R. Nolan, M. Ross, G. Nord, C. Axten,
J. Osleeb, R. Wilson
ASSESSMENT OF RISK OF ASBESTOS-RELATED CANCER BASED
ON AN ANALYSIS OF AIR AND SETTLED DUST SAMPLES
FROM THE 9/11 ATTACK ON THE WORLD TRADE CENTER COMPLEX
Abstract
In the aftermath of the September 11th atrocity in New York City, which destroyed the
Twin Towers, all the other buildings in the World Trade Center (WTC) complex and St.
Nicholas Greek Orthodox Church, questions have been raised concerning the potential for
health effects from the dust. The initial dust cloud caused a horrendously high particulate
exposure, which was both brief and unforgettable. Twenty-four hours after the buildings
collapsed the airborne concentration of dust was markedly lower but it remained uncertain if
exposures to hazardous particles, particularly asbestos, would be elevated from background
during the time required to remove the 1.5 million tons of building debris. This report will
address the questions: What were the asbestos fiber type(s) and their concentration(s) in the
air? How can analysis of settled dust inform us about early asbestos exposures, both on 9/11
and post 9/11, where air-sampling data are either non-existent or limited? How do the outside
ambient airborne asbestos levels determined in Lower Manhattan post 9/11 compare with
historical background levels of asbestos in NYC and elsewhere? What are the asbestos-related
cancer risks as a consequence of the type of asbestos exposures, which occurred post 9/11?
Analysis by analytical transmission electron microscopy (ATEM) of representative
settled dust collected in Lower Manhattan (five days post 9/11) found trace levels of
chrysotile asbestos. No other asbestos fiber type was found to be present in any of the six
settled dust samples. Ironically, the only bulk sample containing no asbestos was fireproofing
removed from a piece of structural steel. Due to the enormous amount of dust, containing
trace levels of chrysotile asbestos, released into the ambient air of Lower Manhattan it is
possible that persistent levels of airborne asbestos detectable above background could occur.
To evaluate this possibility, six air samples were collected in October of 2001 to characterize
the particles in the ambient air in Lower Manhattan starting twenty-seven days post 9/11. The
goal was to determine the non-occupational exposures to the general population, not those
with occupational exposure related to performing demolition and debris removal. The
collection methodology and the microscopy analysis protocol for the air samples allowed for
the examination of all the asbestos fibers in at least 10,000 ml of ambient air. The type of
asbestos and size distribution could be determined.
No asbestos fiber (of any fiber type or length) was found in any air sample one month
post 9/11. The mean ambient airborne asbestos concentration in Lower Manhattan was below
0.00008 f/ml. That was a level consistent with the background pre 9/11 for more than a
decade in NYC. The airborne concentration of asbestos in NYC starting a month post 9/11 is
consistent with what the World Health Organization (WHO) considers the low end of the
background for asbestos in ambient air worldwide. This was markedly lower than the level of
0.01 f/ml, that is usually regarded as safe. However, the initial exposures on 9/11 were
undoubtedly high and probably remained elevated above background for some period of time
during the next twenty-seven days. We will use risk assessment to determine the upper limit
of any increased risk of the two principal asbestos-related cancers – mesothelioma and lung
cancer – among the general population of Lower Manhattan, which might be associated with
the asbestos released from the September 11th attack on the Twin Towers. The important
variables determining these asbestos-related cancer risks are age at first exposure
(mesothelioma), smoking (lung cancer), asbestos fiber type and cumulative exposure
(intensity and duration of exposure to airborne asbestos).
Introduction
The target of the terrorist attacks on September 11th in New York City was the Twin
Towers (WTC1 and WTC2) of the World Trade Center complex. Health concerns have been
raised about the potential for asbestos-related cancer risk from the increase in airborne
asbestos associated with the murderous 9/11 attacks. This report will describe the
characteristics of representative samples of the settled dust released from the collapse of the
Twin Towers and other buildings and ambient air samples. We collected the ambient air
samples starting 27 days after the initial collapse, and during a time when there was still
considerable public health concern about the risk of asbestos-related cancer in Lower
Manhattan due to the 9/11 attacks. The type(s) and concentration(s) of airborne asbestos in
the area surrounding the WTC site were determined and the cumulative exposures to the
general population of Lower Manhattan due to the events of 9/11 were estimated; these are
the important experimentally determined variables for the risk assessment.
