Badger Seal (Mask Fitter)

What is a mask fitter? A soft, flexible and adjustable “frame” that significantly improves the outer seal of a mask.

Why use it? Adding the Badger Seal to a 3-ply disposable mask reduces the effective particle penetration by typically 15x (see the Performance section below).

What makes the Badger Seal unique? It’s cheap (< $1 in materials), easy to assemble, made from readily available materials and tools, comfortable, quickly customizable and open source. There are other fitters out there, though none seem to excel in all of these areas.

What type of mask should I use? The Badger Seal with a 3-layer disposable mask was shown to have great filtration performance while also being low-cost and made from a very breathable material (see the Performance section below). Significant improvements were also seen when the Badger Seal was worn with a 4-layer knit cotton cloth mask or an ASTM Level II mask. Since the outer edges will be much more sealed than usual, you must ensure your mask is made of a sufficiently breathable material.

How can I make a Badger Seal? Click here for the Open Source design including detailed steps, drawings and videos.

Warning: While our fitter seems to improve the seal of a face mask, it has not been tested for effectiveness in preventing illness, and you should use it at your own risk. See legal disclaimer at the bottom of this page.

Obtaining / Purchasing:

Option #1: A Madison-based manufacturer is selling Badger Seals here (separate site).

Option #2: If you are part of UW-Madison, you can use the normal PPE request form here and list “Badger Seal” + the quantity you need in the “other” section.

Option #3: Here’s another “mask fitter” that’s being produced, though it’s not the Badger Seal.

Regardless, please fill out our interest form here and we’ll notify you once other options / updates become available.


Why type is best for you? We are promoting two models: Original and Ear Loop.

[-] Original: Worn as an add-on over a mask. It has a single cord around the back of the head and can be worn comfortably for many hours:

[-] Ear Loop: Attaches to a mask at 3 locations using hook-and-loop material. The entire assembly is worn using standard ear loops. It is very easy to put on and remove, but it generally requires ear savers if worn for more than about ½ hour at a time. People with large necks, beards, etc. find that this model fits better than the original. This model can also be made out of pipe cleaner.

Quick-start Guide (watch first before using):

Detailed How-to Use Guide:

How-to Make Guide (see open source design doc here for more details):


Caption: Visualization using a mannequin that exhales breaths containing fog.

Below are results from tests on a classroom full of mannequins by David Rothamer and Scott Sanders at UW-Madison (reference coming soon). Aerosols were emitted by a mannequin instructor placed at the front of the room. The Effective Filtration Efficiency (EFE) was measured by another mannequin in the corner of the room near the HVAC return.

The Badger Seal (UW Fitter) is shown below to offer significant improvement in the filtration (EFE) for these types of masks tested:

[-] 3-layer disposable mask manufactured by Hodo

[-] ASTM Level II mask manufacturer by Medicom

[-] 4-layer knit cotton cloth mask

When a Badger Seal was worn over a low-cost, 3-layer, disposable mask made by Hodo, it was found to be:

[-] 4x to 8x higher effective filtration efficiency than a 4-layer knit cotton cloth mask (comparison is dependent on the type of material and the number of layers for the cloth mask).  These efficiency gains result in 15 – 18 times lower particle penetration through the mask for both inhalation and exhalation.

[-] Approximately 3x more efficient than a standard ASTM 2 surgical mask worn without a mask fitter because of the typically poor fit.

The Effective Filtration Efficiency of the disposable mask with the fitter was 95% for particles as small as 30 nm (Note: An N95 mask will have an effective filtration efficiency of at least 99% under similar conditions).

Click here for a news article summarizing a research-based guide to choosing the right mask for you, which is based on work done by members of our team and features the Badger Seal.

Caption: The Effective Filtration Efficiency (EFE) of the following four mask types were compared in a classroom setting:  a 3-layer disposable mask manufactured by Hodo, an ASTM Level II mask manufacturer by Medicom, a 3-layer mask made of spun-bond polypropylene (EOC Mask), and a 4-layer knit cotton cloth mask. Each mask was tested with (1) no fitter, (2) the Badger Seal (UW Fitter) and (3) the “Fix the Mask” (FTM) fitter shown here.

Open Source Design:

Click here for the Open Source design including detailed steps, drawings and videos.

There’s also another version here, which doesn’t require tubing.

See our other COVID-19 projects here

This product design and specifications are being provided by the University of Wisconsin-Madison College of Engineering as a free service to the community during this public health emergency.

Regulatory Notice

This product has not been cleared for medical use by the U.S. Food and Drug Administration.  The product should only be used during the declared COVID-19 public health emergency if FDA cleared or approved products are unavailable.  This product has not been evaluated for the prevention of specific diseases or infections, the filtering of surgical smoke or plumes, the filtering of specific amounts of viruses or bacteria, or the killing or reduction of the amount of viruses, bacteria or fungi.  Not recommended for use in a surgical setting where exposure to liquid, bodily or other hazardous fluids may be expected or for use in a clinical setting where infection risk level through inhalation exposure is high.  Not recommended for use in the presence of a high intensity heat source or flammable gas.  [This product is intended for one-time use by a single user.]

Liability Notice

By using this product, the end user assumes all risks associated with its use, including the voiding of manufacturers’ warranties of devices with which the product is used, and agrees to hold harmless and waive any claims against the Board of Regents of the University of Wisconsin System, its officers, employees and agents, and other persons or companies which may have contributed to the design and specifications.  EXCEPT WHERE SPECIFICALLY PROHIBITED BY LAW, NO WARRANTIES OF ANY KIND ARE OFFERED FOR THE PRODUCT DESIGN AND SPECIFICATIONS, INCLUDING WARRANTIES OF NON-INFRINGEMENT AND FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property License

Individuals and organizations are free to use, copy, and share this design and specifications, including for commercial manufacture, without payment of any fees or charges, and for the duration of the declared COVID-19 public health emergency only, but may not assert ownership in the design and specifications, ownership of which belongs to the Board of Regents of the University of Wisconsin System and/or the individuals who created the design.

Notice to Manufacturers

Individuals or organizations that manufacture products utilizing this design and specifications are responsible for any federal or state regulatory requirements that apply to the manufacture of products intended for medical use, and are responsible for informing health care providers to which the product is supplied that they are responsible for decisions regarding appropriate personal protective equipment for their personnel.  The distribution or sale of products manufactured using this design and specifications must include the Regulatory Notice and the Liability Notice as set forth above displayed prominently on packaging and marketing materials, as well as any information required by the FDA or other governmental authorities.