Wearing a mask is one of the simplest and most effective ways to curb the spread of the coronavirus and save lives. But, as a burgeoning number of advisories makes clear, not every mask is helpful.
A recent study suggested that people should avoid the newly popular neck gaiters, which are made of thin, stretchy material.
Researchers at Duke University found that those coverings may be worse than not wearing a mask at all, because they break up larger airborne particles into a spray of little ones more likely to linger longer in the air.
And, in guidance updated late last week, the Centers for Disease Control and Prevention warned against wearing masks with exhalation valves or vents, a type of face covering made for hot and dusty construction work that has become a popular pandemic accessory because of its seemingly high-tech design.
Meanwhile, Joe Biden called Thursday for Americans to be required to wear masks.
Biden, addressing reporters after receiving a briefing from public health experts, said that every American should wear a mask while outside for at least the next three months and that all governors should mandate mask-wearing.
"It's not about your rights," Biden said, standing in front of five American flags at the Hotel du Pont in Wilmington. "It's about your responsibilities as an American."
Trump suggested that a mask mandate threatened to overstep individual freedoms of Americans and said Biden was more interested in keeping Americans "in their basements for months on end" than in listening to medical experts.
"If the president has the unilateral power to order every single citizen to cover their face in nearly all instances, what other powers does he have?" Trump said.
Biden, the former vice president, had previously said that as president, he would seek to require people to wear masks in public. Trump, on the other hand, resisted wearing a mask before shifting on the issue.
On Thursday, he said it's patriotic for Americans to wear masks, but added, "maybe they're great, and maybe they're just good. Maybe they're not so good."
"To Joe, I would say stop playing politics with the virus," Trump said at the White House press briefing.
CDC guidelines read: "The purpose of masks is to keep respiratory droplets from reaching others to aid with source control. However, masks with one-way valves or vents allow air to be exhaled through a hole in the material, which can result in expelled respiratory droplets that can reach others. This type of mask does not prevent the person wearing the mask from transmitting covid-19 to others. Therefore, CDC does not recommend using masks for source control if they have an exhalation valve or vent."
3M, which makes valve masks for construction work, illustrates on its website how they work: inhaled air is filtered through the fabric part of the mask, and hot, humid exhaled air goes out through the valve. The system may be ideal when tearing out a kitchen for remodeling, but the valve defeats the purpose when trying to slow the spread of a virus.
Public health experts recommend mask-wearing to prevent respiratory droplets from spreading into the air when you exhale, speak, cough or sneeze, and the valves allow those droplets through.
The CDC recommends simple cloth masks instead. A few layers of cotton prevent most of the potentially infectious respiratory droplets from escaping into the air around you, and they are also much cooler than the form-fitting N95 masks.
Masks with valves have been banned by the major U.S. airlines, with American Airlines on Wednesday becoming the latest to announce a policy change, citing the new CDC guidance.
Separately, Georgia Gov. Brian Kemp on Thursday said he's dropping a lawsuit against the city of Atlanta in a dispute over the city's requirement to wear masks in public and other restrictions related to the coronavirus pandemic.
Kemp had sued Atlanta Mayor Keisha Lance Bottoms and the City Council to block them from implementing restrictions at the local level.
The Republican governor argued that local governments can't impose measures that are more or less restrictive than those in his statewide executive orders, which have strongly urged people to wear masks but not required them.
Information for this article was contributed by Reis Thebault and Angela Fritz of The Washington Post; by Alexandra Jaffe, Will Weissert, Ben Nadler, Jeff Amy and Kate Brumback of The Associated Press; and by Thomas Kaplan and Glenn Thrush of The New York Times.