A needgap user shared a problem of not being able to find surgical masks to not spread her common cold and cause panic during the COVID-19 pandemic.
I shared with her instructions for DIY coffee filter based face mask, but with an average of 20 μm pore size its ineffective against most aerosols. I had been testing different materials to see if it would make a better filter and I might have something to share.
Before you come coughing at me, please read the caveats carefully of which there will be plenty in this post.
The whole premise of this project is Something is better than Nothing.
It is assumed that a face fitting mask with good seal is better than dirty, loose face mask. Respirator masks are required to be fit tested, there’s no guarantee that even high rated N95 masks procured online or over at retail would serve its purpose without fit testing.
As surgical masks have been hoarded by general population, although it’s effectiveness in protecting against viruses is debated1 health care workers who are working with COVID-19 patients are in dire situation. I’m exploring the possibility of DIY face mask which can be easily built and is effective enough to replace the needs of general population; so that the surgical masks can be left for medical workers.
Surgical masks are by design to be used once and discarded carefully in the medical waste. But since it’s been largely used by untrained population now, surgical masks themselves post health risk to those who wear it and others who come in contact with the contaminated masks.
I’m exploring the alternative of DIY mask which can be reused without loosing its effectiveness.
I make no claims that this mask will protect you against virus or other aerosols. Use this mask at your own risk.
The foundation for the virus deactivation built-in filter is taken from research paper Universal and reusable virus deactivation system for respiratory protection1.
The research shows salt (NaCl) coated filter is very effective in filtering and deactivating H1N1 virus when compared to bare-filter used in surgical masks.
The salt under the aerosol (on filter) dissolve and increases the osmotic pressure to virus. In enveloped virus such as H1N1, cell wall is breached due to osmotic pressure difference within the cell and outside the cell (when both contain salt). SARS-CoV-2 which causes COVID-19 is also an enveloped virus.
When the salt recrystallizes, the salt crystals damage the virus physically.
Salt (NaCl, Phosphate etc.) as virus inactivation agent has been studied for over a century with varied levels of acceptance in the scientific community2. The virus inactivation can happen in other ways from the one discussed above, such as disruption of electrostatic interactions between the proteins3.
Note : These research are about using salt for virus deactivation before it enters the human cell. Salt water gargling will not prevent COVID-19 and has been debunked as a myth.
A small side track,
Talking of proteins, proteins are the key to finding ant-viral drugs for the coronavirus. If you would like to learn how and help by playing a game; check out this tweet thread for more information.
The design for the construction of this face mask is borrowed from Mr. Teslonian4, one of the few remaining Youtubers who make videos without Avatar level production budget.
His design uses Coffee Filter Paper, but I decided to use Qualitative Filter Paper instead.
Qualitative Filter comes in various pore sizes, most commonly from 11 μm (Grade1) to 2 μm (Grade 602 h). Where as Coffee Filter’s pore size averages at 20 μm.
Most School, Colleges have Qualitative Filter paper in stock and if this design is proven effective; face masks can be made from current stocks of qualitative filter paper.
Qualitative Filter paper can be procured in various sizes (diameters), useful for the construction of face masks for different sizes.
Each SARS-CoV-2 virion is approximately 50–200 nm in diameter.
Common surgical masks are made from Polypropylene fibers, they are supposed to filter out particles above 1 micron although its efficiency is widely debated 1. It’s not just the pore size which determines the efficiency of the filtration, the way fibers are arranged in the filter material plays a crucial role.
Filtration happens through various mechanisms such as Inertial impaction, Interception, Diffusion, Electrostatic attraction etc..
It is being assumed that 3 layers of Qualitative Filter paper with salt based virus deactivation layer in the middle provides necessary filtration as required in a face mask.
Qualitative Filter Paper
Mix a 1 tbsp common table salt(NaCl) with 2 tbsp of hot water in a zip lock bag (when warm enough to pour) and soak a qualitative filter paper in it. Ensure that the filter paper is completely soaked in the brine solution.