As the removal of the debris took many months, any increase in the airborne asbestos
concentration associated with the work could have conceivably led to a situation where the
initial hygiene steps taken to control airborne asbestos exposures might prove to be
inadequate. Possibly, allowing the general population, living near the WTC site in Lower
Manhattan, to acquire a cumulative asbestos exposure (with an intensity and duration to a
specific fiber type) that after a latency period of greater than 20 years, might lead to an
increase in asbestos-related cancer among the area’s general population (Hodgson & Darnton,
2000, Nolan et al., 2001). On the basis of the specific asbestos fiber type(s) present and their
post 9/11 concentrations in the ambient air, we will develop a risk assessment for asbestosrelated mesothelioma and lung cancer.
Asbestos health hazards due to the events of 9/11 have been the focus of much media
attention. However, little, if any, attention prior to this report has been given to undertaking
the type of air sampling necessary to perform a modern asbestos-related cancer risk
assessment specific for the cumulative exposure(s) and specific asbestos fiber type(s) caused
by the 9/11 attacks.
History of the world trade center complex
The groundbreaking for North Tower (WTC1) was on April 4, 1966 followed by
the erection of the structural steel starting in August of 1968. By the end of 1970 the
first tenants had moved into the 110-story Tower (at 1,368-feet (ft) it was then the
tallest building in the world) topped with a 360-ft transmitting antennae. By early 1972
the 110-story South Tower (WTC2) was completed with a height of 1362 ft. The
ribbon cutting ceremony was held on April 4, 1973 (FEMA, 2002).
An additional five buildings would eventually be added to complete the complex –
WTC3 a 22-story hotel to the west of the South Tower (WTC1), WTC4 and WTC5 were two
nine-story office buildings, WTC6 an eight-story office building and the final building WTC7
a 47-story office building across the street from the main part of the 16 acre complex was
completed in 1985. The WTC complex contained 12,000,000-ft2 of office space (Fig. 1).
Events of September 11, 2001
The first hijacked Boeing 767-200ER out of Boston’s Logan Airport crashed into the
north face of the North Tower (WTC1) between the 94th and 98th floors at 8:46AM with a
second identical aircraft (also out of Logan) striking lower (between the 78th and 84th floors)
on the south face of the South Tower (WTC2) just 17 minutes later. None of the 157 people
aboard the two aircrafts survived the impact. Of the 58,000 people estimated to be in the
WTC complex that morning approximately 14,000 were thought to have been in the Twin
Towers when the first aircraft struck. Approximately 6,000 were below the impact floors in
the North Tower (WTC1) (see FEMA 2002, for details of the event summary which follows).
Both towers sustained considerable structural damage, and both were severely rocked
by the impact of the aircraft. It has been reported that on impact, the South Tower (WTC2)
swayed in one direction for 7-10 seconds before swaying back, although dramatic to the tower
occupants, the structural strength required for this motion was within the design envelope of
either Tower. Each of the Twin Towers weighed about 500,000 t (or 3,650 times the mass of
the Boeing 767-200ER) and had flexibility in the wind and viscoelastic motion dampers to
minimize their movement, thereby reducing the risk of motion sickness to those occupying
the high floors of the towers on windy days. Each tower was designed to withstand hurricane
force winds of 135 miles per hour (MPH) (or about 5,000 t of lateral load). The damage
caused by the impact of the aircrafts on a relatively low wind day (10-20 MPH) was not
sufficient to knock over or initiate a global collapse in either tower.
Fig. 1. The 7 buildings of the World Trade Center complex stood on 16 acres across West Street form the
World Financial Center (WFC). At the center of each tower was an 87-ft by 137-ft inner core which
differed in orientation between the two towers
All the sides of both towers had widths of 207 ft while the wingspan of the Boeing 767200ER was 156 ft. The first aircraft flying south entered the impact face of the North Tower,
at an angle and destroyed about 31 of the 36 steel supporting columns of the impact face over
4 stories resulting in a partial collapse of the local floors. The 274,000-lb aircraft was
estimated to be traveling at 470 MPH and came to a complete stop within the footprint of the
building. The building load was shifted to the remaining steel columns and the tower
remained standing after an impact significantly beyond what could have been reasonably
anticipated. The Twin Towers were the first buildings, other than those used by the military or
for generating nuclear power, where impact by an aircraft was a design consideration. The
initial fireball (estimated to have consumed 10 to 30 % of the 10,000 gallons of fuel aboard
the aircraft) was followed by an extensive fire initiated by the dispersion of the jet fuel in the
air followed by the ignition of the fuel-air mixture. The sudden increase in the temperature
created a pressure wave, which expanded the burning fuel into a fireball. This dramatic
fireball grew slowly – taking 2 seconds – to reach its full size of approximately twice the
width of the building. This relatively slow rate of expansion is not characteristic of an
explosive device that was considered as a possibility when the fireball was first observed. The
fires in each tower were producing between 3-5 trillion BTU/hr with ⅓ to ½ of the heat being
vented outside and driving the plume of smoke (Fig. 2a).
