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Helmets, whether they be bicycle or motorcycle helmets, are designed to only protect against a single fall. If your is ever hit during an accident, you should definitely replace your helmet even if it looks fine.

This is also why you should never buy a used helmet as it can be very risky.

Helmets also expire after a few years and manufacturers even recommend to replace them after 3 years. You should thus be replacing your helmet at least once every 5 years.