested to capture 0.3 micron particles (the hardest to stop).

 

Home sewn masks are not certified (though, you could send them off for certification if you found and paid a certifying lab to do so), and thus should not be relied on for protection for yourself from inhaling hazardous particles. Furthermore, a respirator (e.g. N95) is NOT the same as a mask…they aren’t even tested the same way! The analogy of comparing apples to oranges comes to mind: both are fruits, but that’s about as far as the comparison goes.

Masks are really designed to protect others from you—the distance that a cloud of droplets can travel can be reduced by more than 50% when wearing a mask [3], whereas respirators are designed to protect you from inhaling hazards.

In the context of COVID, since the virus can spread while a person has no signs nor symptoms, even if you feel perfectly healthy, you might actually be carrying the virus.  Wearing a mask can help protect others from catching it from you.

Works Cited

[1] C. f. D. Control, “Understanding the Difference,” [Online]. Available: https://www.cdc.gov/niosh/npptl/pdfs/UnderstandDifferenceInfographic-508.pdf. [Accessed 28 May 2020].

[2] J. L. Perry, J. H. Agui and R. Vijayakumar, “Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media Filters and Packed Beds of Granular Materials,” NASA, Huntsville, Alabama, 2016.

[3] D. S. Hui, B. K. Chow, L. Chu, S. S. Ng, N. Lee, T. Gin and M. T. Chan, “Exhaled Air Dispersion during Coughing with and without Wearing a Surgical or N95 Mask,” PLoS one, vol. 7, no. 12, p. e50845, 2012.

Share on facebook
Share on google
Share on twitter
Share on linkedin