Report: 1993 WTC Bombing Probably Saved Lives on 9/11

A new report that examines personal and environmental factors impacting the evacuation of the World Trade Center (WTC) on 9/11 finds that the bombing of the WTC in 1993 led to better evacuation plans for many employers in the towers, better signage and better lighting in stairwells, all of which contributed to the relatively high number of survivors of the 9/11 attacks.

On 9/11, an estimated 13,000-15,000 people were successfully evacuated from the two WTC towers. Because full-scale evacuations of such buildings are rare, little is known about how readily and rapidly these buildings can be evacuated and what factors serve as facilitators or barriers to the process. In 2002, the Mailman School of Public Health at Columbia University and the Centers for Disease Control and Prevention (CDC) initiated "The World Trade Center Evacuation Study," a multiyear qualitative and quantitative research study designed to assess factors that affected evacuation of the two WTC towers. A report summarizing qualitative data collected from Phase I of the study was published in the Sept. 10, 2004 issue of the Morbidity and Mortality Weekly Report.

The report analyzed data collected from 56 participants during 2003, approximately 18 months after the events of Sept. 11, 2001. The study participants cited four factors that affected their decision to begin evacuating:

  • Perceived ability to walk down multiple flights of stairs (i.e., more than 80 for certain persons);
  • Experience in evacuation of a WTC tower, including knowledge of stairwell locations and whether individual stairwells led to street level exits;
  • Concern over leaving their work areas without the approval of executives or managers; and
  • Information regarding what had occurred, what floors were involved, and how to respond. Direct evidence of the magnitude of the event (e.g., observing an aircraft strike a building, smelling fuel or feeling a building move) caused some persons to leave immediately.

The qualitative data also suggested that, after a decision to evacuate was made, many persons stopped to attend to last-minute activities such as making telephone calls, shutting down computers or gathering up personal items. Deciding which route to take – stairs or elevators – might have delayed evacuation progress for others. Progress was reportedly slowed for some people because of poor physical condition or inadequate footwear such as high-heeled shoes. Some people also delayed their progress to stop and assist others.

Two major organizational factors affecting evacuation were identified by participants: workplace preparedness planning and training, including evacuation drills and inadequate risk communication. An announcement broadcast in the South Tower shortly after the first aircraft had struck the North Tower urged workers to remain in the building and likely led many to return to their work stations.

Three major evacuation factors in the WTC building environment were identified as structural damage that blocked stairwells; heavy congestion on certain stairways, which in some cases caused evacuees to move back upstairs in hopes of switching to a less congested stairwell; and lack of a back-up communication systems (public address system, elevator telephone system and telephone system). When these systems failed, communication was severely limited.

The high rate of survivability of persons on floors in the North and South Towers that were below the aircraft impacts was attributed to the small percentage (estimated at 20 percent to 30 percent) of the building occupants at their work stations at the time of the first attack, which helped reduced the flow of evacuees on the stairways and hastened evacuation. The lessons learned from the 1993 WTC bombing resulted in extensive fire safety improvements and also are believed to have facilitated the evacuation process. The 1993 bombing led some employers to enhance their emergency preparedness plans, which might have helped evacuate their employees more rapidly. In addition, on 9/11, evacuees almost uniformly acted in an orderly and cooperative manner. The large numbers of first responders on the stairwells, in the lobby and at ground level, might have helped to reassure and motivate evacuees.

"High-rise buildings should be prepared for rapid, total building evacuation," noted researchers. Data from studies such as this one "can help inform builders, developers, insurance companies, employee groups and emergency planners about risk-reduction strategies. Data from such sources also might be of value to regulators at the federal, state and local levels."

Further studies should focus on the development and evaluation of model emergency preparedness programs for high-rise buildings.

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