For years now, research has emerged to show the effects of playing football—including repetitive concussive and sub-concussive hits—may lead to major brain damage. Now, a new study further explores the topic and affirms that players in the National Football League (NFL) could be at a higher risk for brain injuries.

The work published Nov. 28 in JAMA Neurology explored the connection between microglia and brain damage in football players. It was conducted from January 2015 through February 2016.

Previous research has provided evidence that widespread persistent microglial activation has been detected in a number of individuals months to years after a single traumatic brain injury. Researchers from Johns Hopkins University in Baltimore tested this thesis on former and current NFL players alongside other control patients who have never played the sport.

The study, led by Martin G. Pomper, MD, PhD, a radiologist at Johns Hopkins, compared brains of 14 current and former NFL players to 16 non players using positron emission tomographic imaging to measure a marker of activated glial cell response.

Results showed that NFL players who reported an average of seven years since their last self-reported concussion showed higher volumes of microglia in eight of the 12 regions examined compared to patients who didn’t play football.

“The results suggest that localized brain injury and repair, indicated by higher TSPO signal and white matter changes, may be associated with NFL play,” the authors wrote in the study. “Further study is needed to confirm these findings and to determine whether TSPO signal and white matter changes in young NFL athletes are related to later onset of neuropsychiatric symptoms.”