The temperature of the fire was estimated to have a maximum of no more than 1,100 C
and the amount of heat generated was not sufficient to melt steel. However, the towers were
not designed to have large numbers of sequential floors, each of 40,000 ft2, being on fire at
once. The World Trade Center had specifications, which placed limits on the total
combustibility of the contents of any floor; we take this specification as an indicator of the
Twin Towers vulnerability to fire (NY Board of Fire Underwriters, 1975). The nature of the
aircraft impact damage to the building prevented the two principal fire control tactics, water
sprinklers and manual fire fighting, from being used to control the fire. On 9/11, the ability of
the towers to maintain structural integrity after the initial impact was critically related to the
fireproofing.
The Twin Towers were designed as rigid hollow tubes with both an inner and a
perimeter steel structure. Behind the distinctive aluminum façade (19" wide windows set 22"
apart) were the steel supporting columns of the perimeter. The architectural design, by
Minoru Yamasaki and Associates, called for the Towers’ windows to be narrower than the
width of a person’s shoulders and therefore reduce the possibility of anyone looking out from
their great height getting a sensation of possibly falling. The horizontal floor trusses ran from
the outer vertical steel columns to the 87-ft by 137-ft inner core, which carried the gravity
load of the building in addition to housing 99 elevators, 3 exit stairways and 16 escalators
(Fig. 1). The core was a key design feature of the towers eliminating the historical need for
columns every 30 ft or so that would have formed a dense supporting grid on each floor. The
connection between the inner and outer steel structures via the trusses was critical to the
structural strength of the towers and creating the hollow tube, which made the large open
floors possible. This innovative design created open 30,000-ft2 floors, which were a highly
desired feature of the Twin Towers.
Although it was struck second, the South Tower (WTC2) was the first to experience
global collapse at 9:59AM having stood for only 56 minutes after impact, with the North
Tower (WTC1) following about 30 minutes later. The aircraft struck the South Tower on an
angle to the right; largely missing the north to south oriented inner core and pushing the office
furniture and other debris into the northeast corner leading to an intense localized fire (Fig. 1).
A jet engine, landing gear and part of the fuselage of the doomed aircraft came out of that
corner with some parts landing as far as seven blocks to the north of the South Tower
(WTC2).
In addition to weakening the steel columns the heating may have caused the trusses to
sag thereby placing a force on the four bolts securing the trusses to the steel columns for
which their design provided little strength. Each tower’s structural stability was critically
related to the floor trusses that strengthened the building by connecting the steel of the
perimeter tube to the inner core, which was attached by four bolts on each end. The truss
designs provided strength for shear forces and vertical load but not for the tension caused by
the heated truss sagging. A second significant factor could have been the range in temperature
along a piece of steel caused by the non-uniform fire. When a single piece of steel has
temperature differences ranging up to 150 C or more at different points residual stress
develops and weakens the steel. Although the photographic record, particularly of the South
Tower (WTC2) provides much information, the nature of the damage will probably never be
known in sufficient detail to provide definitive explanations for the global failure of both
towers.
In the South Tower (WTC2) the most intense fire appeared to be on the 80th floor along
the north face. A stream of molten metal (possibly aluminum from the aircraft) was seen
streaming from an impact opening just prior to the tower’s collapse. The trusses breaking
away from the steel columns in the perimeter may have triggered the collapse of the floor.
This suggestion is supported by the observed cloud of dust blowing out the side of the
building prior to the collapse of the outer wall in the northeast corner.
