N95 Mask Using CPR Pocket Face Mask and Air Purifier HEPA Filter

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.