A single layer tea towel up against the mouth is difficult to breath through, but it can be made much more comfortable by opening the mouth wide when breathing. This increases the surface area that is available for filtration. This should also reduce the velocity of air approaching and passing through the filter during inspiration and expiration. Given that a lot of studies show increased penetration rates with increases in “face velocity,” this is probably a good thing. The downside is that if there is a hole or other defect in the filter, a wide open mouth is more likely to be breathing through a defective location than a less open mouth.

Tea towel penetration rates compared to surgical masks and FFP2 (surgical N95) mask.

• The study reported findings in the form of a “Protection Factor.” I’m trying to make sense of the math on how it is derived, but a PF of 1 is no mask at all. The tea cloth mask had a PF of 2.2 to 2.5, depending on the activity of the user. The surgical mask had a PF of 4.1 to 5.3. The FFP2 mask had a PF of 66 to 113. The highest ratings for each are probably a better measure of filter media performance. Thus a surgical mask is twice as good (5.3 vs 2.5) as a tea towel. And an FFP2 mask is 45 times better (113 vs 2.5).
  • If you invert the PF numbers (“The inverse of the PF (1/PF) can be interpreted as a probability (that any particle succeeds in moving through the barrier the mask provides”):
    • Tea towel = 1/2.5 = 0.4 = 40% penetration rate = 60% filtration rate
    • Surgical mask = 1/5.3 = 0.189 = 18.9% penetration rate = 81.1% filtration rate
    • FFP2: 1/113 = 0.00885 = 0.1% penetration rate = 99.9% filtration rate
• van der Sande M, Teunis P, Sabel R. Professional and home-made face masks reduce exposure to respiratory infections among the general population. PLoS One. 2008;3(7):e2618. Published 2008 Jul 9. doi:10.1371/journal.pone.0002618
  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440799/

Doubling the layers of a tea towel filter bring filtration performance to the level of a surgical mask for larger particles (bacteria that are ten times larger than COVID-19). Single layer tea towel performance is lower than a surgical mask, but still pretty good, for smaller particles (bacteriophages that are one quarter the size of COVID-19). Some good caveats at the end of the paper about their test materials being “fresh” (i.e. clean) and unworn (i.e. new). A ragged old towel that is wearing through is probably not going to work as well. And cleaning and drying fabrics does tend to tighten up the fibres, which is why your jeans don’t fit as well after washing them. Also good to note is that dried out virus particles may obtain easier passage through a filter.

• Double layer of tea towel has a pressure drop of that is twice as high as a surgical mask. This resistance to breathing would make use uncomfortable and or impossible for practical use unless the surface area for filtration is increased.
• Bacterial penetration test: A single layer of tea towel blocked 83.24% of aerosolized bacteria (0.95 um – 1.25 um, which is ten times larger than the 0.1 um COVID-19) and a double layer of tea towel blocked 96.71%, which is comparable to a surgical mask at 96.35%. A few of the other fabrics were tested by doubling the layers and found no significant improvement, while the tea towel had a significant gain in blocking performance.
• Viral penetration test: A single layer of tea towel blocked 72.46% of aerosolized bacteriaphage (23 nm or 0.023 um, which is four times smaller than the 0.1 um COVID-19) which is good, but not as good as a surgical mask at 89.52% blockage. The study did not test viral penetration against double layers of fabric. Although different methods were used, these numbers are close to those obtained in the van der Sande paper mentioned above.
• Caveats: “The materials used in this published study were fresh and previously unworn. It is likely that materials conditioned with water vapor, to create a fabric similar to that which has been worn for a couple of hours, would show very different filtration efficiencies and pressure drops. In contrast, a study of breathing system filters found a greater breakthrough of bacteriophage MS2 on filters that had been preconditioned. Although the droplet sizes for both virus and bacteria were the same and affected the filter media in a similar manner, it was suggested that the viruses, after contact with the moisture on the filter, were released from their droplet containment, and driven onward by the flow of gas.”
• Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?. Disaster medicine and public health preparedness.
  Davies, Anna & Thompson, Katy-Anne & Giri, Karthika & Kafatos, George & Walker, James & Bennett, Allan. (2013). Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?. Disaster medicine and public health preparedness. 7. 413-418. 10.1017/dmp.2013.43.
  https://www.researchgate.net/publication/258525804_Testing_the_Efficacy_of_Homemade_Masks_Would_They_Protect_in_an_Influenza_Pandemic

