Federal Building and Fire Safety Investigation of the World Trade Center Disaster. September 2005.
Once the 745 first-person accounts were summarized, multiple accounts from the same person were merged into one, which provided accounts for 465 individuals. (Some survivors provided multiple accounts through different sources.) xxv
Before any analysis began, the database was further limited to the 435 civilians who were in either WTC 1 or WTC 2 on that day.
251 occupants of WTC 1 and 184 occupants of WTC 2
Out of 158 people who mentioned their means of egress in WTC2, 18 used the elevators and 26 used a combination of stairs and elevators to leave the tower. In WTC1, out of 202 people who mentioned their means of egress, 198 used the stairs, one used an elevator, and three used a combination of stairs and elevator. This does not include the 22 people who were stuck in elevators when WTC1 was hit.
Overall, 154 mentioned that others were calm. The 41 people who were at the WTC on the day of the 1993 bombing indicated that they were better prepared this time to face the emergency. This past experience, they said, made them readily start their evacuation. page xxvi
Page 1
Despite the massive localized damage caused by the impact, each structure remained standing. However, as each aircraft impacted the building, jet fuel on board ignited. Part of this fuel immediately burned off in large fireballs that erupted at the impact floors. Remaining fuel flowed across the floors and down elevator and utility shafts, igniting intense fires throughout upper portions of the building.
At 9:59 a.m., WTC2, collapsed
At 10:28 a.m., WTC1 collapsed
29 minutes
Approximately 50,000 people worked in each tower (100,000 total), with an estimated 70,000 visitors to the complex during the course of a normal business day (Yamasaki 2002) However, the occupancy of the towers on the morning of September 11, 2001, was not at its maximum capacity, According to USA Today, 5,000 to 7,000 people were in each tower at 8:46 a.m. that morning, the time of the first impact. (Cauchon 2001). It has been suggested that the towers were not at their maximum capacity for several reasons. That morning marked New York City's mayoral primary, and it is assumed that many people stopped to cast their ballots before heading to work. The New York Stock Exchange does not open until 9:30 a.m., therefore many people from trading firms had not come into work yet. Tuesday, September 11, 2001, was the first day of school in several primary school districts, and many parents accompany their children to school on this day. Visitor hours had not started yet, as the viewing platform in the WTC 2 did not open to the public until 9:30 a.m. Perhaps the biggest factor of all was the early hour---many simply had not arrived at work by 8:46 a.m. In addition, dozens of investment firms in the WTC had closed their offices or cut employment sharply. Some offices were leased but empty or under renovation (Cauchon 2001).
Page 2
By certain measures, the evacuation of the WTC towers on September 11, 2001, could be termed a success (Cauchon 2001). Under the impacted floors, nearly every occupant who could physically get out did get out. According to USA Today, in each tower, 99 percent of the civilian occupants below the crash sites survived.
Among the occupants under the impacted floors in WTC1, 72 people died, whereas under the impacted floors in WTC2, 4 people died. Some fraction of the deaths below the impacted floors in WTC1 occurred in the elevators, which were carrying people at the time of impact. [And why would not the same hold true in WTC2?]
Page 7
For instance, 16 survivors provided 5 to 12 different accounts to the media. The study involved no first-person interviews.
Page 8
Despite the drawbacks of using media sources for the basis of research, however, some of the accounts contained such a high level of detail, particularly the ones written by survivors themselves, they provided justification for the analysis of this information. It should be stressed that these media accounts are the only documented descriptions of the WTC evacuation and immediate reactions of the survivors, as no research has been conducted or published 2 years after the events, regrading human behavior surrounding the events of September 11, 2001. Since documenting human behavior is time sensitive and considerable time has passed since the event, it may be said that these initial media accounts may hold significant detailed and accurate information that may only be available in these accounts.
Page 8
30 questions rely on manifest information or elements specifically reported in the account, such as the person's location at certain key moments.
3 questions call for latent information, such as words describing emotions.
Page 9
Questions.
