A top US scientist warned that the new coronavirus can be spread through normal breathing, fueling recommendations that everyone wear masks as several nations posted record death tolls.
With half of humanity under lockdown orders, governments have been racing to find ways to flatten the rise of the virus, which has infected more than one million people around the world.
Anthony Fauci, the veteran US expert who is leading the government's scientific response, backed recent scholarship that found SARS-CoV-2 can be suspended in the ultrafine mist let out when people exhale.
Fauci, head of infectious diseases at the National Institutes of Health, told Fox News that the guidance on masks would be changed "because of some recent information that the virus can actually be spread even when people just speak, as opposed to coughing and sneezing."
The World Health Organization has been more cautious, saying the airborne threat was only known to occur during certain medical treatments, and the United States until now has only advised sick people and their caretakers to cover their faces.
But Vice President Mike Pence said authorities will issue new nationwide advice in the coming days and several local leaders have already gone ahead.
Mayor Bill de Blasio of New York, the coronavirus epicenter of the United States, has recommended that everyone in the nation's largest city wear masks when going outside -- advice backed Friday by the governor of Pennsylvania.
"The only way we can cut the growth of this virus is to act as if we all have it," Governor Tom Wolf told reporters.
But with the United States and Europe facing severe shortfalls in protective gear, Fauci and local leaders urged people to save clinical masks for health professionals and patients.
Turkish President Recep Tayyip Erdogan in a national address ordered all people to wear masks when going to markets and banned anyone under 20 from going outside.
The governor of New York has vowed to take ventilators and protective equipment from private hospitals and companies that are not using them as the state's coronavirus death toll rose to almost 3,000.
Andrew Cuomo complained that states are competing against each other for vital equipment in eBay-like bidding wars.
"If they want to sue me for borrowing their excess ventilators to save lives, let them sue me," Mr Cuomo said.
The executive order he said he would sign represents one of the most aggressive efforts yet in the US to deal with the kind of shortages around the world that have caused healthcare workers to fall sick and forced doctors in Europe to make life-or-death decisions about which patients get a ventilator.
The number of people infected in the US has reached 250,000 and the death toll climbed past 6,000, with New York state alone accounting for more than 2,900, a surge of over 560 dead in just one day.
Most of the dead are in New York City, where hospitals are being pushed to breaking point.
The move by Mr Cuomo came as the Covid-19 outbreak snapped the United States' record-breaking hiring streak of nearly ten years.
The US government said employers slashed more than 700,000 jobs in March, bringing a swift end to the nation's lowest unemployment rate in 50 years.
The true picture, though, is far worse, because the government figures do not include the last two weeks, when nearly 10m Americans applied for unemployment benefits.