At least one in four former NFL players who died between 2016 and 2021 was found to have had chronic traumatic encephalopathy (CTE), according to a new study.
Out of the 878 former players who died during that timeframe 235 donated their brains, researchers said, with 215 of those donated brains found to have CTE.
The study's lead author Daniel Daneshvar, the chair of physical medicine and rehabilitation at Mass General Brigham and an associate professor at Harvard Medical School, said it was difficult to determine how common CTE was as it could only be diagnosed after death.
"People who donate their brains are more likely to donate if they have some concern, some reason for donating, if they think that either they or a loved one is having problems with their memory or thinking or concentration or behaviour changes," he told ABC News Breakfast.
"And so you can't just look at the rate of CTE amongst people who donate to get a sense of how common this disease is."
Dr Daneshvar said the "possible prevalence of CTE" among NFL players was "very concerning".
"In the general population, less than 1 per cent of people get this disease pathology in their brain," he said.
The study from Mass General Brigham, Boston University, and the Concussion & CTE Foundation also found dementia was common among brain donors, with researchers urging prevention measures that focus on cutting down on repetitive head impacts across all levels of the sport.
"Similarly, we and others have shown that in NFL players, specifically, there is about a four times higher rate of death from neurodegenerative disease than the general population," Dr Daneshvar said.
"The bulk of the head impacts that these former NFL players experienced didn't occur at the NFL level.
"They occurred at the collegiate level ... and at the high school level and in many cases the youth level.
The NFL's 2011 Collective Bargaining Agreement limited the number of contact practices during the season. The Ivy League banned full-contact tackling during practices five years later.
The Supreme Court cleared the way for the NFL's settlement of concussion-related lawsuits with thousands of retired players in 2016.
An NFL spokesperson said the league continuously strives to make the game of football safer, "including by implementing strategies to reduce concussions and head impacts".
"The NFL remains committed to ensuring that the NFL community has access to a robust — and expanding — set of resources to enhance their physical and mental wellbeing," they said.
"We encourage former players to utilise these resources to identify and seek treatment when they are concerned about their health."
Dr Daneshvar urged current and former athletes experiencing cognitive symptoms to seek a neurodegenerative disease expert evaluation.
"We know that nearly 70 per cent of head impacts are occurring in practice.
"So without changing a single minute of game time at any of those levels, we can be substantially reducing the cumulative head impact burden that these individuals are experiencing throughout their lives."