The skylobbies on floors 44 and 78 served the occupants of floors 43 and 77, respectively.
Page 11
Personal web sites and e-mails written by survivors themselves were also used and are of particular interest, as they have not been altered by media editors in any way, but appear in their full original format. During the three months following the evnts, over 280 first-person accounts were collected.
Page 13
In WTC1, 90 people 936 percent) were from upper floors (77 to 110) 79 people (31 percent) were from mid levels (43 to 76) and 58 people (23 percent) were from the lower floors of WTC1. Another 22 people (9 percent) were in elevators and two people did not specify a location.
Page 14
The person who used the elevator for evacuation reported that he was in an elevator when the building was struck, and the elevator stopped on one of the floors. He was able to move people from that floor to the lobby. Two of the three who used both elevator and stairs were initially trapped in an elevator behind a 50th floor restroom. After freeing themselves, they were directed by firefighters to an elevator to the 44th floor, from which point they walked down.
The third person who used both stairs and elevators rode with a person he was assisting from the 52nd floor to the 44th floor. Unable to find a working elevator on the 44th, he walked down the rest of the way
Page 15
Interestingly, only 25 people made any mention of building alarms in their evacuation accounts. Of those, eight in WTC1 and one in WTC2 reported hearing alarms but did not specify where. Two in WTC1 and one in WTC2 heard alarms while on their floors and one person in each tower stated that they did not hear alarms.
Among occupants who initially decided to stay, it is noteworthy to mention a group in WTC1. Two survivors reported that a group of about 16 employees gathered in a conference room on floor 64 of WTC1. The group stayed in the room discussing the situation for approximately 1 hour before deciding to evacuate the building.
Page 17
Of the 22 evacuees who were reported encountering jammed or locked doors, 20 were in WTC1, and all but three were located on upper floors. Of the 25 evacuees who reported being trapped, nine were in elevators, eight were trapped by debris or smoke when their building was hit, five were trapped in the collapse of WTC2, and three were trapped when WTC1 collapsed.
Page 19
Some 47 of WTC2 people (16 percent) were still inside WTC1 on lower levels from the basement to the 42nd floor, and three people (1 percent) were on mid levels (43 to 76) in WTC1 when WTC2 fell.
[At 9:59 a.m., WTC2, collapsed
At 10:28 a.m., WTC1 collapsed
29 minutes]
Page 21
The majority of people who placed phone calls that morning did son once they were outside (93 people or 54 percent); however, many did not get through.
Page 24
Only nine percent, 41 people, reported being present during the 1993 bombing and evacuation of the WTC.
Four survivors reported seeing photoluminescent stripes on the stairs, railings and doors---an improvement the Port Authority made following the 1991 bombing. As one survivor stated, "All you had to do was follow those yellow-green stripes. They were wonderful."
A Decade After 9/11, One Survivor's Tales of a Miraculous Recovery ...
FM Issue: Emergency Measures In A Post 9/11 World
snopes.com: Adam Mayblum Tower Escape
Gold, Scott. "You're on the 87th Floor, and Something's Terribly Wrong..." Los Angeles Times, 16 September 2001 (p.A1)
Kugler, Sara, "E-Mail from WRC Arrack Survivor Touches Hundreds Worldwide." The Associated Press. 17 December 2001.
Miller, Kimberly. "Son Was Slowly Climbing Down as Tower Fell." The Palm Beach Post. 17 September 2001 (pA8).
Walsh, Mary Williams. "At 8:48 AM, 2 Normal Guys Met in a Moment of Transformation." The New York Times. 16 September 2001 (p. A1).