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This is an improvement on my N95 Mask Using CPR Pocket Face Mask and Air Purifier HEPA Filter design. Compared to that design:

• Breathability is greatly improved by increasing the surface area of the airflow through the filter medium. With a larger diameter canister (pill bottle), lower efficiency filter media (higher resistance to breathing) can be used as surface area for airflow is increased.
• The filter medium is not as subject to deformation that could reduce performance or release potentially hazardous reinforcement material (e.g. fibreglass).
• Construction still requires minimal skill and basic tools (a hot nail could be used in place of a drill), though assembly is less straight-forward. Decontamination will be more difficult.

Parts:

• CPR Pocket Face Mask
• Pill bottle
• Plastic sandwich bag
• Elastic bands
• Filter media. This could be of any material available, from better than nothing to excellent. In this example, the patch of filter media was harvested from a True HEPA filter unit for a home air purifier. WARNING: This filter media probably contains fibreglass, which is something you do not want to inhale. Hot gluing the edges of the patch and sandwiching the patch between two layers of cloth might help. But I am playing it safe and moving on to other materials that are guaranteed to be non-hazardous.

Tools:

• Scissors
• Drill or hot nail

Future work:

• Larger canister to support use of double layer of tea towel, which has terrible resistance to breathing, but apparently pretty good filtration performance.
  • See follow-up post: Efficacy of Double Layer of Tea Towel as Filter Medium
• Salt functionalized filter element inside or outside canister.

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WARNING: HEPA filters often contain fibreglass, which can cause lung damage and skin irritation. You should make sure that the fibreglass fibres from the filter media harvested from HVAC filters has been contained to your satisfaction. I have had it suggested that one should hot glue the edges or use a food-safe epoxy and to sandwich the filter media layer with two layers of cloth. If you know of a standard containment method or test protocol to see if fibres are properly contained, please comment below or use the contact form. In the meantime, I AM NOT USING HARVESTED HEPA FILTER MEDIA ANYMORE and NEITHER SHOULD YOU unless you can be certain that you have contained any fibreglass.

Summary: N95 mask assembled with basic tools and minimal skill from commonly available CPR pocket face masks and commonly available filter media sources. The design appears viable, but breathability will be dependent on filter material selection and the inner diameter of the filter/flap unit airway where it meets the mask body.

Resistance to breathing was unacceptably high when I used the MCR Medical RM-2033 pocket face mask with the filter media extracted from the Holmes aer1 True HEPA Allergen Remover filter (Product code: HAPF300AH). With the design tested, the filter surface area actively used was 176mm^2 (15mm diameter airway aperture). Breathability was improved when the filter media was simply held up against the mask body with fingers such that the airway aperture and filter surface area employed was 415mm^2 (23mm diameter airway aperture in mask body). Breathability through non-woven polypropylene was acceptable for short durations. Breathability through a light T-shirt was wonderful. The reinforcement layer material for the filter is unknown, but suspicious (could be fibreglass), and fragments breaking off may cause lung irritation if inhaled. 

Some very good safety points on using HVAC filters due to fibreglass content:

Design could be viable with less resistive filter media or with a CPR pocket mask whose aperture through the filter/flap assembly was wider.

Filter media not secured to canister yet. Top of bottle sealed to plastic bag with elastic band.

