Simply stated helmets do not prevent concussion.
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"While helmets do not prevent a brain injury, they do reduce the risk of a structural brain injury by as much as 85%. Helmets also do reduce the risk severe physical injuries to your head such as a skull fracture, along with other structural damage that can occur in brain injuries. On the other hand, helmets do not prevent injury to the brain or brain functionality such as concussion, or closed head injuries. "
The American experience was that giving grid iron players a helmet encouraged them to use their heads as battering rams and actually increased the risk of head injury.