Mask Confusion Settled: Here’s How Effective They Are
New study confirms consensus that masks protect the wearer and others
Face masks are widely considered by infectious disease experts to be a key layer of protection against catching or spreading SARS-CoV-2, the virus that causes Covid-19. A new study confirms this consensus and provides helpful details for buying or making a mask.
Well-fitting masks can easily trap 50% or more of infectious particles, according to a series of lab tests led by Linsey Marr, PhD, an aerosol expert at Virginia Tech. 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.
The findings are detailed in a preprint paper that has not yet been peer-reviewed. But several scientists not involved in the work touted its validity and importance.
“This detailed lab study shows how masks can be effective bi-directional filters when worn properly,” says Kimberly Prather, PhD, an aerosol expert and atmospheric chemist at the Scripps Institution of Oceanography. “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.”
Sizing up the problem
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.
The gold standard for masks is the N95, which is tested to trap 95% of particles 0.3 microns in size. 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.
While SARS-CoV-2 particles are only about 0.1 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.
For particles 0.3 microns and smaller, homemade masks did poorly. But at 2 microns and up, several of the masks blocked 50% or more of particles inhaled or exhaled.
Material from vacuum bags worked best, filtering 80% of 2-micron particles, and more than half of particles 0.5 microns and bigger.
At least 50% of particles were blocked, inbound and outbound, by surgical masks, the ubiquitous disposable coverings, along with homemade, sewn masks created using instructions from the U.S. Centers for Disease Control and Prevention (CDC).
A non-sewn CDC-inspired mask made from a cotton T-shirt performed surprisingly well, Marr said.
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
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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.
The results from Marr’s team support other studies suggesting cloth face coverings indeed provide two-way protection, says David Aronoff, MD, director of the Division of Infectious Diseases at the Vanderbilt University School of Medicine.
“While the performance of many masks is a bit better as ‘source control,’ protecting people around the wearer, it is also clear that such masks can reduce the likelihood of infection to the uninfected wearer as well,” says Aronoff, who was not involved in the research. “This is a well-done study that should provide more rationale for us each to do our part and wear face coverings when around others in public.”