At least some N95 Masks are treated on the outside layer of the mask (facing the world) with an antiviral agent composed of a 1.78% citric acid solution according to a document validating a new N95 to a predicate device. #MAM researched this information further and created a recipe for a DIY Anti-Viral Mask Spray using a 2% w/w Citric Acid solution intended to be an extra layer of protection from the virus. The EPA charts that list which cleaners deactivate COVID-19 list many cleaners with citric acid as the active ingredient, and seems to corroborate the idea behind the mask spray.
This excerpt is taken from the document linked above and a statement from the N95 manufacturer: “ActiproteCtTM UF N95 Respirator is a single-use, disposable respirator coated with Virucoatrm on the outer layer (active ingredient: citric acid, 1.8%, a pH lowering agent) and is not an antiviral drug. Actiprotect kills (inactivates) 99.99% of influenza A viruses (tested against influenza A subtypes HINI (2009 pandemic strains not tested)) within one minute of contact with the surface of the respirator. In vitro testing has demonstrated 99.9 9% kill (inactivation) on the surface of the outer layer when tested in vitro against the following influenza viruses (tested against Influenza A subtypes (and strains): HlNl(JPN/35/2007, JPN/36/2007, 2009 pandemnic strains not tested), H2N2 (A2/JP/305/57), H3N2 (Hong Kong 8/68) and including bird flu subtypes: H5N1 (VNH5Nl-PR8/CDCRG.), H5N9 (Turkey A/Wisc/68, Myna”
REPLICATION OF N95 ANTIVIRAL LAYER
As established, N95 masks have an antiviral coating of 1.78% citric acid solution on the outside (facing the world) side of the mask and it is known to kill 99.9% of many strains of Influenza A w/in one minute of contact. This can be replicated by creating a similar citric acid solution from two household products including lemon juice concentrate and citric acid crystals (used for canning and descaling coffee machines).
LEMON JUICE CONCENTRATE
The ReaLemon brand was found to contain 4.65% (+/- 0.005) citric acid and an average of 1.24-g (+/- 0.003) of citric acid per 25-mL sample. Compared to the ~5% in the Signature Kitchens brand, it is a decent difference. Based on these results, the ReaLemon brand stays more true to the average regarding citric acid content by a standard deviation of 10.9. The Generic brand varied by a standard deviation of 14, proving to not be very reliable.”
It can be garnered from this information that lemon juice concentrate is roughly 4.5 to 5% citric acid. If you want to make a 2% v/v solution then you would want to dilute the lemon juice concentrate with distilled or R/O water. Distilled or R/O water is used when making the citric acid solution with crystals to avoid a reaction between the metals in tap/spring water and the citric acid (as citric acid is used as a metal cleaner) and it seems safe to say the same applies here. It's also important not to store in a metal container or stir with metal utensils.
CITRIC ACID CRYSTALS
Citric acid crystals are sold for many purposes. The most readily available is going to be in a supermarket/hardware store around canning supplies or around cleaners. The forthcoming information is based on the food-grade citric acid crystals intended for canning. This was determined to be the safest approach since this will be on our faces.Here are some photos of the citric acid for canning you will notice that ¼ tsp equals 1g.
Making a 2% w/w solution equals 2g of citric acid dissolved in 100g (or 100ml) of distilled water so it is important to pay attention when measuring. 100ml is equivalent to 3.38 fl oz and we know from the packaging that 1g of citric acid =¼ tsp. So for every 100ml of distilled water, we have to add 1/2 tsp of citric acid to maintain the 2% w/w solution strength.
To make a larger amount since it is shelf-stable we used 1L or 1000 mL of water.
When preparing this solution it is important to use distilled or R/O water because tap and spring water has metals in it and it reacts with the citric acid. It's also important to boil the water in a non-metal pot.
To the 1L of water we added 5 tsps,or 10 times the amount for 100mL, of citric acid crystals.
Remove the solution from the heat and stir the crystals in until dissolved with a non-metal utensil and store in a non-metal container.
As the N95's Anti-viral coating is found on the outside of the mask, #MAM recommends spraying the outside of a clean cotton mask with the DIY Mask Spray and allowing it to dry before inserting the filter and using the mask.