The science is clear: Face masks can prevent coronavirus transmission and save lives.
A preliminary analysis of 194 countries found that places where masks weren't recommended saw a 55% weekly increase in coronavirus deaths per capita after their first case was reported, compared with 7% in countries with cultures or guidelines supporting mask-wearing. A model from the University of Washington predicted that the US could prevent nearly 67,000 coronavirus deaths by December if 95% of the population were to wear face masks in public.
But not all masks confer equal levels of protection.
The ideal face mask blocks large respiratory droplets from coughs or sneezes — the primary method by which people pass the coronavirus to others — along with smaller airborne particles, called aerosols, produced when people talk or exhale.
The World Health Organization recommends medical masks for healthcare workers, elderly people, people with underlying health conditions, and people who have tested positive for the coronavirus or show symptoms. Healthy people who don't fall into these categories should wear a fabric mask, according to WHO. The Centers for Disease Control and Prevention also recommends cloth masks for the general public.
But even cloth masks vary, since certain types are more porous than others.
"It depends on the quality," Dr. Ramzi Asfour, an infectious-disease physician in Marin County, California, told Business Insider. "If you're making a cloth mask from 600-thread-count Egyptian cotton sheets, that's different than making it from a cheap T-shirt that's not very finely woven."
Over the past few months, scientists have been evaluating the most effective mask materials for trapping the coronavirus. The most recent research, published by Duke researchers last week, found that using polyester spandex neck fleeces as face coverings may actually increase the rate of droplet transmission during speech. Here are the results of the best studies on masks so far, with materials from most to least protective.
Two medical-grade masks, N99 and N95, are the most effective at filtering viral particles.
There's a reason agencies recommend reserving N99 and N95 masks for healthcare workers first: Both seal tightly around the nose and mouth so that very few viral particles can seep in or out. They also contain tangled fibers to filter airborne pathogens.
A June study published in the Journal of Hospital Infection evaluated more than 10 masks based on their ability to filter airborne coronavirus particles. The researchers found that N99 masks reduced a person's risk of infection by 94% to 99% after 20 minutes of exposure in a highly contaminated environment. N95 masks offered almost as much protection — the name refers to its minimum 95% efficiency at filtering aerosols.
The recent Duke research, meanwhile, showed that less than 0.1% of droplets were transmitted through an N95 mask while the wearer was speaking.
Disposable surgical masks are a close second.
Surgical masks are made of non-woven fabric, so they're usually the safest option for healthcare workers who don't have access to an N99 or N95 mask.
An April study found that surgical masks reduced the transmission of multiple human coronaviruses (though the research did not include this new one, SARS-CoV-2) through both respiratory droplets and smaller aerosols.
In general, surgical masks are about three times as effective at blocking virus-containing aerosols than homemade face masks, a 2013 study found. But healthcare workers should still have access to them first.
"The official guidelines are cloth masks because we don't want to take those masks away from medical workers who might need them more," Asfour said.
"Hybrid" masks are the safest homemade option.
In a June paper that hasn't yet been peer-reviewed, researchers in the UK determined that "hybrid" masks — combining two layers of 600-thread-count cotton with another material like silk, chiffon, or flannel — filtered more than 80% of small particles (less than 300 nanometers) and more than 90% of larger particles (bigger than 300 nanometers).
They found that the combination of cotton and chiffon offered the most protection, followed by cotton and flannel, cotton and silk, and four layers of natural silk. The researchers suggested that these options may even be better at filtering small particles than an N95 mask, though they weren't necessarily better at filtering larger particles.
The team also found that two layers of 600-thread-count cotton or two layers of chiffon might be better at filtering small particles than a surgical mask.
A layer of silk or three layers of cotton or are also highly protective.
Dr. Anthony Fauci at a Senate hearing on June 30.
WHO recommends that fabric masks have three layers: an inner layer that absorbs, a middle layer that filters, and an outer layer made from a nonabsorbent material like polyester.
According to a University of Illinois study that's still awaiting peer review, three layers of a 100% cotton undershirt may be just as protective as a medical-grade mask. The researchers also found that a single-layer, 100% silk shirt may be equally protective. Silk in particular has electrostatic properties that can help trap smaller viral particles.
Vacuum-cleaner bags are a DIY alternative to surgical masks.