UPDATE: External canister design with improved airflow and reduced filter media deformation can be found here: Improved N95 CPR Pocket Face Mask Using External Canister. This will allow you to use other filter media or multiple layers of different media that are harder to breathe through.

Could also create a filter bag of higher surface area over the entire filter/flap assembly, placing the filter/flap unit into the bag, then using the filter/flap to lock and seal the filter bag. Filter bag could be partially of filter media and partially of plastic. Make it reasonably small in volume compared to normal tidal volume of 500mL to avoid creating a rebreather mask.

Design Goal: A nearly snap-together N95 mask using commonly available materials that replicates the simplicity of the recently publicized concept of using disposable hospital supplies like an anaesthesia mask and ventilator filter. No special tools or skills should be required to assemble or test.

Real-life Viability Status: 75% ready.

Proposed Components:

• CPR pocket face mask – Found in better first aid kits. Uses an air bladder to provide a good and relatively comfortable seal around the face. Often equipped with a light elastic strap that is intended to keep the mask from falling off a patient’s face, not to effect a tight seal. They have a similar decontamination protocol to the one proposed in the hospital supplies based filter mentioned above (isopropyl alchohol and/or dish soap and/or mild bleach solution). They can be purchased for between $3 and $10 USD
• N95 or better filter media harvested from an air purifier filter unit – DO NOT USE THIS UNLESS YOU CAN BE CERTAIN THAT THE FIBREGLASS IS CONTAINED. Found in large quantities in “true” HEPA grade filters used in home air purifier units. The box for the filter I have has such claims as “For Best Performance Change Filter Every 12 Months” and “Removed up to 99.97% of airborne allergens as small as 0.3 microns (um) from air passing through the filter. A $10 USD filter I purchased online had a 9cm x 22.5cm airway, but was constructed of pleated material with a pleat depth of 2cm. I counted about 51 pleat ridges, with each pleat about 9cm wide and 2cm deep. Thus the potential surface area of filter media in the filter is approximately 9cm x 2cm/side x 51 pleats x 2 sides/pleat = 1836 cm^2. Assuming (need to research this further) that each filter requires a square patch of 10 cm^2 of material (3.3 cm x 3.3 cm or 1.25″ x 1.25”), a single $10 filter unit could yield enough filter media to construct 100-184 mask filters with an operating lifespan of 1 year, though this lifespan is based on using it in an air cleaner, rather than breathing through it with moist air. You would also need to consider how to decontaminate the filter itself if you plan to use it for more than a day. This is a filter media cost of $0.05-$0.10 USD/filter. Patches of filter media can probably be extracted with ordinary scissors.
• Filter mounting/sealing system – CPR pocket face masks often have a removable/replaceable filter and flap unit that is friction fit to a hole in the front of the mask body. It is designed to protect the rescuer providing the ventilations by preventing the rescuer from inhaling the patient’s breath or bodily fluids. It also allows patient expiratory air to vent to the outside though an exhaust port that is closed by the flap when the rescuer is providing ventilations. And it allows a spontaneously breathing patient to inhale through the exhaust port. There are two options at this point:
  • Option 1: Rebuild filter/flap assemble as filter-only. Break the filter/flap unit open, remove the flap, replace the existing filter with the N95 media harvested above, seal the exhaust port, and insert a small patch of the filter media harvested from the air filter. This would conserve filter material. But it will also destroy the functionality of the filter/flap unit for CPR purposes and possibly damage it beyond repair for our own purposes by compromising its sealing properties. I do not plan to try this.
  • Option 2: Jam filter media into mask body airway and keep/seal it in place with the old filter/flap unit. Remove the filter/flap unit from the mask body. Cut out a patch of filter media slightly larger than the hole in the mask body. The airway hole on the CPR mask body I have has a diameter of 23 mm. Place the patch over the hole on the outside of the mask. Re-insert the fliter/flap unit over the patch to hold the patch in place and seal it with a friction fit. Tried this today. See below for results.