Once one floor collapsed, whatever structural strength remained in the damaged tower
was insufficient to prevent a rapid global failure. At the time of the aircraft impact,
approximately 2,000 people were left in the South Tower (WTC2). Most (if not all) persons
below the impact floors were evacuated safely while of the 500 estimated to be trapped above,
only four survived.
Three factors are thought to be important with regard to the rapid collapse of the South
Tower (WTC2): 1. The second aircraft was estimated to be traveling 120 MPH faster than the
first plane (thereby increasing the kinetic energy of the faster aircraft by 50 % compared to
the one striking the North Tower). 2. The damage occurred lower in the building making the
overhead mass greater on the impact and fire damaged area. 3. The fire was particularly
intense in the northeast corner, where the failure of the structural steel columns and trusses
(weakened by a combination of both the heat and temperature ranges caused by the nonuniformity of the fire) may have initiated the global collapse.
The initiating event for the collapse of the North Tower (WTC1) is thought to be
different from that of the South Tower. Here the damage to the inner core was more extensive
due to the direct impact of the aircraft, which only traveled about 60-ft before striking into the
inner core of the North Tower (WTC1). The damage to the inner core caused by the impact of
the 274,000-lb aircraft traveling at 470 MPH was likely to have been extensive (Fig. 1). The
collapse of the North Tower seems to have been related to the failure of the other end of the
trusses from the South Tower (WTC2). Perhaps damage to the inner core, which had been
severely impacted by the aircraft and was burdened with carrying the significant gravity load
of the tower, explains the different initiating mechanism for the global collapse of the second
tower. The evidence for this inner core failure in the North Tower (WTC1) rests in part on the
observation that the transmitting antenna began to fall prior to the collapse of the outer walls.
Based on this observation the structural engineers speculate that the collapse of the North
Tower (WTC1) was triggered by an inner core failure. This inner core failure led to
1,400 deaths.
Both towers collapsed in approximately 11 seconds and the wreckage was traveling at
about 120 MPH when it hit the ground with impact magnitudes of just over 2 on the Richter
Scale. An air pressure wave was created by the collapse of each tower sending dust clouds in
all directions. As the material in the 110-story towers was mostly open space, the debris pile
was hardly six stories high. It is estimated that 2,830 lives were lost – 2,270 building
occupants, 157 airplane crew and passengers and 403 firefighters, police and emergency
personnel (FEMA, 2002). Estimates made two months after 9/11 placed the loss of life at
3,900, by mid-August, 2002 the estimate had fallen to 2,819, slightly lower than what FEMA
reports.
Sources of the cloud dust and the aftermath
No masonry was used in the steel construction of the Twin Towers and therefore the
mainly 4-inch light concrete floors (40,000 ft2 per floor), fireproofing (5,000 tons), insulation
and interior dry walls were the main sources of the dust (Table 1). The initial dust cloud
caused a horrendously high concentration of airborne particulate and combustion products,
which was both extensive and unforgettable. Within minutes, the air pressure generated by the
collapsing tower raised a dust cloud, which bellowed up over 1000 ft (see the American
International Building, 70 Pine Street, with a tip height of 952 ft in Figure 2a). In a series of
seven photographs taken from the Brooklyn College Campus (seven miles from Ground Zero)
over the first eight minutes after the South Tower’s collapse the extent of the mixing and the
size of the dust cloud are readily apparent (Fig. 2a-g). On a crystal clear day with low wind it
rapidly expands, in 8 minutes, to a size sufficient to obliterate any view of Lower Manhattan
(Fig. 2g). The dust cloud moved down the street like a wall of volcanic ash (Fig. 3), reaching
such a height that no skyscraper (several over 800-ft in height) was visible in Lower
Manhattan. After five hours the dust had cleared sufficiently (Fig. 4) for the skyline to be
partially visible again, although missing the two tallest and largest buildings in NYC. The
mixing indicates the settled dust collected for our study, five days later at several locations
more than 8 blocks from the WTC, should be representative of the stable particulate matter in
the dust cloud.