The Journal of Hospital Infection study found that vacuum-cleaner bags (or vacuum-cleaner filters inserted in a cloth mask) reduced infection risk by 83% after 30 seconds of exposure to the coronavirus and by 58% after 20 minutes of exposure in a highly contaminated environment. The material was almost as good at filtering aerosols as surgical masks, the researchers found.
That could be enough protection to stop an outbreak. A May study found that universal mask-wearing would bring an epidemic under control even if the masks were only 50% effective at trapping infectious particles.
Tea towels and antimicrobial pillowcases aren't ideal materials, but either is better than a single layer of cotton.
Tea towels and antimicrobial pillowcases were the next-best alternatives to vacuum-cleaner bags or filters, the Journal of Hospital Infection study found. Tea towels need to be tightly woven to confer protection, the researchers said.
Antimicrobial pillowcases (usually made of satin, silk, or bamboo) were preferable to a standard cotton pillowcase, they found.
Wrapping a scarf or cotton T-shirt around your nose and mouth isn't particularly effective at filtering the coronavirus, but it's still better than nothing.
Actor Johnny Depp uses a bandana as a face mask at the High Court in London on July 7.
The UK researchers found that a single layer of 80-thread-count cotton was among the least effective materials at blocking coronavirus particles both large and small.
Scarves and cotton T-shirts reduced infection risk by about 44% after 30 seconds of exposure to the coronavirus, the Journal of Hospital Infection study found. After 20 minutes of exposure in a highly contaminated environment, that risk reduction dropped to just 24%.
But that's better than zero.
Even a loosely fitted cotton mask "substantially decreases" the spread of viral particles when an infected person coughs or sneezes, researchers in India recently determined. They found that infectious droplets could travel up to 16 feet when a person wasn't wearing a mask, compared with just 5 feet when particles leaked out the sides of a face mask.
Knitted masks and bandanas aren't very protective, either.
Fashion designer Yr Johannsdottir shows one of her knitted masks at her studio in Reykjavik, Iceland, on May 11, 2020. Jeremie Richard/AFP/Getty Images
The Duke researchers found that bandanas reduced the rate of droplet transmission by a factor of two — that's slightly less than knitted masks, but still more effective than no mask at all.
Single-layer cotton masks are preferable to single-layer paper masks.
A man reading a newspaper in Lisbon, Portugal, on May 20.
The UK researchers found that people who wore cotton masks had a 54% lower chance of infection than people who wore no masks at all. People who wore paper masks had a 39% lower chance of infection than the no-mask group.
Unlike a surgical mask, which is typically pleated and made of three layers of fabric, paper masks are thinner, so they confer less protection.
The CDC advises against wearing masks with built-in valves or vents.
Masks with valves or vents filter incoming air to protect the wearer — but the air that comes out may still pose a risk to people nearby.
"Masks with one-way valves or vents allow air to be exhaled through a hole in the material, which can result in expelled respiratory droplets that can reach others," the CDC website reads. "This type of mask does not prevent the person wearing the mask from transmitting COVID-19 to others."
Neck fleeces made of polyester spandex may be worse than no mask at all.
Driver Ross Chastain wears a neck fleece before the NASCAR Xfinity Series Shady Rays 200 at Kentucky Speedway on July 9.
The Duke researchers found that neck fleeces made of polyester spandex could actually increase the rate of droplet transmission during normal speech compared to no mask at all.
Instead of blocking large droplets, neck fleeces appear to disperse these droplets into smaller, more numerous particles. Since small droplets tend to linger in the air for longer, the researchers suggested neck fleeces may be "counterproductive." But that likely depends on their material and how many layers of fabric they have.
But how you wear your mask matters, too.
A woman with a face mask pulled under her chin in Rome on April 29
The protectiveness of a mask — including N95 and surgical masks — declines considerably when there is a gap between the mask and the skin.
"It's about the seal of the mask," Asfour said. "You have to make sure there's no air leak."
Even so, research has shown that wearing masks improperly or sporadically could still reduce transmission. In a July editorial published in the Journal of the American Medical Association, CDC Director Robert Redfield predicted that the universal adoption of face masks could bring the US's outbreak under control in as little as four weeks based on case numbers and transmission rates at that time.