When should you wear a face mask? Official guidance on how to wear them

mirror.co.uk:

Face masks become a more regular site on our streets, in our supermarkets and on public transport. People have turned to them as one of the ways to help protect themselves during the coronavirus pandemic, and they’ve become so popular it’s very hard to buy any.

But when should you wear one? And who should wear one? Mayor of London Sadiq Khan believes the public should be wearing facemasks when they leave the house., however the UK's chief medical officer Chris Whitty has said evidence of their effectiveness is minimal.

The World Health Organisation (WHO) has issued advice on who should use them, and when. Its advice covers who should wear them, when they should be worn, and how to wear them - in order to avoid masks causing infections