Table 1
Composition of common building materials, known to have been used,
in the construction of the Twin Towers of the World Trade Center
Wall board
Portland cement
Fireproofing
75-90 wt % gypsum (calcium sulfate), 10-25 wt % bentonite clay (aluminum silicate) and
or similar clay fillers, <1 % paper facing and <0.1 % quartz
60-67 wt % lime (calcium oxide) 17-25 wt % quartz (SiO2), 3-8 wt % aluminum (Al2O3),
0.5-6.0 wt % iron oxide (Fe2O3), 0.1-4 % magnesium (MgO), alkalis (K2O,Na2O) and
1-3wt % SO3
Four different fireproofing materials were used during the construction of the World Trade
Towers:
 20 % chrysotile asbestos, 60-65 % mineral wool (specifically slagwool) and the
remainder of a binder of gypsum and Portland cement
 80-85 % of mineral wool (specifically slagwool) and binder
 lightweight gypsum plaster and vermiculite aggregate with about 13 % chrysotile
asbestos
 a “hard coat” consisting of 80 % chrysotile asbestos set in a matrix of Portland cement
A day later, the airborne concentration of dust was markedly lower but visually
elevated above the usual background level (Fig. 5a). If levels of airborne asbestos
remained significantly increased above background for some period of time it could
present a public health concern. The task of removing the 1.5 million tons of debris
required 20,000-30,000 truckloads and was completed in June 2002 (Fig. 5b-d).
During the removal of the debris, the ongoing fires (occurring in the massive six sub
story complex beneath the western part of the WTC site) continued to release
additional air pollutants. The movement of heavy equipment and other vehicles could
promote reentrainment of the settled dust known to contain traces of chrysotile
asbestos; even allowing for efforts to suppress the dust by keeping the streets wet and
the use of trucks capable of removing the dust by vacuuming (Fig. 5d). Five days after
9/11, smoke continued to billow from the WTC site as it would until early December
when the underground fires were finally extinguished (Fig. 6). Airborne dust related to
the event was widely distributed and would continue to settle out of the air for some
period of time creating the potential for cumulative asbestos exposure to occur which
might carry a significant increased risk of asbestos-related cancer.
Materials and methods
Collection of Representative Settled Dust Samples Post 9/1.1 Six bulk samples of the
settled dust representative of the dust cloud released into Lower Manhattan from the collapse
of the Twin Towers and one sample of intact fireproofing were collected. In total, seven bulk
samples were collected: two settled dust samples from motor vehicles, one sample of
fireproofing from a piece of structural steel, three settled dust samples from the elevated
footbridge crossing the Westside Highway north of the WTC at Chambers Street (Table 2).
The dust inside the enclosed footbridge was thought to be finer in size as it had diffused
inside the bridge while the two settled dust samples on the motor vehicles settled directly out
of the air. An additional random, directly settled, dust sample was collected at a similar
distance southeast of the WTC (Table 2).
Table 2
Settled dust and bulk samples were collected the area of the collapsed Twin Towers
Sample No. 1
Sample No. 2
Sample No. 6
Sample No. 3
Sample No. 4
Sample No. 5
Sample No. 7
The following six samples were collected on September 16, 2001
Fireproofing on a steel girder on a truck
Dust collected from an auto van parked on White Street
Dust collected from a truck parked on Chambers Street between the Westside Highway and
Greenwich Street
Dust collected from step railing inside footbridge across the Westside Highway from the
Borough of Manhattan Community College to Stuyvesant High School
Dust collected on long railings inside footbridge across the Westside Highway from the
Borough of Manhattan Community College to Stuyvesant High School
Dust collected inside footbridge across the Westside Highway from the Borough of
Manhattan Community College to Stuyvesant High School
The following sample was collected on October 7, 2001
Dust collected from street 150 yards southeast of the site of the World Trade Center
Fig. 2 a-g. As the South Tower (WTC1) collapsed the lighter colored dust cloud associated with the construction material became
more visible. The lighter colored cloud quickly reached higher then the tallest building in Lower Manhattan, 952 ft at 70 Pine
Street. Within 8 minutes the entire skyline of Lower Manhattan had disappeared in a cloud of dust
Fig. 3. The dust cloud moving south in very sharp zones on Trinity Place
Fig. 4. 09/11/01. (5 hours after collapse)
Fig. 5a. Twenty four hours after the collapse of Twin Towers
Fig. 5b. The smoking debris at the WTC site post 9/11
Fig. 5c. Although the Twin Towers did not tip over the debris from their collapse
caused considerable damage to the surrounding area
Fig. 5d. Trucks on Chambers Street waiting to pick up debris from World Trade
Center during the first week in October, 2001. Note that the streets surrounding
the WTC were kept continuously wet to suppress the dust
Fig. 6. View south along the Westside Higway (recently renamed the Joe DiMaggio Higway)
on September 16, 2001. Note WTC site remains on fire
Continuous Scan X-ray Powder Diffraction (XRD). The XRD analysis of the settled
dust samples will allow the identification of the crystalline phases present at concentrations of
approximately 1 % by mass or greater. Each sample was prepared for analysis by being
dispersed in acetone with gentle grinding in an agate mortar and pestle. Once the dust sample
was uniformly dispersed, the suspension was transferred using a disposal pipette to a low
background silicon plate sample holder. The acetone was allowed to evaporate settling the
suspended particles onto the plate. Additional aliquots of the suspension were transferred until
the silicon plate had a thin layer of settled dust suitable for analysis. A Philips Analytical Xray X’Pert MPD Diffractometer operating with the X’Pert software was used to collect the
diffraction patterns. Each sample was continuously scanned from 5-85 2θ using variable
slits, generator voltage of 45 kV and generator current of 40 mA, and a step size 0.02 2θ.
