Most injuries to the head are superficial and recovery is quick. As such, these injuries need be of no more concern than injuries to other parts of the body.
A minority of head injuries have more serious consequences, and may lead to death or chronic intellectual disablement, usually as a result of brain damage. Most brain injuries sustained in road crashes involve rotational forces, which cycle helmets are not designed to mitigate. Indeed, concern has been expressed that helmets might make some injuries worse by converting relatively harmless direct forces to rotational ones.