Which easy-to-buy materials are best suited for home made masks based on how well they filter particles?

After Business Insider first reported on a group of fashion designers who discovered certain blue shop towels filter particles far better than a cotton bandana, one of them was approached by a manufacturer of N95 mask-testing equipment.

He offered to test any consumer material the designer was investigating, he told Business Insider.

She sent him 20 materials that people on the internet have been using for masks, from paper towels to vacuum cleaner bags. He also tested her shop-towel masks.

Here are the results of those tests, ranked by how well they filter tiny, potentially-dangerous particles as well as by effectiveness balanced with breathability.

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After Business Insider first reported on a group of fashion designers who designed an effective face mask using blue shop towels, one of the designers received an outpouring of responses.

Chloe Schempf Chloe Schempf The designer, Chloe Schempf, was raising money to conduct more official filter-testing on various mask materials. Her question: Could any material available to the public block particles almost as well as N95 masks?

The president of particle-testing equipment company TSI, Tom Kennedy, heard her story and reached out.

He told Schempf that TSI would be happy to test the new mask — and any other materials — using the same machine that N95 mask makers employ to certify that their respirator masks meet OSHA standards: the TSI 8130a.

Kennedy tells Business Insider that he was impressed with Schempf and the team's approach to finding mask-making material

"They did a really nice job," Kennedy said. "They had a sense of the science."

Schempf sent Kennedy every material she had seen discussed on the internet as being potentially useful as a mask, from blue shop towels and bandanas to paper towels and vacuum cleaner filters. She sewed them into swatches with various combinations, and sent samples of the designers' own home-made mask, too. Their mask includes a pocket where the user can add filter material, such as the blue shop towel.

Blue shop towel mask homemade particle tests Chloe Schempf Kennedy conducted a blind test.

"She sent us 20 samples, only numbered," he said. "We didn't know what the materials were."

Schempf has published a website with a more details on the tests results, but she also shared the results with Business Insider.

We have compiled the info into two charts.

The first one ranks how well each combination of materials filters potentially-dangerous small particles. The second one ranks the materials by balancing how well they filter with how hard they are to breath through.

Kennedy also shared lots of other information for people trying to make the most effective masks possible at home:

Fit is as critical as material choice. If the mask is not snug around the nose, cheeks and chin, and unfiltered air leaks in from around the mask, the material doesn't matter.

"At the end of the day, the mask has to fit extremely well, otherwise you get airflow around the mask or through nose bridge," says Kennedy.

Filtering must be balanced with breathability. Obviously, if a filter is suffocating, no one will want to wear it. The harder it is to breath through, the more unfiltered air will likely be sucked in from around the mask's edges.

The gold standard for filtering hazardous particles is the N95 mask and its quality can't be matched at home. The N95 respirator mask filters at least 95% of particles that measure 0.3 microns. There's a lot of science to get to that 95-percent-or-better number that home sewers simply can't replicate. This includes factors like the special electro-static material used in the masks, as well as considerations taken to mitigate the impact of moisture build-up from the user's breath.

The best combinations of materials that Kennedy tested only filtered between 80 and 85% of particles in the air. Moreover, it's not a linear progression: A mask that filters at 95% efficiency is vastly more protective than one that filters at 80%, while a mask that filters at 80% is only modestly more protective than one that filters at, say, 60%. So even 80% is not to be confused with close-to-N95 protection.

This is the official TSI 8130A particle testing machine used to test home made mask materials. TSI While the CDC recommends that a bandana is far better than no mask at all (Kennedy's test found that bandanas filter less than 10% of particles), people in higher risk situations — like retail workers who can't obtain N95 masks —are still better served by a mask that filters better, Kennedy says.

The COVID-19 virus is smaller than 0.3, at about 0.1 microns, but 0.3 microns is a sort of magic size to test for, according to Kennedy. That's because this size particle is the most difficult to capture, he says. Particles that are bigger, as well as particles that are smaller will be trapped by filters that capture 0.3 microns. So this test of fabrics, "certainly covers the size range for the virus," Kennedy says. (Here's an article that explains the theory behind that.)

A Suay employee models the blue shop towel mask Suay Sew Shop

Some of the manufacturers of the materials Schempf tested are warning people that they haven't been tested as safe to use in a mask. This includes makers of furnace filters, vacuum cleaner bags, and blue shop towels.

Some experts caution that breathing in tiny fibers of these commercial materials could be unsafe.

The designers' created a mask that includes a pocket so people could add extra filter material if they chose to. Even so, its wise to check with the manufacturer of the filter material before putting it in a mask.

20 materials ranked by the % of particles filtered, from least to most effective, according to TSI tests

MATERIAL % of particles filtered RESISTANCE (breathability) (MMH20) 2 layers Kona 100% Quilters Cotton 7.02 6.49 Folded Bandana 9.65 3.38 2 layers Braun Coffee Filter 14.1 16.42 1 layer Kona 100% Cotton + 1 layer Flannel 15.39 8.08 2 layer Pellon Med.Weight Fusable Interfacing 17.24 1.08 4 layer Viva Classic Paper Towels 22.54 3.37 3 layer Hanes 100% Cotton T-shirt 27.98 8.88 2 layer Viva Classic Towel + 1 layer Swiffer Dry Sheet 30.01 2.16 2 layer Wypall towels x80 30.58 6.1 1 layer Uline Lightweight Old Absorbent Pad (not dimpled) 35.02 2.67 2 layer Bandana + 1 layer craft felt + 1 layer Toolbox Shop Towel 36 9.78 2 layer Hanes 100% Cotton knit + 1 layer Zep Towel 39.08 11.54 2 layer Toolbox Shop Towels 41.94 11.98 2 layers 100% Bamboo Rayon + 2 layer Swiffer Dry Sheets 44.77 14.72 1 layer Zep towel + 1 Layer Toolbox towel 55.66 11.57 1 layer Evolon 100g 57.56 11.2 1 layer Zep Towel + 1 layer Radnor LW Oil Absorbent pad 66.49 11.44 2 layer Kona 100% Cotton + 99.7% Vacuum bag lining (no paper) + 1 layer Swiffer Dry Sheet 68.48 10.82 2 layers Kona 100% Cotton + 1 layer 1500 Filtrete Furnace Filter 80.85 8.15 1 layer Filti + 1 layer 6.5g Cotton Rip Stop 84.03 13.33

20 materials ranked by a balance of breathability and particle filtering protection, according to TSI tests

For this ranking, we penalized the materials that were hard to breath through by subtracting their resistance rating from their particle filter percentage performance. This gives us a crude ranking of materials that balance breathability with filtering.