Method

• I employed Option 2, summarized above – Jam filter media into mask body airway and keep/seal it in place with the old filter/flap unit.
• Construction
  • Parts:
    • Holmes aer1 True HEPA Allergen Remover filter (Product code: HAPF300AH)
    • MCR Medical Model RM-2033 Adult/Child CPR Rescue Mask Kit
  • Tools: Scissors. Plastic zipper storage bags to hygienically store extracted samples and the filter.
  • I washed my hands with soap and water and sanitized the scissors with isopropyl alcohol, ensuring that the alcohol had evaporated before allowing the scissors to make contact with the filter media. If I choose to make the filter media I have extracted available to others to breathe through, it should be as sterile as possible.
  • I used ordinary scissors to cut through 5-6 pleats in one zone between filter support columns from the front and back sides of the filter. I then cut through the ridges of the end pleats of my cut area to release the sample of media from the filter. In retrospect, a sharp knife should have been used to cut through the ridges.
  • The media sample extracted from the air purifier filter was 0.36mm thick and about 5cm wide by 30cm long (150 cm^2) when flattened. Ten more such samples could be extracted of that width. Narrower samples could be extracted from other parts of the filter.
  • The filter media was of multi-layer construction. There was a softer cloth-like layer on one side and and a stiffer, possibly fibreglass reinforcement layer on the other.
  • I then cut out an approximately 5cm x 5cm patch of filter media. The edges should probably be hot glued (like in the original filter unit) and the media sandwiched in two layers of cloth. This will hopefully control inhalation of fibreglass. I don’t know and if you don’t, you shouldn’t. SEE WARNING AT THE TOP OF THIS PAGE.
  • I removed the filter/flap assembly from the filter mask and set it aside.
  • I centered the 5cm x 5cm patch on top of the airway hole in the filter mask. The softer layer was facing towards the mask. The more rigid reinforcement layer faced away from the mask. Given that I do not know what the reinforcement layer is made from and it could be fibreglass, I decided it would be best not to ensure that I am directly inhaling fragments that could break off while the filter media is deformed and crushed into place. Inhaling fibreglass can cause irritation and damage to the airway/lungs. SEE WARNING AT THE TOP OF THIS PAGE.
  • I then pushed the filter/flap assembly back into the airway hole as if I was reassembling the CPR pocket face mask, but with the filter media in place.
  • Ready to test.

Testing

• Seal test – This test does not require the N95 filter media. We are testing the seal of the mask to your face. Remove the filter/flap unit from mask. Cut out a piece of plastic cling wrap that is twice as large as the opening (or use a lightweight plastic bag). Put on the mask. Breath normally to ensure that the airway is large enough for you to be comfortable. Lightly press the piece of plastic wrap over the opening in the mask where the filter/flap unit was. Attempt to inhale. If a vacuum is not created, you do not have a good seal. Attempt to exhale. If the mask does not blow off of your face, you do not have a good seal. It might be good to add a second elastic band as the supplied elastic is not designed to create a seal, but to keep it from falling off of a patient’s face during CPR.
• Filter test
  • Test for resistance. Is breathing comfortable?
  • Apply the smoke test. I haven’t done this part yet, but will update this with my results today.

