PHILADELPHIA - OCTOBER 17: DeSean Jackson #10 of the Philadelphia Eagles is laid out by Dunta Robinson #23 of the Atlanta Falcons during their game at Lincoln Financial Field on October 17, 2010 in Philadelphia, Pennsylvania. Both players were injured on the play and had to be helped off the field. (Photo by Al Bello/Getty Images)

Source: CDC

PHILADELPHIA - OCTOBER 17: DeSean Jackson #10 of the Philadelphia Eagles is laid out by Dunta Robinson #23 of the Atlanta Falcons during their game at Lincoln Financial Field on October 17, 2010 in Philadelphia, Pennsylvania. Both players were injured on the play and had to be helped off the field. (Photo by Al Bello/Getty Images)

OAKLAND, CA - AUGUST 18: Quarterback Troy Aikman #8 of the Dallas Cowboys ices the back of his neck and drinks a cup of Gatorade as he sits on the bench to recover from a concussion during a preseason game against the Oakland Raiders at Oakland/Alameda County Coliseum on August 18, 1990 in Oakland, California. The Raiders won 16-14. (Photo by George Rose/Getty Images)

ST. LOUIS - SEPTEMBER 23: Third baseman Chris Donnels #18 of the Arizona Diamondbacks lies on the infield after he suffered a mild concussion while fielding a bunt in the third inning of the MLB game against the St. Louis Cardinals on September 23, 2002 at Busch Stadium in St. Louis, Missouri. The St. Louis Cardinals beat the Arizona Diamondbacks 13-1. (Photo by Elsa/Getty Images)

ASPEN, CO - JANUARY 25: Lindsey Jacobellis of Stratton, Vermont, exits the superpipe after shattering her helmet and suffering a concussion as she fell during practice for the Women's Snowboard Superpipe at the ESPN Winter X Games 11 January 25, 2007 in Aspen, Colorado. (Photo by Doug Pensinger/Getty Images)

WORCESTER, UNITED KINGDOM - FEBRUARY 11: Jason Robinson of Sale lies on the ground after suffering a concussion during the Guinness Premiership match between Worcester Warriors and Sale Sharks at Sixways on February 11, 2006 in Worcester, England. (Photo by Warren Little/Getty Images)

CHARLOTTE, NC - FEBRUARY 22: Gerald Wallace #3 of the Charlotte Bobcats is strapped to a gurney after suffering a concussion from a hit to the face by Mikkie Moore #33 of the Sacramento Kings during a game at Bobcats Arena on February 22, 2008 in Charlotte, North Carolina. (Photo by Streeter Lecka/Getty Images)

GLENDALE, AZ - OCTOBER 05: Starting Quarterback Trent Edwards #5 of the Buffalo Bills suffers a concussion after getting hit by Strong Safety Adrian Wilson #24 of the Arizona Cardinals during the first half of their NFL Game on October 5, 2008 at Stadium in Glendale, Arizona. (Photo by Donald Miralle/Getty Images)

WASHINGTON - OCTOBER 28: Retired National Football League player Merril Hoge (R) testifies before the House Judiciary Committee about football brain injuries on Captiol Hill October 28, 2009 in Washington, DC. A recent NFL study of retired players suggested that N.F.L. retirees ages 60 to 89 are experiencing moderate to severe dementia at several times the national rate. Merril nearly died in …

ATLANTA - NOVEMBER 08: Clinton Portis #26 of the Washington Redskins watches from the sidelines after suffering a concussion against the Atlanta Falcons at Georgia Dome on November 8, 2009 in Atlanta, Georgia. (Photo by Kevin C. Cox/Getty Images)

WASHINGTON - SEPTEMBER 23: Stanley Herring, chairman of the Subcommittee on Education and Advocacy, Head, Neck and Spine Committee of the NFL and team physician for the Seattle Seahawks and Seattle Mariners, holds up a Centers for Disease Control poster about concussions while testifying before the House Education and Labor Committee about the Protecting Student Athletes from Concussions Act on …

WASHINGTON - SEPTEMBER 23: Former NFL athlete and current executive board member for the NFL Players Association Sean Morey testifies before the House Education and Labor Committee about the Protecting Student Athletes from Concussions Act on Capitol Hill September 23, 2010 in Washington, DC. After 12 successful seasons in the NFL, Morey decided to stop playing after doctors told him the risks …