The September 11 terrorist attacks on the World Trade Center appeared to have left one in eight Lower Manhattan residents with signs of post-traumatic disorder (PTSD), according to a study published online this week in the Journal of Traumatic Stress.

PTSD is an anxiety disorder that can occur after you have been through a traumatic event such as combat or military exposure, child sexual or physical abuse, terrorist attacks, sexual or physical assault, serious incidents (car crashes), natural disasters such as fire, tornado, hurricane, flood or earthquake.

These events cause people fear for their lives, living them feeling helpless. Strong emotions caused by the event create changes in the brain that may result in PTSD.

Not everyone going through a terrible event is at risk of PTSD. How likely people are to get PTSD depends on many things such as how close they were to the event, how strong their reactions were, how intense the trauma was or how long it lasted, how much they felt in control of events, how much help and support they got after the event or if they lost someone they were close to or were hurt.

People with PTSD experience irritability or anger, sleep difficulties, trouble concentrating, extreme vigilance, flashbacks and nightmares.

The New York City health department analyzed 11,000 residents through the World Trade Center Health Registry and found 12.6 percent of all respondents showed signs of PTSD as many as three years after the attacks.

The study was the first one to measure the attack’s long-term effect on the mental health of the community.

Factors such as sex, race, study level and income appeared to have influenced the respondents’ chances to develop PTSD include

More exactly, women were more likely to be affected, with 15 percent reporting symptoms compared with 10 percent of men.

Hispanics (24.7 percent) and blacks (20.6 percent) were more affected than whites (10.7 percent) and Asians (8.9 percent).

Only 11.1 percent of respondents with more than college diploma were affected compared with 18.3 percent of respondents with less than a high diploma.

Only 11.3 percent of hose earning more than $50,000 to $74,000 compared to 19.8 percent of those earning less than $25,000 a year.

About 38 percent of those injured in the attacks were most likely to be still suffering PTSD. About 17 percent of those who witnessed violent deaths and 17 percent of those caught in the dust cloud after the towers collapsed were showing signs of PTSD.

All these people “require more in-depth mental health monitoring, independent of the larger metropolitan area” the study authors concluded.

These figures add to hundreds of military troops coming back from Iraq and Afghanistan who develop PTSD symptoms yearly, and also to hundreds of other people affected by the Hurricane Katrina three years ago.