Test Results

• Seal test on myself (40 year old male, tall, thin, oblong face) of the CPR pocket face mask with a piece of plastic wrap over the airway was conducted last night. It was a success. I would suffocate if the plastic wrap was kept in place. Exhalation pushed the mask off of my face if the plastic wrap was kept in place.
• Filter tests
  • Resistance. Very high resistance to breathing was found when using the filter media harvested from the Holmes aer1 True HEPA Allergen Remover filter (Product code: HAPF300AH). Breathing was possible, but not comfortable. Exhalation tended to push the mask off of my face, though it was not strongly attached as it was only held on by the single light elastic strap. Given that air purifiers are electrically powered units with blower fans, the ability to comfortably breathe through a 176mm^2 (15mm diameter airway aperture) surface area of their filter units was probably not a concern of their design engineers.
    • Comparison: Resistance to breathing was lower using a filter media made from a patch of material from a 0.25 mm thick non-woven polypropylene reusable shopping bag / tote that I had on hand. Of course the shopping bag is not a HEPA filter…yet.
    • With the filter/flap assembly removed and the filter media simply held up against the mask body airway (23mm diameter circular airway aperture with an area of 415mm^2) with fingers, resistance to breathing was improved, but was still not adequate for any usage beyond a few minutes. With this particular model of mask, a different means of locking in and sealing the filter media would be necessary to use the larger airway aperture.
  • Smoke test. Forthcoming to validate filter media performance against manufacturer claims (which will be of value for other designs) and to test sealing of filter with the mask (which will validate this design, but not make up for the poor breathability of the media tested today).

Conclusions (so far)

• Pass. The CPR pocket face mask appears to be a viable method of obtaining a seal to the face and allowing the quick deployment of a choice of filtering media for experimentation or live use.
• Fail. Use of the filter media from the Holmes aer1 True HEPA Allergen Remover filter (Product code: HAPF300AH) is not recommended as the heavy breathing effort would one would quickly fatigue respiratory muscles and one would have to quickly choose between respiratory failure (that’s bad) and removing the mask. Anyone who is claustrophobic would quickly freak out. Also, I am unsure of the nature of the reinforcement material and its potential for irritating or damaging the airway if broken off particles are inhaled. Probably better to use with a larger surface area opening and with a mounting system that does not rely on heavily deforming the material to lock it in place and seal it.

Further work

• Finding a better filter material. Other, higher efficiency HEPA filters may have a lower resistance to breathing.
• Could also adapt a filter bag of higher surface area over the entire filter/flap assembly, placing the filter/flap unit into the bag, then using the filter/flap to lock and seal the filter bag. Filter bag could be partially of filter media and partially of plastic. Make it reasonably small in volume compared to normal tidal volume of 500mL to avoid creating a rebreather mask.
• Smoke test.

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The below video features a relatively comfortable, re-usable N95 (or higher) mask that can be constructed easily in minutes using “disposable” materials on hand in most hospitals. The video also includes a simple decontamination protocol, but I would be very careful to ensure that you do not apply alcohol to the filter media, as contact with alcohol will usually destroy a significant portion of the filter’s capability.

The concept is excellent. But the average person does not have access to anaesthesia masks or ventilator filters. In a follow-up post, I will document my development of a similar N95 mask using a commonly available CPR pocket face mask.

Making Your Own Reusable Elastomeric Respirator For Use During Covid-19 Viral Pandemic N95 Shortage

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Polypropylene (reusable shopping bag) surgical masks and possible DIY enhancements to filtering ability such as hydrocharging