Becareful not to get the salt in-between locking mechanism of the plastic bag, as that may make the bag difficult to use for the next time.
You can keep the filter paper soaked in brine inside the bag itself for few hours or even a day to ensure that it has soaked well.
Note that the filter paper would still be wet when you take it outside the bag and that’s fine; you can build the mask when it’s still wet. But ensure it’s not dripping brine solution, you can hold the filter paper on the top of the bag to ensure excess is drained.
Qualitative Filter Paper in Salt Brine Solution
Two longitudinally folded pieces of Aluminium Foil Buy from a 5” wide strip is used for structural rigidity at nose and chin regions. The salt brine soaked filter paper is wedged between two dry filter papers.
Aluminium Foil Pieces for Nose and Chin
Thread and Tape
Filter paper are folded at the edges in opposite directions with aluminium foil inside and taped to form a good seal.
Same is repeated at the other edges, but with threads in between instead of the aluminium foil.
The filter paper is then folded in half, such that threaded sides are next to each other and the gap before the thread is closed with the tape (Don’t worry, it should become clearer with the video in the next section).
Make sure that all gaps except the area where expansion is needed i.e. cheek area, are sealed completely with tape. The cheek area would get sealed automatically by your cheek and the mask.
You can see Mr. Teslonian’s video for step by step construction video on how to make this mask, only change being the filter used.
Here is the photo of the final constructed mask.
Final DIY Qualitative Filter face mask
Once you have finished the mask, breath through it; You should now breath through the filter papers only, if you find any large gaps seal it with the tape.
Never thought this would give a good reason to shoot a selfie video since I stopped posting any personal picture online (and removed all earlier ones, but one) after Deep Fake became consumer friendly.
The mask is making a good seal around the nose, mouth regions as evident in this video by the movement of the mask when breathing.
Effectiveness of DIY Qualitative Filter based face mask in filtration of the aerosols and deactivation of the virus with the salt coated filter layer in the middle is unknown. The effectiveness needs to be tested in a proper research lab as done with the research1 quoted in this project.
Qualitative (not to be confused with the qualitative filter paper) and Quantitative fit test of the DIY Qualitative Filter based face mask needs to be conducted to check the effectiveness of the protection offered by it.
If you have any queries on DIY Qualitative Filter Face Mask, contact me on Twitter.
Please don’t forget to share this content with anyone who may find it useful.
 Quan, F., Rubino, I., Lee, S. et al. Universal and reusable virus deactivation system for respiratory protection. Sci Rep 7, 39956 (2017). https://doi.org/10.1038/srep39956
 Virus inactivation by salt (NaCl) and phosphate supplemented salt in a 3D collagen matrix model for natural sausage casings. Wieringa-Jelsma T1, Wijnker JJ, Zijlstra-Willems EM, Dekker A, Stockhofe-Zurwieden N, Maas R, Wisselink HJ. https://www.ncbi.nlm.nih.gov/pubmed/21632134
 Influence of salts on electrostatic interactions between poliovirus and membrane filters. P A Shields and S R Farrah. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC242318/
I did not monetize this content earlier, as I did not want to gain anything out of this crisis.
But, this content has blown out and I’ve already reached my bandwidth limits even though majority of my visitors are from single western country as of now.
This website is optimised and is delivered via a cached CDN system.
Bandwidth usage on Cached CDN System for past 24 hours as on 23 March IST
Post cache, the content is delivered directly from the hosting system.
Bandwidth usage on Hosting System for past 24 hours as on 23 March IST
I expect to surpass my bandwidth limits as I’m writing this, so I have to include some monetization to negate for the bandwidth costs(if possible) for continuing to serve this content.
Use the common table salt (Sodium Chloride) [I did not include links to not confuse with other available salts.]
23 March 2020 - Added Monetization for Bandwidth reasons.
27 March 2020 - Added better image to show the measurements and added more statements regarding sealing this mask completely.
28 March 2020 - Added instructions about wet filter paper.