The best and worst face masks, ranked by their level of protection

There's a reason why agencies recommend reserving N99 and N95 masks for healthcare workers first: Both seal tightly around the nose and mouth so that very few virus particles can seep in or out. They also contain tangled fibers that filter airborne pathogens.A study published last month evaluated more than 10 different masks based on their ability to filter airborne coronavirus particles. According to the researchers, 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. Another recent study also determined that N95 masks offer better protection than surgical masks. 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. 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 more effective at blocking virus-containing aerosols than homemade face masks, according to a 2013 study. 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 paper that hasn't yet been peer reviewed, researchers in the UK determined that hybrid masks — which combine 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).The combination of cotton and chiffon offered the most protection, according to the findings, followed by cotton and flannel, cotton and silk, and four layers of natural silk. The researchers even suggested that these options may 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.The 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 non-absorbent material like polyester.According to a University of Illinois study that's still awaiting peer review, three layers of either a silk shirt or 100% cotton T-shirt may be just as protective as a medical-grade mask. Silk in particular has electrostatic properties that can help trap smaller virus particles. The study that compared 10 masks found that vacuum-cleaner bags (or vacuum-cleaner filters inserted into 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 determined 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 were the next-best alternative to vacuum-cleaner bags or filters, the same study found. Tea towels need to be tightly woven, the researchers added, in order to confer protection.Antimicrobial pillowcases (usually made of satin, silk, or bamboo) were preferable to a standard cotton pillowcase.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 around 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 virus particles when an infected person coughs or sneezes, Indian researchers recently determined. According to their study, infectious droplets traveled about three times farther (up to 16 feet) when a person wasn't wearing a mask, compared to 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% 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 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 now suggests that wearing masks improperly or sporadically could still reduce transmission. In a Tuesday 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 outbreak under control in as little as four weeks.

The best and worst face masks, ranked by their level of protection

Two medical-grade masks, N99 and N95, are the most effective at filtering virus particles.

Disposable surgical masks are a close second.

"Hybrid" masks are the safest homemade option.

Three layers of cotton or silk are also highly protective.

Vacuum-cleaner bags are a DIY alternative to surgical masks.

Tea towels and antimicrobial pillowcases aren't ideal materials, but they're better than a single layer of cotton.

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.

Single-layer cotton masks are preferable to single-layer paper masks.

How you wear your mask matters, too.