The study, published in The BMJ, examined the brains of 235 of the 878 former NFL players who died between 2016 and 2021.

The finding adds to ongoing research into the long-term effects of repeated head impacts in professional football. Chronic traumatic encephalopathy, or CTE, is a degenerative brain disease linked to concussions and other head trauma.

There were 215 (91.4%) donated brains that were diagnosed with CTE.

To take players who didn't donate their brains into account, researchers estimated a range for possible CTE prevalence between 24.5% and 97.7%.

"The true prevalence likely lies somewhere in the middle," they said, which would mean the actual percentage could be much higher than 24.5%.

The disease is likely caused by repeated head injuries, leading to the death of nerve cells in the brain. There is currently no way to definitively diagnose CTE in a living person.

The NFL has implemented various rule changes and protocols in recent years aimed at improving player safety and reducing head injuries. These include modifications to tackling techniques and stricter concussion management guidelines.

Former players and their families have pursued legal action against the league over health issues attributed to on-field impacts.

In the study’s conclusion, it found that NFL players have a higher prevalence of CTE at death than has been previously shown in multiple community-based studies.

The data indicated that CTE severity at death is significantly associated with dementia in NFL player brain donors, and that dementia is common in these donors.

Further research is needed to clarify the relation between CTE and dementia, particularly with a high lifetime exposure to repetitive head impacts.