Fifteen years on from the attacks on the World Trade Center, the firemen, police officers and other emergency service personnel who risked their lives to help those caught up in the chaos continue to suffer the emotional and cognitive consequences of the trauma. According to a new study published this week in the journal Alzheimer’s and Dementia, 9/11 rescue workers with post-traumatic stress disorder (PTSD) may be at risk developing cognitive impairment and dementia at an abnormally young age.

PTSD is a complex condition that includes an array of disturbing symptoms, including flashbacks and other emotional and behavioral responses to traumatic events. Common among war veterans, the condition has been linked to early dementia, although scientists cannot yet explain why this is the case.

In light of this alarming phenomenon, the team behind this latest study recruited 818 World Trade Center rescue workers from a clinic that specializes in treating those who suffer from PTSD as a result of 9/11. When assessing the mental health of these individuals, the researchers discovered that 12.8 percent – or around one in seven – showed signs of cognitive impairment, while 1.2 percent had scores suggesting possible dementia.

“These numbers are staggering, considering that the average age of responders was 53 during this study,” explain the authors in their write-up. Indeed, dementia is typically defined as an age-related decline in cognition, usually occurring at a much later stage in life than one’s early fifties.

Furthermore, given that around 33,000 people have been treated for PTSD and depression as a result of their involvement in the 9/11 attacks, the researchers worry that a mere extrapolation of their data could indicate that up to 5,300 could be suffering from cognitive impairment and 810 from dementia.

Though much remains poorly understood about the neural mechanisms underlying both PTSD and dementia, study author Sean Clouston insists in a statement that “if our results are replicable, doctors need to be aware of the impact of cognitive impairment among individuals who have experienced traumatic events leading to PTSD.”