A loose helmet may well come off in a crash. At the least, it will wobble about on your head while you are riding and disturb you, reducing your concentration. The way to check whether a helmet is likely to come off in a crash is to grab the back of it while you are wearing it and try to pull it up over the top of your head. If the helmet lifts more than a hand's span it is too loose. If it comes off, it is miles too loose!
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