References
1. 9/11 in numbers, observer.guardian.co.uk, 8/18/02 [cached]
2. Closure from 9/11 elusive for many, USAToday, 9/3/01 [cached]
3. A Miracle's Cost, time.com, 9/1/02 [cached]
4. Forensic Identification of 9/11 Victims Ends, abcnews.com, 2/23/05 [cached]
5. For Many on Sept. 11, Survival Was No Accident, usatoday.com, 12/20/01 [cached]
6. , New York Times, 4/19/02
09/04/2002 - Updated 09:04 PM ET, USA Today,
05/17/2004 - Updated 09:33 PM ET
Elevators were disaster within disaster
21 trapped people improvised escapes
Excruciating wait nearly fatal
Plunge just the start of nightmare
Video: World Trade Center worker Mark Jakubek helped free people trapped in an elevator
Day 2: Life, death on the 78th floor
Inches decide life, death on 78th floor
Delay meant death on 9/11
Desperation forced a horrific decision: burn or jump
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2636613/#bib5
Estimating the World Trade Center Tower Population on September 11, 2001: A Capture–Recapture Approach
Joe Murphy, MA
The author is with RTI International, Chicago, IL.
Corresponding author.
Requests for reprints should be sent to Joe Murphy, MA, RTI International, 230 W Monroe St, Suite 2100, Chicago, IL 60606 (e-mail: jmurphy@rti.org).
Peer Reviewed
Accepted May 18, 2008.
Abstract
I applied the capture–recapture method to estimate the World Trade Center tower population at the time of the September 11, 2001, terrorist attacks. Available lists helped identify 8965 survivors and 2152 confirmed casualties. The capture–recapture model suggested that an additional 4435 survivors were present, putting the total count of all present at 15 552 (95% confidence interval = 15 216, 15 897). An accurate estimate represents the potential number at risk for trauma as a result of direct exposure to the events of the day.
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On September 11, 2001, 2 airplanes crashed into the World Trade Center towers in New York City as part of a terrorist attack. In addition to the loss of life, survivors were exposed to hazardous dust and debris from the collapse of the buildings, and many people endured psychological trauma.1–4 No one has definitively determined the number of persons present in the 2 towers at 8:46 am on that day when the first airplane crashed into the North Tower, and no one may ever be able to do so. Several estimates have been made previously,5–7 but none made use of capture–recapture methods, which are useful in estimating population size when enumeration by more-direct methods is not feasible.8–10
My objective was to apply the capture–recapture method to 3 list sources of individuals determined to be present inside the World Trade Center towers on September 11 to estimate the number present when the first airplane struck. This number represents a population at risk for long-term health effects as a result of direct exposure to the events of the day.
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METHODS
The World Trade Center Health Registry (WTCHR) is a database used for following up with individuals exposed to the disaster of September 11 to evaluate the short- and long-term physical and mental health effects. Exposed groups, including rescue and recovery workers, residents, students and school staff, building occupants, and passersby, were located and interviewed. Potentially eligible individuals were identified through a multitude of list sources and could also register through the project Web site or toll-free telephone number. In all, 71 437 interviews were completed, including 8965 with individuals present in the World Trade Center towers at 8:46 am on September 11.
Those present in the towers—a group with high potential for physical and psychological trauma as a result of exposure to the events of the day—were identified from 3 different list sources for the WTCHR. The first list included 3622 individuals who volunteered by Web site or telephone to complete the WTCHR survey (the self-identified list), and were confirmed to be present in the buildings on September 11. The second list included data supplied by businesses (the business list), with office space in the World Trade Center towers identifying employees who were present on the morning of September 11. Interviews conducted with 1282 individuals from this list confirmed that they were present in the buildings on September 11. The third list, obtained from the Port Authority of New York and New Jersey (the Port Authority list), included individuals with security access to the towers as of September 11; from this list, 5247 were confirmed to have been present. I identified individuals included on multiple lists by comparing names, addresses, telephone numbers, dates of birth, and social security numbers with an algorithm developed by Choicemaker Technologies (New York, NY).11
Capture–recapture methods that used log-linear modeling estimated the number of individuals not on any list. The log-linear approach models the logarithm of the expected value of each observed list source. Seven samples were observed: z100, z010, z001, z110, z101, z011, and z111, where, for instance, z011 is the number of individuals in samples 2 and 3 but not in sample 1. The missing sample, z000, refers to those not on any list. The most general log-linear model for the 3-sample case is as follows:
where log E(zijk) = logarithm of the expected value of each observable category, andI(A) is an indicator function for event A.8
Models were fit with R version 2.6.0 (available at http://www.r-project.org.index.html). which uses the generalized linear model (glm) function to provide population estimates and fit statistics.12 The models were run with all combinations of interactions between the list sources except for the 3-list interaction term, which was assumed to be 0 (u123 = 0).8 The model with the smallest Akaike's information criterion was selected as best.13,14 The estimated number of people not on any list was added to the number on any list and the casualty count to obtain the overall population estimates for the World Trade Center towers at 8:46 am on September 11.
