NEW YORK (AP) -- New York City Mayor Bill De Blasio has asked New Yorkers to wear a face covering when they go outside and will be near other people.

He cited research showing asymptomatic people could be spreading the coronavirus without realizing it. De Blasio said at a press briefing that until now, "there just wasn't evidence" to support the move.

"When you put on that face covering, you're protecting everyone else," he said.

The mayor said it could be a scarf or a bandanna or anything homemade, but it should not be a surgical mask needed by front-line medical workers.

A recent study by researchers in Singapore became the latest to estimate that somewhere around 10% of new infections may be sparked by people who carry the virus but have not yet suffered its flu-like symptoms.

In response to that study and others, the Centers for Disease Control and Prevention changed how it defined the risk of infection for Americans. The agency's new guidance targeted people who have no symptoms but were exposed to others with known or suspected infections. It essentially says that anyone may be a carrier, whether that person has symptoms or not.