It’s NFL season again. In the Minnesota Vikings’ first game of the season, quarterback Kyler Murray’s debut run made it just 11 plays before a violent shoulder to the right side of his helmet sent him to the turf.

The blow was severe enough to trigger a strict concussion protocol designed to ensure potential trauma to the player’s brain is given every chance to recover.

Officials will be watching Murray closely to determine when he’s fit enough to return, though the knock to his brain may have already contributed to lifelong damage that could affect his mental well-being long after retirement from the sport.

A new study emphasizes the risk professional sportsmen like Murray face every time they play, estimating that at least 18% of the 1,712 former NFL players who died between 2008 and 2021 had chronic traumatic encephalopathy when they died.

Shockingly, the true figure may be just shy of 100%, with 215 of the 235 brains donated in the 2016-2021 period showing a serious degree of damage.

Chronic traumatic encephalopathy (CTE) has a lot in common with other neurodegenerative diseases like Alzheimer’s. Every time brain tissue is given a good shake, structural proteins called tau can misfold and accumulate in toxic bundles, increasing the risk of nerve cells dying. This makes it a difficult condition to diagnose easily in a living patient. Only on death can specialists spot the signs of its pathology.

Still, a handful of studies suggest that few of us are ever hit hard enough or often enough to trigger the build-up of dangerous proteins. A study published in 2018 found that only one of the 164 donated brains in its study had CTE. Another study from 2021 found three of the 532 brains scanned showed signs of the disease.

Football players frequently experience impacts that briefly subject their brains to as much as 180 times the pull of gravity (G). The brain can cope with around 85 to 90 G before it risks a concussion. The math isn’t on their side.

A 2023 analysis of 631 American football player donations in the Understanding Neurologic Injury and Traumatic Encephalopathy brain bank found 451 – just over 70% – had signs of CTE.

Brain donations from deceased football players have risen in recent years, prompting researchers to take a fresh look at the risk and extent of CTE with respect to dementia diagnoses.

Based on death certificates alone, a total of 1,712 people who played at least one NFL game since helmets became compulsory in 1949 died between 2008 and 2021. Of those, 338 donated their brains to one of three US "brain banks."

If we assume those donors fairly represent the total number who died, 93% of NFL players showed some signs of CTE pathology.

There’s good reason to believe that donors are a slightly different breed of player. They tend to have played longer than non-donors, for example, and have participated in more Pro Bowl games.

If we were to assume instead that donors self-selected as a unique group, the 315 brains with CTE only represent 18% of the total.

There could also be a few differences in volunteers who decided to donate once word hit the press that CTE was a big deal. Limiting the findings to when most of the brains were donated, which was between 2016 and 2021, the numbers range from just 24.5% to an incredible 97.7%.

Those are some truly staggering statistics, especially given the increased chance of developing a neurodegenerative condition following CTE.

Among the people who had donated their brains to research, nearly 60% had been diagnosed with dementia, while just under 19% had a neurodegenerative condition listed as a cause of death.

With the concussion protocol only implemented in 2011, it’s too soon to tell if it will make a significant difference to the likes of Murray. Hopefully, decades into the future, studies just like this one will compare the numbers and find that our increased awareness of the full impact of concussions means far fewer players will pay for their moment of glory with a dementia diagnosis in years to come.

This research was published in the BMJ.