Paper Towel Soaked in Salt Water May Improve Masks' Ability to Block Viruses, Bacteria

By Linda Carroll

June 02, 2020

(Reuters Health) - When paper towels are soaked in a saline solution and then allowed to dry, they can effectively filter out particles comparable in size to the SARS-CoV-19 virus, a new study suggests.

The ability of kitchen paper towels, laboratory paper towels and surgical mask filters to block tiny particles was dramatically improved by salt soaking, researchers report in the American Journal of Infection Control.

The researchers see the salt treatment both as a way of improving DIY cloth masks and also extending the life of surgical masks.

"Ideally people should have an adequate supply of masks and be able to change them out in-between patients," said the study's lead author, Jonathan Carnino, a research assistant in the department of pulmonology at the Boston University School of Medicine. "But in this pandemic, an extra layer of protection makes it more acceptable to be reusing masks throughout a shift. It's an added level of safety for those risking their lives on the front lines."

An earlier study had shown that salt treatment of surgical masks improved their ability to block various strains of the flu virus, Carnino said. "In that article, they saw a dramatic difference in filtration abilities," he added. "You can't expect every single healthcare worker to cut open their masks and take out the filters. We wanted to use common household products."

The SARS-CoV-2 virus is reportedly 70-90 nanometers (nm) in diameter, the authors note, and E. coli bacteria are similar in size at 20-200 nm.

How would soaking a paper towel in saline improve its ability to block such tiny particles?

"Salt in water forms molecular bonds and it creates a lattice (in the paper towel) when the salt is applied through an aqueous solution," Carnino said. "When a nanoparticle hits the treated paper towel, the salt dissolves for a second and when it dries, creating a lattice again, it encapsulates the virus."

The earlier study combined salt with water and a surfactant, which Carnino and his colleagues believe helped spread the salt around the paper towel better. So, for their first experiment the researchers duplicated that mixture and soaked small pieces of a kitchen paper towel, a lab paper towel and a surgical mask filter.

Those bits of material were allowed to dry. Then the researchers dripped small drops of fluid containing fluorescent outer membrane vesicles (OMVs) that were close in size to SARS-CoV-2 particles.

"We then looked under a microscope to see how many particles had penetrated," Carnino said. "There was a dramatic difference between the standard surgical mask filter and the pretreated ones."

In fact, the pretreated paper towel was a better filter than the untreated surgical mask filter.

Because it's unlikely that many will have a surfactant in their home, the researchers repeated their experiments without the compound and once again, the pretreated paper towel performed better than an untreated surgical mask.

For those who want to try the pretreatment method, Carnino supplied a recipe:

"1) Measure out 100 mL of H2O (preferably distilled H2O as that was used in our study)

2) Heat H2O lightly so it is hot to touch

3) Mix in 30 grams or 5 teaspoons of table salt until completely dissolved

4) Soak household paper towel pre-cut to the size and shape of a surgical mask or N95 respirator in the solution for five minutes to ensure the solution completely soaks the material

5) Allow to completely dry

6) Secure the pre-treated paper towels to the outside of a homemade mask, surgical mask, or N95 respirator and replace approximately every two hours or sooner if soiled."

In the case of surgical masks, Carnino and his colleagues suggest cutting a paper towel down to the size of the mask, treating it with salt, and then attaching it to the outside of the mask where it will block particles for up to two hours. It can then be replaced with another salt-treated paper towel filter.

The new study is "an interesting proof-of-concept," said Dr. Catherine Clase, an associate professor of medicine at McMaster University in Hamilton, Ontario, Canada, and a nephrologist at St. Joseph's Hamilton Healthcare, who coauthored a recent paper reviewing evidence on the effectiveness of cloth masks.

"Pretreatment of paper towels with saline solution reduced transmission of small particles in an experiment using a drop of liquid," Dr. Clase said in an email. "It will be interesting to see whether this translates into blocking particles in an aerosol experiment, which would teach us more about how this might work for a mask material. It's good to see the authors reiterating the importance of reserving medical masks and N95s for healthcare and other high-risk workers."

The novel coronavirus is thought mainly to be transmitted via respiratory droplets (5 micrometers diameter and larger) and finer aerosols (less than 5 micrometers).

The experiment didn't look at the ability of salt-treated masks to block particles in aerosols, Carnino noted. But, "my assumption is that the droplet (used in the experiment) is a much more intense test," he said.

SOURCE: American Journal of Infection Control, online May 25, 2020.