The most dangerous result of a head injury in hockey can be classified as a concussion. Most concussions occur during player-to-player contact rather than when a player is checked into the boards. Checks to the head have accounted for nearly 50% of concussions that players in the National Hockey League have suffered. In recent years, the NHL has implemented new rules which penalize and suspend players for illegal checks to the heads, as well as checks to unsuspecting players. Concussions that players suffer may go unreported because there is no obvious physical signs if a player is not knocked unconscious. This can prove to be dangerous if a player decides to return to play without receiving proper medical attention. Studies show that ice hockey causes 44.3% of all traumatic brain injuries among Canadian children. In severe cases, the traumatic brain injuries are capable of resulting in death. Occurrences of death from these injuries are rare.