MARINA DEL REY, Calif. - An estimated 350,000 athletes endure head injuries while playing sports every year in the United States, and that's only counting the ones who lose consciousness after impact.

Including the rest of the dings, pings and "rung bells" that don't knock people out but still result in concussions, the total could be adjusted as high as 3.8 million.

"A gross underestimation," is what concussion expert Gerry Gioia of the National Children's Medical Center in Washington called the 350,000 figure. "We don't know the true denomination of this problem."

That's what makes sports concussions, in the words of agent Leigh Steinberg, "a health epidemic, the consequences of which are a ticking time bomb that may not be seen in their totality for 10, 15 or 20 years."

Steinberg and NFL Hall of Fame quarterback Warren Moon, who suffered the first of his half-dozen concussions at age 11, headlined a summit Friday designed to draw attention to the growing problem of concussions in sports.

It's a problem most widely recognized in the NFL, where the suicide of former defensive back Andre Waters and the story of former Patriots linebacker Ted Johnson's hurried return to the field made recent headlines.

Waters' niece, Kwanna Pittman, said the symposium was meaningful "because, no, it didn't help Andre, but it can help someone else."

Indeed, it's an issue that goes well beyond the NFL -- to every sport, at almost every level.

Among the presenters Friday was Deborah Goldberger, whose 17-year-old daughter, Stephanie, has had four concussions as a soccer goalie. The last one forced her to give up the game. Seven months later, she's still tired, has headaches and has to go to sleep most nights at 8:30 p.m.

"It impacts everything you do on a daily basis," Goldberger said.

For every concussion identified in the NFL, Gioia said, another 56,500 occur among kids. Some are so hard to recognize -- such as the basketball player who falls and bumps his head on a teammate's knee but gets right back up -- that it's difficult to know when they happen and what to do.

"We've got to manage it better and educate people about it better," Gioia said, "so we don't end up with pro athletes saying, 'After my 15th, I stopped counting.'"

Determining and differentiating the immediate and long-term effects of concussions is a constantly shifting science.

Steinberg touted numbers from a series of recent studies, hoping the seriousness of the new stats will intensify the debate and bring quicker change.

"What are the stakes?" Steinberg said. "It's one thing to go out and play football and understand that when you turn 40, you can bend over to pick up your child and have aches and pains. It's another thing to bend down and not be able to identify that child."

One study showed athletes are six times more likely to suffer a second concussion if they return within a week of the first one. A client of Steinberg's, former 49ers quarterback Steve Young, retired after precisely that kind of quick return and repeat injury.

Other studies showed that in cases where athletes had three or more concussions over their lifetimes, they were five times more at risk for early onset Alzheimer's disease, three times more at risk of significant memory loss and four times more likely to have severely elevated depression.

The NFL and the NFL Players Association have done some things to lessen the impact of head injuries -- most notably with rules changes and the gradual elimination of Astroturf as a playing surface. They're looking at changes in helmets and mouthpieces to lessen the severity of hits to the head.

Steinberg some day also would like to see neurologists present on every NFL sideline and more effective use of baseline tests that establish players' capabilities before they get injured and can be used as markers for the tests they take after concussions.

Not as easily changed is the football-player mentality. From a young age, they're taught to play in pain, to sacrifice their bodies, to stay on the field and to make their coaches happy.

"Most athletes are in a state of denial," Steinberg said. "They're taught to ignore pain."

He hopes some of the most recent studies will lead NFL management to conclude rushing players back isn't in anyone's best interest. After all, is getting a player back for one week worth risking him for the rest of the season -- or his career? It's a question that has gained more traction as salaries and signing bonuses have exploded over the past decade.

Almost as difficult as changing the mind-set is determining what is or isn't a concussion, which translates into the kind of medical attention paid to an injured player.

To illustrate this challenge, one doctor showed two plays from a recent Oregon-Arizona college football game. One showed a player's helmet getting popped off after a particularly vicious hit made under his chin. Another showed one player knocking his teammate to the ground in a congratulatory body bump after a good defensive play.

The player in the first example walked away with a cut lip. The player in the second example had a concussion because his head banged against the turf as he received the congrats.

"It goes to show that we can be fooled by what we see," said Kevin Guskiewicz, a specialist from the University of North Carolina who studies the biomechanics of sports concussions.