Long-term Posttraumatic Stress Symptoms among 3,271 Civilian Survivors of the September 11, 2001, Terrorist Attacks on the World Trade Center

Laura DiGrande; Yuval Neria; Robert M. Brackbill; Paul Pulliam; Sandro Galea


Am J Epidemiol. 2011;173(3):271-281. 

In This Article


In the largest cohort of civilian survivors of the WTC attacks assembled to date, we found that the majority of survivors experienced multiple 9/11-related posttraumatic stress symptoms 2–3 years after the attacks. Almost 1 in 6 participants screened positive for probable PTSD. This finding underscores the long-lasting mental health effects of mass violence but is lower than the rate reported in similar research on other terrorist attack survivors.[6] Despite differences in levels of exposure, study design, screening instruments, and length of follow-up, assessments of bombings in France between 1982 and 1987 found PTSD rates of 31% and 18% among survivors 2–3 and 8 years later, respectively.[27,28] Six months after the 1987 bombing in Enniskillen, Northern Ireland, 50% of survivors were determined to have PTSD.[28] In the United States, 34% of survivors of the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City had PTSD 6 months after the attack, and PTSD was chronic in 100% of cases, lasting ≥3 months.[30] In a follow-up study 17 months after the bombing, 89% of cases were unremitted.[31]

One explanation for the lower prevalence of PTSD documented here compared with the few other extant postterrorism studies is that many survivors of the attacks on the WTC did not realize that their lives were in extreme danger during the event. Reports from evacuees included descriptions of calm descent.[32–34] Many survivors did not imagine that the towers would collapse, and it was not until after a safe evacuation that the magnitude of the situation was apparent. It is possible that awareness of life threat is a driver of high risk for PTSD,[35] and the absence of such knowledge accounts for the lower prevalence documented here compared with other terrorism studies. Still, longitudinal study of WTC survivors to determine whether symptoms increase or decrease over time is warranted; research on the Oklahoma City bombing showed that symptoms and functional impairment of bombing survivors might not be diagnosable for years after an attack.[31]

As expected, we estimated a higher prevalence of PTSD among tower survivors than among the general population after 9/11.[9–13] Although population studies demonstrated that rates of 9/11-related PTSD declined over time, they also found that those with the highest exposure burden experienced the worst mental health. For example, in a small group of individuals who were physically in any part of the WTC complex at the time of the attacks and part of a representative sample of New York City residents, 36.7% had PTSD within 6 months of 9/11,[13] and among primary care patients who reported being in the WTC or lower Manhattan on 9/11, 17.1% had PTSD 1 year after the attack.[25]

We examined demographic risk factors and found that their relation supported the general association of social disadvantage with adverse mental health indicators.[6] Our observations about female gender were consistent with findings from other post-9/11 studies,[36] and could have been explained by women's higher prevalence of pretrauma psychological disorders, ancillary stressors, caretaking responsibilities, and gender role expectations.[37] African Americans and Hispanics were more likely to have PTSD than were whites, similar to what was seen in other studies.[38] Explanations for this observation include differential vulnerability,[39] pre-9/11 exposure,[39] immigrant status,[25] pre-9/11 mental health problems,[39] lack of social supports, and perievent emotional reactions.[39] However, by far the strongest demographic risk factor in our study was low income. We observed a pronounced dose-response relation between income and PTSD that was independent of other covariates. Potential explanations for this relation include findings that poor populations tend to avoid seeking[40] or receiving[41,42] treatment from mental health specialists, as well as broader sociologic theories on the effect of marginalization, lack of resources, and powerlessness on coping with negative life events.[25,43–45]

It has been well-documented that the extent of exposure to a disaster is one of the most important factors in the development of disaster-related PTSD. However, most studies used relatively crude measures of exposure, often combining the loss of a loved one or property and participation in rescue efforts into one broad categorization of direct exposure. Our large cohort allowed us to focus on experiences by civilians. PTSD risk was greater among survivors whose experiences indicated severe life threat, as defined by location in the towers, time of evacuation initiation, or dust cloud exposure. We also found strong relations between having witnessed horror or having sustained injuries and PTSD, consistent with other studies on human-made disasters.[12,13,46–48] One potential explanation as to why those who witness horror are more susceptible to PTSD might lie in a biologic understanding of PTSD etiology, as images of grotesque and unimaginable scenes are encoded into memory and may be relived upon stimuli.

We found an interesting relation between coworker casualties and PTSD. In a meta-analysis, Rubonis and Bickman[7] found that the number of human casualties clearly affects the relation between disasters and consequent psychological problems. It is possible that WTC fatalities within one's company indicated increased life threat for survivors, but bereavement for colleagues who perished may have also contributed to this finding.[7,46]

The cumulative effect of severe exposure was found to be significantly associated with PTSD in stepwise regression, demonstrating that although tower survivors may have shared a collective experience, individuals who were exposed to several of the most troubling and life-threatening events during the disaster were at the greatest risk of PTSD. This cumulative effect is an important observation that has implications for future disaster response and that warrants further attention. We are aware of only 1 prior study related to the WTC disaster that used a comparable measure of exposure severity.[49] Brackbill et al..[50] found that residents with a very high exposure level (i.e., ≥6 of 14 events, including having been afraid of being killed, having a friend or relative killed, having been forced to move, having financial difficulties, etc.) were significantly more likely to screen positive for PTSD at a 1-year assessment.

Strengths of the present study include the procurement of a large cohort of civilians who survived the WTC attacks that had characteristics comparable to the NIST's smaller egress study, the use of a validated measure of PTSD, and detailed characterizations of 9/11 exposures that were not explored in previous studies. A central limitation to this study was the recruitment strategy, which may have produced selection bias and limited generalizability to all those who evacuated the towers on 9/11. The true effect of this bias is unknown because of the lack of information in the literature on all tower survivors, but if those most affected were more likely to participate in our study, our estimates would have been inflated. Alternatively, if those most affected avoided participation, true symptom prevalence would have been underestimated. Our findings were also based on a cross-sectional survey, which typically makes the temporal relation of exposures and outcomes unclear. However, exposures assessed here were ones that occurred on 9/11, and all stress symptoms were reported as occurring in the last 4 weeks before participation. This study relied on self-reporting, which could have affected the precision of true symptom prevalence. Although the PCL-S is one of the most widely used screening instruments today, it is inevitably less precise than a clinical interv iew. Confounding and effect modification could have led to nonrandom error in our risk factor analyses: Lack of information on preevent psychological functioning and trauma, subjective appraisal of life threat, bereavement, social support, and mental health services may account for some of the observed relations between the risk factors examined here and PTSD.[6] Further, our data collection instrument did not include other psychiatric disorders, such as depression, generalized anxiety disorder, or substance abuse, that are commonly observed after trauma.[12,30]

As the long-term effects of the WTC disaster emerge,[50,51] the results from this study suggest that some survivors of the WTC disaster will continue to report psychological symptoms years after their exposure to the events of 9/11. The implication of this finding is that the impact of terrorism on survivors, particularly those in low socioeconomic positions, could be substantial, as PTSD is known to be comorbid with other disorders and harmful behaviors that affect daily functioning, wellness, and relationships.[52]

As disaster literature moves toward understanding long-term risks in the general population, this study reminds us that the relation between direct exposures and PTSD is clear and suggests potential avenues for planning policy to reduce the burden of terrorism-related psychopathology. Disaster preparedness training for the civilian workforce should incorporate disclosure of potential experiences that contribute to PTSD risk such as evacuation, injury, and personally witnessing the horrors of death and destruction. The additive effect of such direct exposures should also be taken into consideration during postdisaster treatment.


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