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There are more reasons than ever to wear a mask, according to the U.S. Centers for Disease Control and Prevention.
Masks provide some coronavirus protection to the wearer, not just to others, the CDC says in an updated science brief.
Until recently, CDC has said the purpose of wearing a mask is to lower the amount of virus particles coming from the person wearing the mask.
But a torrent of research conducted since the pandemic began has shown that masks offer a degree of protection from the virus to the person wearing the mask, too.
"I think there's always been some appreciation that there was personal protection from the mask itself, but there was no way to quantify that," said Carle Illinois School of Medicine associate professor William Scott. "There was no real research to know for sure if that was truly happening."
Since April, when CDC first recommended people wear cloth face coverings, many groups of researchers have raced to find out how well they work.
Pretty well, it turned out. Research first focused on particles coming from the wearer. Masks with layers of fabric could reduce them by 50% to 70% in some studies.
More recently, when scientists started looking at whether they helped the person wearing the mask, "our data was kind of surprising, to find out that there was some protection," Scott said. Some studies found cloth masks could filter out half of the very fine particles that carry the virus.
But it is hard to put a firm number on how well masks work, experts caution, because materials vary widely and researchers have studied the question in different ways.
Evolving advice
Advice has changed over time as scientists have learned more over the course of the pandemic.
Health officials initially had discouraged people from wearing masks. There was not much evidence at the time that they would help.
Also, when the first cases in the United States appeared in late January, people began hoarding surgical masks and N95 respirators. It created shortages of critical protective gear for front-line health care workers.
"Seriously people - STOP BUYING MASKS!" U.S. Surgeon General Jerome M. Adams wrote on Twitter in February. "They are NOT effective in preventing general public from catching #Coronavirus, but if health care providers can't get them to care for sick patients, it puts them and our communities at risk!"
But when scientists learned that people can be infected with the coronavirus without knowing it, the CDC started recommending people wear homemade cloth face coverings.
It was not clear how much it would help, but something was considered better than nothing.
The time is now
The case became clearer as time went on. In May, two hair stylists in Springfield, Missouri, caught the virus but did not pass it on to any of their 139 clients. Everyone at the salon wore masks.
In July, the CDC gave cloth masks a full endorsement in an editorial in the medical journal JAMA.
"Universal Masking," the editorial title proclaimed, "The Time is Now."
As the data continued to come in, the CDC updated its recommendations.
While protecting others still gets top billing, the CDC's latest guidance says, "A cloth mask also offers some protection to you, too. How well it protects you from breathing in the virus likely depends on the fabrics used and how your mask is made (e.g. the type of fabric, the number of layers of fabric, how well the mask fits)."
"Things have evolved because our information and our knowledge of the situation evolved," said University of Cincinnati biologist Patrick Guerra.
Behind the curve
But some say the CDC has been too slow to respond to new information. The agency was criticized for how long it took to recognize that the virus could spread through airborne transmission.
And it has not been promoting the full potential of masks to slow the pandemic, said Harvard University epidemiologist Michael Mina.
"We know masks work," he said, the only question has been how well.
Mina compares them to wearing seat belts.
"When you get in a car accident, does it mean everyone who gets in a car accident will survive because they're wearing a seat belt or have an airbag? Absolutely not," he said. "But we know it cuts risk."
"The CDC has just been behind on this," he added.