NFL team helmets are displayed at the NFL headquarters in New York in 2015. Photo by Brendan McDermid/Reuters

Weeks before the NFL season begins, new research reminds us of the physical cost of America’s most popular sport.

A new study of 111 former NFL players found all but one had a degenerative brain disease as known as chronic traumatic encephalopathy (CTE). These injuries may have started early in the players’ careers, based on the findings published in the Journal of the American Medical Association. The results also suggest those who play American football have a higher chance of developing long-term neurological conditions, like CTE.

“This is the largest study to date on CTE,” Michael Alosco, a clinical neuropsychology fellow Boston University and study co-author said. “It provides a rich source of data on what CTE look like on the brain and how the symptoms play out over the course of years.”

CTE is a disease that occurs in people who have endured repeated brain trauma. Symptoms of neurodegenerative diseases, such as CTE, not only affect one’s behavior, memory and mood, but can also promote dementia, the report stated.

Alosco and his colleagues studied the brains of 202 deceased players of American football across all levels of play, including those who participated in football before high school, in high school, semi professionally, in the Canadian Football League and in the NFL. All the players involved in the study had donated their brains for research.

Overall, 87 percent — or 177 players — had CTE. But the rate was significantly higher — 99 percent — in former NFL players. On average, participants with mild signs of CTE had played for 13 years, while subjects with severe CTE had careers lasting 15.8 years.

“We have followed the studies and the science on this serious issue closely, which is why we have advocated for and mandated the health and safety advancements in the NFL in recent years,” Kaitlin Murphy, a spokeswoman for the NFL Players Association said in a statement to PBS NewsHour. “Our union will continue to look at the developments in the medical and scientific community to fight for additional changes in the future.”

Last year, Jeff Miller, the NFL’s executive vice president of health and safety, admitted that a connection existed between football and brain damage in response to a Boston University study that found CTE in 90 of 94 former NFL players.

But in response to Tuesday’s study, the NFL stated, “There are still many unanswered questions relating to the cause, incidence and prevalence of long-term effects of head trauma such as CTE.” The NFL did not respond to the NewsHour’s request for comment.

The study exposes what happens during glimpses into this disease’s during its early stages and its progressions, by looking at a large number of people who endured the same type of repetitive head trauma within the same sport. Three out of 14 (56 percent) high school players had mild CTE pathology, whereas severe CTE was found in the majority of former college (91 percent), semi-professional (64 percent) and professional players (99 percent).

Brain injuries can promote mental conditions like bipolar disorder, Alzheimer’s disease and dementia, Anne McDonnell, executive director of the Brain Injury Association of Virginia, told the NewsHour.

Out of the 27 former players participants with mild CTE pathology, 96 percent had behavioral or mood symptoms while they were alive. That rate was similar for the 84 participants with severe CTE pathology. Of those, 89 percent had behavioral or mood symptoms, 95 percent showed a decline in cognitive ability, and 85 percent had signs of dementia.

“Damage to the brain, even a mild injury, can cause a decrease in the life span and puts people in the risk for dementia and Alzheimer’s,” McDonnell said.

Suicide was the most common cause of death for those with mild CTE, pathology whereas those with severe CTE typically passed away due to complications with neurodegenerative diseases, like pathology was neurodegenerative such as dementia or pParkinson’s disease. Former Pittsburgh Steeler Terry Long, former Chicago Bears defensive back David Duerson and Adrian Robinson, who played for several NFL teams throughout his career, are all examples of players whose suicides have been linked to brain disease.

Dr. Gregory J. O’Shanick, president and medical director of the Center for Neurorehabilitation Services in Virginia, said he isn’t sure if the NFL will do anything further with this information but that the study outlines path to addressing the long-term consequences of head injuries, like concussions.

“What this does show though is that in essence, you have nearly 100 percent evidence in abnormality in these players,” O’Shanick said. “[But what deserves more scrutiny] is the one individual who has no evidence of CTE. That’s the person that needs to be focused on to see what can be duplicated … to hopefully protect others.”

WATCH: High school football players discuss the pressure to stay in the game, despite concussions

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