There are places where face coverings are required eg public transport /shops. Some people are rightly exempt. There is no obligation to display a sign saying that that person is exempt however. This makes it difficult to ask someone to wear a face-covering without fear of causing offence.
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Over 1,000 people have died of coronavirus on 20 January alone. Many of us may be vulnerable, unable to avoid entering a shop or travelling on public transport. Those who should and can wear a face covering must, and those who cannot, should feel no shame in displaying a recognised sign. If they can register to self-certify for one (or face a fine), a widely available, free of charge sign will avoid those rightly exempt from feeling judged, promote good practice, help enforcement and avoid deaths.