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RESULTS
A total of 8965 persons identified for the WTCHR indicated being in one of the towers on the morning of September 11. Among this group, 3622 volunteered for the interview via Web site or telephone (the self-identified list), 5247 were included on the Port Authority list, and 1282 were included on the business list. Table 1 presents the number included in each combination of lists, and the results of the capture–recapture model are shown in Table 2. The model with SI × BL and BL × PA interactions had the lowest Akaike's information criterion and was selected as best. Adding the 2152 occupant casualties15 to the 8965 included on a list source and the 4435 estimated individuals not included on any of the list sources resulted in an estimate of the World Trade Center tower population on the morning of September 11 of 15 552 (95% confidence interval [CI] = 15 216, 15 897).
TABLE 1
World Trade Center Health Registry Tower Occupants on September 11, 2001, by List Source
TABLE 2
Capture–Recapture Model Estimates of the World Trade Center Tower Population on September 11, 2001
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DISCUSSION
Debate exists concerning the applicability of capture–recapture to epidemiology; indeed, capture–recapture estimates should be interpreted with great caution.16Although these methods are not directly comparable to those used in previous studies to measure the World Trade Center tower population on September 11, capture–recapture provided results similar to those from previous analyses. For instance, the Port Authority of New York and New Jersey (A. Reiss, Port Authority of New York and New Jersey, written communication, March 2005) counted 14 417 card accesses to the towers on Tuesday, May 8, 2001, between 12:00 am and 8:47am, the closest date to September 11 for which data were available. USA Todaymatched floor plans, architectural drawings, and photographs to the accounts of many survivors and estimated that between 12 000 and 16 000 people were in the towers at the time of the first crash.5 The National Institute of Standards and Technology contacted a stratified random sample of individuals from a list of persons with building access as of September 11 and estimated that 17 400 (95% CI = 16 260, 18 620) were present.6
The 3 necessary assumptions of the capture–recapture methodology are perfect matching, independence of lists, and homogeneity.9 Perfect matching assumes that individuals on different lists can be matched without error. Despite considerable effort devoted to proper matching in this study, some true matches or nonmatches may have been missed. Independence of lists assumes that the probability of an individual appearing on 1 list does not depend on inclusion on another list. List dependence is controlled for when 3 or more lists are used and interactions between are included in the model, as was done in this study. Homogeneity assumes that the probability of inclusion on a list does not vary among individuals. Possible heterogeneity in the capture probabilities was explored with the Rcapture plot descriptive function,12which suggested that heterogeneity was not a problem for this analysis. However, individuals may have varied in their capture probabilities in an unmeasured way.
Although the capture–recapture method has limitations and may not provide a definitive estimate of the World Trade Center tower population on September 11, this study confirmed that it was a useful method in this context that provided estimates similar to those obtained in other studies.
Acknowledgments
This research was supported by the Agency for Toxic Substances and Disease Registry (ATSDR; cooperative agreement U50/ATU272750), and the brief underwent peer review and received clearance by ATSDR, the Centers for Disease Control and Prevention, and the National Institute for Occupational Safety and Health before submission for publication.
The author thanks Lisa Thalji and Carla Tinberg for their helpful review and comments and all World Trade Center Health Registry enrollees for their participation.
Human Participant Protection
This study was reviewed and approved by the institutional review boards of the New York City Department of Health and Mental Hygiene and RTI International.
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References
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