FILTERS for homemade masks and respirators

Filters can be divided into two types.
Physical, which create a mechanical barrier, and filters which deactivate and destroy virus particles by virtue of their chemical composition.
Currently, there’s no certified filter material that could be used as a face mask filter. What we can offer are filter alternatives, or theory about filter material viability.

1) Meltblown filters

It’s possible to create filters from polypropylene meltblown non-woven fabric.
Wide availability of such filters may be a matter of weeks. In Czech and also from local sources in foreign too.

2) Salt filters (NaCl)

THEORY IN PRACTICE? The authors of a canadian study found out that if you drench your filter in a strong solution of kitchen salt, you’ll create a great antiviral protection layer. The filtration rate for viral particles hovers between 40-70% after 24 hours of use (85% right at the beginning), but, most importantly, the viral particles of flu that did pass through weren’t able to cause the illness! 99.9% of viral particles that land on the filter are destroyed within 5 minutes, all of them are destroyed within the hour.
The second awesome thing is that the protection works even for damp masks and for the whole day. Naturally it wasn’t tested against the current coronavirus strain, but there’s a massive chance that it will work just the same (the principle is physical sterilization - high osmotic pressure destabilizes proteins).
It has been cited by some major media in Czech:, Dení,

Article about salty filter is also from US
disclaimer: the method is periodically updated by professor Polzer, therefore there may be some major changes in later versions of the same method.
It has been tested in practice by wearing a black 100% cotton mask with a salt filter - it didn’t get damp even after a day of use, and there was no skin irritation from wearing these masks for a week.

3) Nanosilver filters (Ag)

Beware of snakeoil salesmen with nanosilver filters.
It has been proven that silver nanoparticles do kill bacteria. However, we have no studies about their alleged immediate antiviral effect.
Materials with nanosilver do exist, but they’re used as antimicrobial fiber or sterile dressings.
There is a chance that they are cytotoxic, and that small particles that you breathe in could harm your lungs.

4) Alternative materials - cabin filter, cat toilet filter

Unused cabin filters with activated carbon may be cut up and used as filter in face masks, especially if they are made according to the HEPA standard. While they are only certified for use for particles 3 times bigger than our coronavirus particles, they may still filter it partially by electrostatic attachment. They are also easier to breathe through than vacuum cleaner filters, or other similar DIY solutions. It is suggested you iron the filter gently before sewing it into your mask, so it’s easier to handle. Cat toilet filters should theoretically work exactly the same. We have tests underway for various brands, and we will publish our results as soon as there are any.

5) Electrostatic filter

The principle of proffessional respirators often isn’t a dense fiber structure (that’s impossible to breathe through, and gets damp immediately, which usually increases the likelihood of infection), but rather ELECTRICALLY CHARGED fiber. It doesn’t even need to be dense. The electric charge attracts the particles, that would otherwise pass through, but in this case, they will attach themselves to the fiber.
By testing, I found out that polyester is best at keeping its electrical charge, as it is best able to keep its charge, even in damp conditions. There will still be about a half of the charge even after four hours of use.
The filter material discharges slowly by air (in tens of hours), but the effect is sped up by water in your breath. That’s why your mask needs to be as light and thin as possible.
Be careful about contact with a grounded conductor (your body conducts electricity too!)

Technical University of Liberec make own nanofilters:

Proffesor from University of Alberta describe results of study na salt filter

Where can I buy filters?

It’s likely that certified meltblown filters may become available to the public in a matter of weeks. If you don’t have access to professionally made respirators, you’ll have to settle for alternatives. You can either use untested, but likely decent cabin filters or kitty litter filters, or you’ll have to use some WIDELY AVAILABLE MATERIAL or a mask without a filter. It’s also possible to make your own salt filter or electrically charged filter at home.

How do I make a salt filter?

This is an adapted method, outlined directly in the study, where they even test the function of said filter.
EDIT 4.4.2020
1) Cut up a single use surgical mask and use the middle layer. If you don’t have that, use 15x20 cm [6x8 in] another kind of synthetic fabric, for example non-woven garden fabric.
2) Prepare 300g of salt [1 US cup] and a tablespoon of Tween 20 . They used polysorbate 20 (Tween 20) in the article, jde ale jen o to, snížit povrchové napětí vody tak, aby roztok mohl obalit všechna vlákna. You can use washing powder or dish detergent-but it is up to you a your health condition.
3) Put a litre of hot water into a litre measuring cup [just get a 2 pint pitcher and you’re good], add Tween 20 (or washing powder or dish detergent) wait until it dissolves, and add the salt. Mix well until all of the salt dissolves. The hotter the water, the easier this step is, but don’t burn yourself.
4) Drench the fabric in 25 ml of the solution (put 5 teaspoons into a bowl or on a plate) and leave at least 8 hours. This applies to fabric that’s 15x20 cm [6x8 in], scale accordingly by area for smaller pieces. Amount of the solution should be 0,08ml/cm2 of fabric
5) Put 50 ml of the solution (10 teaspoons) on a second plate, put the fabric in that, and put it into an oven at 40°C [104°F, and yes, it’s not a typo] until all of the water evaporates. This applies to fabric that’s 15x20 cm [6x8 in], scale accordingly for smaller pieces. Amount of the solution should be 0,16ml/cm2 of fabric
6) Put it into your cotton mask with a filter pocket (15x20 cm [6x8 in]) and it could last for a day.
7) After you come home, put the whole mask in boiling water for 30 minutes, then remove the filter, and then you can impregnate it again with the salt solution. One liter should last you 14 days.
Apply to filters only. If you use it directly on the mouth mask, it will eat your face off. And that’s not an exaggeration.
If it’s hard to breathe through the fabric, use half the solution per filter. There will be less salt, but it will still work. We have no results for other concentrations.

How do I make an electrostatic filter?

IS NOT TESTED YET!!! It is just a concept.
1) Ideally, use 100% polyester. You have to charge the material electrically, for example, by rubbing it on a balloon, until the balloon attracts your hair.
2) Put it into the filter pocket of your mask. Professionally made respirators use 5 layers or so.
3) The best material is apparently black non-woven garden fabric. It’s black, and it dries out fast.
4) Sterilize the filter afterwards. You can’t sterilize it by boiling or steaming - therefore, do not share the filter within the household.