Narrator: This is Science Today. Each year there are about 300 thousand sports-related concussions in this country - about a third of them occurring on the football field. But it hasn't been until fairly recently that doctors realized even getting knocked around without losing consciousness can injure the brain. Martin Holland, a clinical professor of neurosurgery at the University of California, San Francisco says researchers now also realize that one concussion increases the chances of getting a second one, which can lead to second impact syndrome.

Holland: The post-concussive syndrome after second impact injury can be more severe. The recovery after the second injury is longer. That's one of the reasons why we don't want to let athletes go back too soon, because this is well established, especially in the younger populations.

Narrator: Studies in the past found more than half of high school football players returned to the game the same day as a loss of consciousness.

Holland: We have to educate - educate trainers, educate coaches, educate parents and educate the players themselves in terms of somebody should realize 'look, I've just got my bell rung, I'm gonna stay out of the next couple of games.'

Narrator: For Science Today, I'm Larissa Branin.