The general rule is that you have a big impact while you're wearing the helmet, then it shouldn't be trusted to offer you complete protection again. This does not include lesser impacts like dropping the helmet on the floor or bumping into the door frame as you walk through it.
This also only really applies to single use helmets that have a compressed styrofoam core. Other helmets (like my Protec classic) use sponge-type foam and a thicker plastic shell and can take multiple impacts without losing the ability to protect your head (unless you damage the shell)
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