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 percent weekly increase in coronavirus deaths per capita after their first case was reported, compared with 7 percent in countries with cultures or guidelines supporting mask-wearing.
A model from the University of Washington predicted that the US could prevent at least 45,000 coronavirus deaths by November if 95 percent 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 Organisation 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 Centres 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. Here are their results so far, from most to least protective.
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 fibres to filter airborne pathogens.
A study published in the Journal of Hospital Infection last month 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 percent after 20 minutes of exposure in a highly contaminated environment. N95 masks offered almost as much protection - the name refers to its minimum 95 percent efficiency at filtering aerosols.
Another recent study also determined that N95 masks offered better protection than surgical masks.
Surgical masks are made of nonwoven 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, officially called 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.
In a recent 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 percent of small particles (less than 300 nanometres) and more than 90 percent of larger particles (bigger than 300 nanometres).
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.
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.
A University of Illinois study that's still awaiting peer review found three layers of either a silk shirt or a 100 percent cotton T-shirt may be just as protective as a medical-grade mask. Silk in particular has electrostatic properties that can help trap smaller viral particles.
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 percent after 30 seconds of exposure to the coronavirus and by 58 percent 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 percent effective at trapping infectious particles.
Tea towels and antimicrobial pillowcases were the next-best alternatives to vacuum-cleaner bags or filters, the same 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.
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 percent 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 percent.
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 travelled 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.
The UK researchers found that people who wore cotton masks had a 54 percent lower chance of infection than people who wore no masks at all. People who wore paper masks had a 39 percent 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 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 suggested that wearing masks improperly or sporadically could still reduce transmission.
In an editorial published Tuesday 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.
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