Residents in London and New York are being told to cover their faces with masks or other items of clothing to protect against coronavirus.
New rules requiring New York residents to cover up go into effect today, as social distancing measures have been extended for another month by New York Governor Andrew Cuomo.
Everyone must wear a mask when in a public place and unable to maintain a distance from others. Children under 2 and those with medical reasons are exempt in the city.
New York has been in lockdown since May 22 and will stay that way until at least May 15 as it battles the worst coronavirus outbreak in the US.
Earlier this month the US Centre for Disease Control recommended wearing face masks in public but states have made their own rules and President Donald Trump said he would not do it.
In London, Mayor Sadiq Khan broke ranks with medical bosses and urged people to wear masks or face coverings in public if they cannot maintain a safe distance after at least 20 transport workers died of coronavirus.
The UK's lockdown will last until at least May 7.
"I'm lobbying the Government to advise people to wear non-medical face coverings where it's impossible to keep a safe social distance," he said on Twitter.
"These would NOT be medical masks – which must be reserved for health and care workers who desperately need them – but scarves or reusable face coverings to reduce the risk of the virus spreading. It is time to act."
It comes after at least 20 bus drivers were killed by the virus in recent weeks. Khan, whose father was a bus driver, said it "breaks my heart."
"It could easily have been my dad and his friends," he said.
"Our transport workers are heroes and we must do everything we can to protect them."
The World Health Organisation advises to only wear a mask if you are ill with Covid-19 symptoms (especially coughing) or looking after someone who may have Covid-19.
On its website, WHO stated that wearing multiple face masks was "not effective" against the novel coronavirus and could actually be harmful if they were worn incorrectly or contaminated.
Australia's public health authorities are "actively looking" at whether they should change official advice, but at this stage Australians are advised they are not necessary.
Commonwealth Deputy Chief Medical Officer Paul Kelly has said face masks were crucial for health workers, but "not recommended for the Australian public".