• UPDATE #2: The middle layer of commercial surgical masks is made of webs of non-woven polypropylene, the same material used to make the vegetable cover cloth used by vegetable farmers to protect plants from frost and insects. Though further research is needed, it appears that vegetable cloth might be useful in making DIY surgical masks with the same protective qualities of commercial surgical masks. Hydrocharging the vegetable cloth couldn’t hurt.
  “Both the 1816 and 1818 masks are three layer masks with non-woven polypropylene filter webs”
  https://multimedia.3m.com/mws/media/82004O/common-questions-3m-tie-on-surgical-masks.pdf 
  Though not medical grade, and possibly of different consistency, this is ground cloth / vegetable cloth / tobacco cloth. One of the major brands is Agribon, produced by Berry Plastics.
  Berry Global Produces Materials Critical for Coronavirus Defense; Expands Wipes Capacity
  http://ir.berryglobal.com/news-releases/news-release-details/berry-global-produces-materials-critical-coronavirus-defense
  https://youtu.be/m4O-3wQQjpA?t=60
• UPDATE #1: The below technique applies to “microfiber webs” of melt-blown non-wovn polyproylene, which is a more fibrous material (more like gauze) than shopping bags and is what is often used as the lining of the baby side of disposable diapers. Although I doubt spraying your shopping bag with water can make things worse, they probably won’t have the same properties.
• Untested proposed technique – Make surgical masks (or filter media) out of reusable shopping bags (constructed of non-woven polypropylene). Then treat the mask fabric material to improve filtration performance by spraying both sides with water at 25 PSI (170 kPa) from a pressure washer / power washer / garden hose nozzle. Accelerating the drying time is less of an issue as this is not a mass production process and we are not planning to immediately put the fabric into large rolls. But do not apply heat. I do not know if this method will survive sterilization by boiling in water, so it may not be applicable to healthcare settings. But for your own individual use, it is as good as your own hygiene practices.
• Real-Life Viability Status: Half-baked. Not tested yet.
• A reusable shopping bag in my home had a fabric thickness of 0.36 mm to 0.40 mm, but had a weave pattern stamped into it such that I could compress it to a thickness of 0.24 by squeezing the micrometer calipers with my fingers. The web material in the study had a thickness of 0.1 cm (1mm).
• Non woven polypropylene (NWPP) is commonly used in surgical masks and can be easily sewn into masks that can block the spread of droplets from coughs and sneezes.https://www.project-cloth-masks.com/ – Using non-woven polypropylene
  https://www.project-cloth-masks.com/about – Research
• Hydrocharging, the use of a high pressure water jet to enhance the filtration capability of non-woven polypropylene (NWPP) is detailed in a 3M patent application (see below). Note that a patent application (even if granted) is not proof that a technique actually works or works well, just some legal assurance that the technique is new, so we should dig up some research on actual efficacy. Always independently test new concepts from strangers on the internet.
  “Surprisingly, it has been found that merely by impinging these jets of water or stream of water droplets onto the nonwoven microfiber web, the web develops filtration enhancing electret charge. The charging can be further enhanced by subjecting the web to corona discharge treatment prior to impingement by the water. Preferably, the web is formed from melt blown polypropylene microfibers, poly(4-methyl-l-pentene) microfibers or blends thereof. The term “hydrocharging” will be used herein to describe this method.”
  “Each sample passed beneath the spray bar at a rate of 3.5 m/min, and was treated once on each face, vacuum extracted [Alex’s note: this is to speed up drying in mass production] and dried at 70 °C for one hour…hydrocharging (at pressures of at least about 170 kPa [Alex’s note: This is 25 PSI.]) develops useful levels of electret enhanced filtration characteristics”
  “FIG. 4 is a perspective view of a pump action sprayer useful in the present invention”
  Method of charging electret filter media
  Agadjivand, S. A.; Jones, M. E.; Meyer, D. E. PCT International Application WO 9505501, 1995.
  • https://patents.google.com/patent/WO1995005501A2/en
  • https://patentimages.storage.googleapis.com/e4/7b/61/4bd0a413bd14df/WO1995005501A2.pdf
• “Another method of charging which appears to rely heavily on triboelectric effects is hydrocharging (10). Hydrocharging involves the use of a high pressure water jet directed through a nonwoven filter media in order to impart charge. It is reported that quite good filtration media can be made using the hydrocharging technique (10). A drawback of the hydrocharging technique is the drying of the charged filters. If the drying is done at an elevated temperature, thermal charge carries are liberated within the dielectric leading to charge dissipation. If the web is insufficiently dried, the residual moisture can lead to fungal or bacterial contamination of the filter media during storage.”
  Electret Media For HVAC Filtration Applications
  By David L. Myers and B. Dean Arnold, Nonwoven Fabrics Research and Development, Kimberly-Clark Corporation, 1400 Holcomb Bridge Rd., Roswell, GA 30076
  • https://www.jeffjournal.org/INJ/inj03_4/p43-54-myers.pdf

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