A helmet should not:

You will get the greatest amount of protection from a helmet if it meets U.S. Department of Transportation (DOT) and state standards, fits snugly all the way around, and has no obvious defects. A helmet worn at the time of a crash should be replaced.

Meet U.S. DOT standards.
Fit snugly.
Be without obvious defects.
Have been previously worn in an accident.

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