Catastrophic injuries are not common in American football. According to the National Center for Catastrophic Sport Injury Research, there were 468 non-fatal injuries resulting in permanent neurological damage across all high school sports in the United States from 1982 to 2011. In football, catastrophic injuries are rare but are devastating when they occur. The rate of catastrophic head injuries has remained low since the introduction of the modern football helmet in the 1970s, but rates of injury are much higher at the high school level than the college level. A 2007 study found that, in high school and college football, there are an average of 7.23 catastrophic head injuries per year: there were 0.67 injuries per 100,000 high school players and 0.21 injuries per 100,000 college players. Over a 13-year period from September 1989 to June 2002, there were 94 players who sustained catastrophic head injuries—8 of these players died as a result of the injury, 46 sustained permanent neurological damage, and 36 made a full recovery. 56% of these players had a history of head injuries, 71% of them occurring in the same season as their catastrophic injury, and most of the catastrophic injuries resulted from being tackled or making a tackle. The study recommended that players exhibiting neurological symptoms should be strongly discouraged from returning to play.