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Mask Confusion Settled: Here’s How Effective

A new study confirms this consensus and provides helpful details for buying or making a mask.

Masks made of two or three layers of cotton and other filtering materials can capture up to 90% of the particles. The effectiveness was similar for exhaling and inhaling, Marr and her colleagues found.

Such careful lab studies provide much needed critical mechanistic insights into why masks are critical for reducing the spread of SARS-CoV-2,” says Prather, who was not involved in the study. “In addition, this study provides guidance on the best materials and number of layers that are most effective in filtering micron-sized aerosols that could contain infectious virus.”

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When people talk or just breathe, a constant stream of respiratory droplets are emitted, which from an infected person also pack tiny coronavirus particles. Larger droplets, roughly the diameter of a human hair, fall to the ground quickly, typically within six feet or so. Smaller droplets, called aerosols, are invisible and can remain airborne and infectious for minutes or hours.

And while such efficiency is vital in a hospital setting, N95s are in short supply, and aerosol experts agree that lesser products — surgical masks and homemade masks — can be highly effective.

1 microns in diameter, they’re trapped inside larger aerosols that contain water, salts, and other particles. The virus-packing aerosols thought to be most important for Covid-19 infections range from 1 to 2 microns, Marr says, though her lab tests — using mannequins to mimic breathing in and out — measured effectiveness against particles ranging from 0.04 microns to more than 100 microns.

Materials matter

Material from vacuum bags worked best, filtering 80% of 2-micron particles, and more than half of particles 0.5 microns and bigger.

Efficiency dropped significantly with masks made entirely of acrylic materials, and for loose-fitting face coverings like bandanas, where aerosols easily enter or exit the bottom.

Marr has a recommendation for making the ideal three-layer mask, based on her lab results and other studies:

Inner: Soft, flexible, tightly-woven fabric such as cotton

Middle: Material designed to filter particles, like MERV 12 filter material (which can be hard to find and should be rated as safe for mask use) or a vacuum bag

Outer: Soft and tightly-woven material, again like cotton

Despite some outlier claims that masks don’t work, leading experts on the transmission of airborne viruses are unified in proclaiming the value of masks for lowering risk of Covid-19 spread. A new study from the CDC adds to the evidence by showing that infection rates in Kansas were lower in counties that enforced mask mandates.

“No one intervention — masking, social distancing, hand washing, indoor ventilation — will stop the spread of Covid-19 alone,” Marr says. “The mask is one of the many interventions that we need to combine together.”

The tactics have been described as working like layers of Swiss cheese.