One mask is good. Would two be better?

As coronavirus cases continue to surge around the world, some prominent people – including the new US president, Joe Biden – have begun to double up on masks, a move that researchers say is increasingly being backed up by data.

Double-masking isn't necessary for everyone. But for people with thin or flimsy face coverings, "if you combine multiple layers, you start achieving pretty high efficiencies" of blocking viruses from exiting and entering the airway, says Linsey Marr, an expert in virus transmission at Virginia Tech, the US university, and an author of a recent commentary laying out the science behind mask-wearing.

Of course, there is a trade-off: at some point “we run the risk of making it too hard to breathe”, she says. But there is plenty of breathing room before mask-wearing approaches that extreme. A year into the Covid-19 pandemic, the world looks very different. More than 90 million confirmed coronavirus infections have been documented worldwide, leaving millions dead and countless others with lingering symptoms, amid ongoing economic hardships and shuttered schools and businesses. New variants of the virus have emerged, carrying genetic changes that appear to enhance their ability to spread from person to person.

And while several vaccines have now cleared regulatory hurdles, in many countries the rollout of injections has been sputtering and slow – and there is not yet definitive evidence to show that shots will have a substantial effect on how fast, and from whom, the virus will spread.

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Through all that change, researchers have held the line on masks. We "will not need to be wearing masks forever", says Dr Monica Gandhi, an infectious-disease physician at the University of California, San Francisco, and another of the new commentary's authors. But for now they will need to stay on, delivering protection both to mask-wearers and to the people around them.

The arguments for masking span several fields of science, including epidemiology and physics. A bevy of observational studies have suggested that widespread mask-wearing can curb infections and deaths on an impressive scale, in settings as small as hair salons and at the level of entire countries.

One American study, which tracked state policies mandating face coverings in public, found that known Covid cases waxed and waned in near-lockstep with mask-wearing rules. Another, which followed coronavirus infections among healthcare workers in Boston, noted a drastic drop in the number of positive test results after masks became a universal fixture among staff. And a study in Beijing found that face masks were 79 per cent effective at blocking transmission from infected people to their close contacts.

Recent work by researchers like Marr is now pinning down the basis of these links on a microscopic scale. The science, she says, is fairly intuitive: respiratory viruses like coronavirus, which move between people in blobs of spittle and spray, need a clear conduit to enter the airway, which is crowded with the types of cells the viruses infect. Masks that cloak the nose and mouth inhibit that invasion.

The point is not to make a mask airtight, Marr says. Instead, the fibres that make up masks create a haphazard obstacle course through which air – and any infectious cargo – must navigate. “The air has to follow this tortuous path,” Marr says. “The big things it’s carrying are not going to be able to follow those twists and turns.” Experiments testing the extent to which masks can waylay inbound and outbound spray have shown that even fairly basic materials, like cloth coverings and surgical masks, can be at least 50 per cent effective in either direction. Several studies have reaffirmed the notion that masks seem to be better at guarding people around the mask-wearer than mask-wearers themselves. “That’s because you’re stopping it right at the source,” Marr says. But, motivated by recent research, the US Centers for Disease Control and Prevention has noted that there are big benefits for those who don masks as well.

The best masks remain N95s, which are designed with ultrahigh filtration efficiency. But they remain in short supply for health workers, who need them to safely treat patients. Layering two less specialised masks on top of each other can provide comparable protection. Marr recommends wearing face-hugging cloth masks over surgical masks, which tend to be made with more filter-friendly materials but fit more loosely. An alternative is to wear a cloth mask with a pocket that can be stuffed with filter material, like the kind found in vacuum bags.

But wearing more than two masks, or layering up on masks that are already very good at filtering, will quickly bring diminishing returns and make it much harder to breathe normally. Other tweaks can enhance a mask’s fit, such as ties that secure the fabric around the back of the head, instead of relying on ear loops that allow masks to hang and gape. Nose bridges, which can help the top of a mask to fit more snugly, offer a protective boost as well.

Achieving superb fit and filtration "is really simple", Gandhi says. "It doesn't need to involve anything fancy." No mask is perfect, and wearing one does not obviate other public-health measures, like physical distancing and good hygiene. "We have to be honest that the best response is one that requires multiple interventions," says Jennifer Nuzzo, a public-health expert at Johns Hopkins University. – New York Times