San Francisco 49ers linebacker Chris Borland gave up the final three years of his $2.9 million rookie contract Monday when he retired from the NFL after just one season due to concerns about potentially suffering a long-term neurological injury. But the 24-year-old may have also added as many as 20 years to his overall lifespan by making the difficult decision to walk away from America's most popular professional sport.

A third-round pick in the 2014 NFL Draft, Borland was paid approximately $420,000 and a $617,436 signing bonus during a successful debut season in which he started eight games at inside linebacker. Fellow 49ers linebacker Patrick Willis’ surprise retirement after the season at age 30 all but assured Borland a full-time starting job and a shot at an even more lucrative second contract. But Borland, a University of Wisconsin graduate, researched the neurological disorders that often affect former NFL players, such as chronic traumatic encephalopathy (CTE), and determined an ephemeral chance at fame and fortune wasn’t worth possible brain damage.

“We’re so used to seeing players ‘retired’ by their teams, it’s rare to see a player retire himself and go out on his own terms. … I don’t remember anyone coming to me in my time with the Packers and saying, ‘Because of what I know about concussion risk and future head trauma, I’m not going to play anymore,’” said Andrew Brandt, an ESPN analyst and former vice president of the NFL’s Green Bay Packers.

A Rare Stand

Borland’s decision, while inherently logical, is a rarity in a league where the lack of fully guaranteed contracts calls job security into constant question. Players unwilling or unable to physically compete are regularly cut from team rosters, with the mantra “next man up” running constantly through the minds of NFL coaches and personnel executives. Borland himself admitted he played through a possible concussion suffered in training camp last season so as to make the 49ers’ roster.

“I just thought to myself, “What am I doing? Is this how I’m going to live my adult life, banging my head, especially with what I’ve learned and know about the dangers?” Borland told ESPN’s “Outside the Lines.”

Those dangers of an NFL career are well-documented. As of 2013, more than 100 former players had sued the league in connection with head injuries suffered on the field, according to the Boston Globe. Many said the NFL did not properly educate players on the negative long-term effects of brain injury. Countless others have suffered significant injuries to their legs or developed an addiction to painkillers. A $1 billion settlement that the NFL reached with players in August 2013 is still being finalized.

The numbers get worse. A 2009 study commissioned by the NFL found former players developed Alzheimer’s disease or memory loss at 19 times the normal rate of the average American aged 30 to 49, according to the New York Times. A 2012 study by the American Academy of Neurology found that NFL players who played at least five seasons in the league from 1959 to 1988 were three times more likely to die from brain damage and nearly four times as likely to die from Alzheimer’s disease or from Lou Gehrig's disease, which is also called amyotrophic lateral sclerosis (ALS).

Unsurprisingly, the prevalence of these neurological disorders has affected the average NFL player’s life expectancy. The NFL’s players union in 2013 commissioned Harvard University to spearhead a 10-year study on long-term health risks of playing in the league. While the average white American male lives to age 78 and the average black American male lives to age 70, Harvard researchers found that professional football players in North America “have life expectancies in the mid- to late 50s,” Harvard professors said in a pre-project summary, according to the Boston Globe. That means Borland or any other player who chooses to retire at his age could add decades to his lifespan.

'I'd Have To Take On Some Risks'

The scientific community’s understanding of CTE is still in its infancy, but the degenerative brain condition played a major role in Borland’s decision to retire and is a source of increasing concern among NFL players. CTE, which is purportedly brought on by repetitive head trauma, causes depression, aggression and strange behavior in those it affects. Former Chicago Bears safety Dave Duerson, former Atlanta Falcons safety Ray Easterling, former Pittsburgh Steelers center Mike Webster and former San Diego Chargers linebacker Junior Seau were each posthumously diagnosed with CTE. Duerson, Easterling and Seau committed suicide.

“I’ve thought about what I could accomplish in football, but for me, personally, when you read about Mike Webster and Dave Duerson and Ray Easterling, you read all these stories, and to be the type of player I want to be in football, I think I’d have to take on some risks that, as a person, I don’t want to take on,” Borland said.

And being the type of player Borland wants to be doesn’t necessarily guarantee lifelong financial security. The average NFL player’s career lasts for three-and-a-half years, with an average annual salary of $1.9 million and $6.7 million in career earnings. All three of those figures are the lowest averages, by far, of players in America's four major team sports. Statistics show that NFL players between ages 28 and 35 earn the most money, according to Business Insider. But most players are out of the league before then.

“That’s the real problem with nonguaranteed salaries,” Brandt said. “Guys are willing to play through and not worry about the long-term risk, knowing that they can be cut at any time.”

Borland earned a bit more than $1 million during his one-year NFL career, which means he likely left up to $5.5 million in career earnings on the table when he decided to prioritize his health. There are indications that Borland isn’t alone in his thought process -- he’s the fourth football player age 30 or younger to retire from the NFL this offseason, along with former teammate Willis, 27-year-old former Pittsburgh Steelers linebacker Jason Worilds and 26-year-old former Tennessee Titans quarterback Jake Locker. Both Worilds and Locker were free agents who had garnered interest from other teams.

But for now, these players are outliers among the thousands of individuals willing to risk their long-term health for a shot at NFL stardom. Among them is Seattle Seahawks linebacker Bobby Wagner, who commented on Borland’s retirement on Twitter on Monday night.

“No offense to anyone but I’m playing until I can’t anymore,” Wagner wrote. “I love this game [too] much.”