Why NFL rookie Chris Borland decided to retire after one season

"Too much unknown" is prompting Chris Borland, one of the most promising NFL rookies, to retire from football after one season with the San Francisco 49ers. The 24-year-old linebacker's announcement brought the long-term effects of head trauma back into the spotlight, as Borland made the personal decision that the risk of brain damage "wasn't worth" the rewards of playing the game.

"People talk about knowing the risk going in, and I think guys understand, of course it's not good for you, but I don't think even the top neurologists truly understand the risks and the connections," Borland said Thursday on "CBS This Morning." "So, that's what I found in my research, and there's just too much unknown for me, and there have been too many tragedies for me to be comfortable playing."

During fall training camp last year, Borland said he sustained a hit that "was nothing out of the ordinary for a linebacker."

"There's a lot of vernacular in football about getting your 'bell rung' or getting 'dinged,' and it was one of those instances. The hit itself wasn't cataclysmic, it just changed the way I approached the game," Borland said.

The incident triggered him to ask, "Is this the route I want to go? How many times am I going to do this, for how long, and what are the real consequences?"

"Subsequently, I did a lot of research and ultimately came to the conclusion that no, it wasn't worth it for me personally," Borland said.

Borland didn't expect to attract as much attention as he did for his decision.

"Last week I had a conversation with a neurologist, and we had a good conversation about a lot of things, and then I said, 'Is this a lightning rod issue? Is this going to gain a lot of attention?' And he said, 'No, you'll be at the ticker at the bottom of ESPN one day,'" Borland said. "It's obviously gone further than that, which wasn't my intent, and I'm not really particularly interested in having in-depth conversations about it. However, while it's in the spotlight, I think there's been enough former players who have suffered and future players whose health might be at risk, so it's important to talk about the information that's available."

Among former NFL players who experienced brain trauma was Junior Seau, a star linebacker in the '90s and 2000s who died from a self-inflicted gunshot wound in May 2012. A study at the National Institutes of Health found that Seau had abnormalities in his brain consistent with chronic traumatic encephalopathy (CTE), a neurodegenerative disease linked to repeated concussions and head injuries.

More than a year before that, in February 2011, Dave Duerson, who rose to fame as a Pro Bowl safety in the '80s, shot himself in the chest, leaving a note asking for his brain to be donated to science. Researchers found "indisputable" evidence that Duerson had a moderately advanced case of CTE.

More recently, 27-year-old Sidney Rice of the Seattle Seahawks announced his retirement in July 2014 after experiencing multiple concussions. In an interview with CBS News, Rice said he was impacted by the stories of former NFL players like Hall of Famer Tony Dorsett, who was also diagnosed with having signs of CTE.

"You have these guys that have been going to the same house for 25 years. And all of the sudden they get to a certain point on their way home and they have to call their wives to get the directions home. So that is something that really hit home for me after having experienced so many concussions," Rice said.

After Borland's announcement, Dr. Joseph Maroon, Pittsburgh Steelers' team neurosurgeon and member of the NFL Head, Neck and Spine Committee, said on "NFL Total Access" that football has "never been safer." He also said CTE was being "overexaggerated and it's being extrapolated to youth football and to high school football" and that there are more injuries to kids when they fall off of bikes or playgrounds than there are in youth football.

"You can ride a bicycle and the act of riding a bicycle isn't causing brain trauma. Yeah, you could fall, but that's if something goes wrong. Everything could go right in football and it's still dangerous, which isn't an indictment of the game," Borland said. "I think if you love it and you think it's worth it, you should play. The important factor is that it's an informed individual choice."

When asked if the sport can be changed to address concerns like his, Borland said it's a big topic that he wasn't sure he could tackle.

"But I think there's a lot of things that can change," he said. "I think waiting is a good idea. Brevity might be a good idea, just playing a smaller amount of time. But no, I think the game is inherently dangerous, which isn't all bad, we don't need to be overly cautious, but you should be smart."

Borland said the 49ers and his teammates have been supportive and understanding of his decision.

Current and former NFL players piped in on social media, recognizing Borland's concerns.

"The most meaningful thing has been former players who have struggled who have reached out. That's been really touching," Borland said.

As for what's next, Borland is going back to school.

"I have some interesting opportunities. I was a history undergrad, and there's some things I could do in academics or business," Borland said. "I need to learn more, but there's a lot on the table."