Director-General Tedros Adhanom Ghebreyesus said people over age 60 or with underlying medical conditions also should wear masks in situations where social distancing can not be maintained.
But in an update affecting healthcare workers, the WHO now recommends that in areas with widespread virus transmission, all people working in clinical sections of a health facility should wear medical masks - not just those dealing with patients with Covid-19.
She said part of the reason for the widening of WHO's advice on face masks was the increasing evidence COVID-19 can be spread by people before they have symptoms.
The updated guidance also contains new information on the composition of fabric masks, based on academic research requested by the WHO.
"And we specify a fabric mask - that is, a non-medical mask", the WHO's technical lead expert on COVID-19, Maria Van Kerkhove, said.
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Based on new academic research, Tedros said the World Health Organization advised people to wear masks consisting of at least three layers of different materials, as outlined in the guidelines, if wearing fabric variants.
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Over 60s with underlying health conditions should go further, the WHO said, and wear medical-grade masks to give themselves better protection.
'Based on this new research, World Health Organization advises that fabric masks should consist of at least three layers of different material.' said Dr Tedros said.
The UK largely followed the same line until the transport secretary, Grant Schapps, announced on Thursday that face masks on public transport would be compulsory in the UK, with fines for those who disobeyed.
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Masks can also create a false sense of security, leading people to neglect measures such as hand hygiene and physical distancing.
Director-General, Tedros Adhanom Ghebreyesus, listed new precautions on Friday at virtual news conference.
With a fabric mask, "right now we only know that it provides a certain level of barrier, but we don't know if that's enough to prevent", Baller said. "So our goal fully remains suppressing transmission and save lives - and masks are part of that package of interventions".
However, the United Nations health body warns that masks are not a replacement for physical distancing, hand hygiene and other public health measures, and they are only of benefit as part of a comprehensive approach in the fight against COVID-19.
Ryan added that wearing masks needs to happen alongside other elements such as "case finding, cluster investigation, widespread testing, isolation of cases and quarantining of contacts".
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In the USA, legislation around masks is decided on a state-level - or even at a county-level in some cases.