US government researchers found that two masks are better than one in slowing coronavirus spread, but health officials stopped short of recommending that everyone double up.
The Centres for Disease Control and Prevention on Wednesday gave the results of a laboratory experiment that spaced two artificial heads two metres from each other.
Researchers then checked to see how many coronavirus-sized particles released by one were inhaled by the other.
The researchers found that wearing one mask, surgical or cloth, blocked about 40 per cent of the particles coming in.
When a cloth mask was worn on top of a surgical mask, about 80 per cent were blocked.
When both the exhaling and inhaling heads were double-masked, more than 95 per cent of the particles were blocked, said the CDC’s Dr John Brooks.
“The first challenge is to get as many as people as possible masking," Dr Brooks said.
"And then for those that do mask, to help them get the best benefit out of that mask."
The study had many limits. The researchers used only one brand of surgical mask and one kind of cloth mask, and it is not clear if results would be the same with every product.
But it echoes earlier research that suggested two masks were better than one.
“It works,” Dr Brooks said.
The agency has also updated its guidance on wearing two masks.
If done correctly, a cloth mask worn over a surgical mask can tighten the gaps that can let virus particles in around the mask’s edges, it said.
The agency said it was taking down a do-it-yourself page that went up last year when masks were in short supply, and it was encouraging people to take steps to interrupt viral transmission.
Some Americans have already started doubling up on masks.
Experts believe this is at least partly out of concern over new strains of coronavirus that have been found to spread more easily than the one that has driven the US epidemic for the past year.
CDC guidance has evolved over the course of the epidemic.
Wearing masks has long been common in some countries during respiratory outbreaks, especially in parts of Asia, but not in the US.
When the Covid-19 crisis began and masks disappeared from shop shelves, US health officials discouraged the general public from wearing them.
“Seriously people, stop buying masks,” then-surgeon general Jerome Adams wrote in a tweet almost a year ago.
Two months later, after it became clear that infected people who did not exhibit symptoms could spread the virus, the CDC began recommending that people wear masks in public.
More people are wearing masks and some places have made them obligatory, but many Americans continue to shun them.
A recent University of California survey suggested that only about half of US adults wear masks when in close contact with people outside their household.
Discussions about double-masking and higher-quality masks are important, said Dr Isaac Bogoch, an infectious diseases scientist at the University of Toronto.
“But if a significant proportion of your population isn’t wearing a mask in the first place, then you’re having the wrong conversation,” Dr Bogoch said.