How to I know? Well, to put it bluntly I'm forking lucky not to have lost an eye but my left cheek and corner of my mouth know about it and it's bloody painful.
Worst still is that I was wire wheeling at the time too.
Small price to pay for a mask/shield compared to what could be a possibility of an irreversible cost.
On that note, Merry Christmas and Happy New Year - have a safe one whether you're in the shed or on the roads

Bennie