Preparation of the Bulk Samples by Analytical Transmission Electron Microscopy
(ATEM.) Each of the seven bulk samples was suspended in distilled water. The particles were
dispersed with the use of ultrasound to form a uniform dispersion. Initially, the particles were
suspended at a fairly high particle concentration to obtain a suspension representative of the
bulk sample. Once uniformly dispersed, it was diluted to a lower concentration and a 10 ml
aliquot was taken using a pipette and transferred to a 200-mesh copper locator grid. Each grid
contained a thin film of formvar coated with carbon to act as a support for the particles. Each
sample preparation was examined using a JEOL-2010 ATEM. The water was allowed to
evaporate and the grid was re-coated with a thin film of evaporated carbon to reduce charging
by the electron beam.
Collection of the Air Samples Pre and Post 9/1. During the month of October, high
volume ambient air samples were collected at a site in Lower Manhattan. The air filters were
prepared by direct-transfer and analysis of these air samples by ATEM determined the types
and concentrations of asbestos to which the general population of Lower Manhattan was
exposed. Historical air samples collected in NYC and in the chrysotile-mining town of Asbest
City in the Sverdlovsk Region of the Russian Federation were used respectively as low and
high background controls. The air samples collected in the chrysotile asbestos mining
community was used to represent the high end of background for airborne chrysotile asbestos.
The results of these analyses and other exposure data in the literature are needed to undertake
a risk assessment for the two major asbestos-related cancers – mesothelioma and lung.
Preparation of the Air Filters and Controls for ATEM Analysis. Each membrane filter
was removed from the sampling cassette, and a wedge-shaped section (~0.25 % of the total)
was cut. A carbon evaporator was used to coat each wedge of filter with carbon. The carboncoated section of the membrane filter was then cut into squares, approximately 3 by 3 mm,
and placed carbon-side-up on a formvar-coated 200-mesh copper locator grid. The grids were
placed on an aluminum wire mesh in a petri dish. Acetone was used to dissolve the membrane
filter (made of carboxymethylcellulose). The electron microscopy grids were examined using
a JEOL-2010 ATEM equipped with an ultra thin window to a lithium-drifted silicon detector.
A Princeton Gamma Tech-IMIX energy dispersive X-ray spectrometer was used for elemental
analysis. The technique of fiber counting used is consistent with that recommended for
AHERA (1986) (see Federal Register 1987, ISO 1995 for a detailed protocol). Two types of
historical blank controls were used to determine if, prior to sample collection, some filters
were contaminated with asbestos (closed blanks) or asbestos simply fell on the filter (open
blanks) at the sampling site.
Experimental results
Analysis of One Fireproofing and Six Settled Dust Samples by XRD. The peak positions
and intensities in each XRD pattern were determined and matched to reference standards to
identify the major crystalline phases present. The same three major crystalline phases were
identified in each of the seven bulk samples: gypsum, calcite and quartz (Table 3). Gypsum
and quartz were expected from the known composition of the construction materials (Table
1). Calcite is commonly used in many products including paint, paper and insulation. Two
diffraction patterns comparing the two settled dust samples collected from the two motor
vehicles on Chambers Street and White Street (north of the WTC) are shown in Figure 7a.
These two settled dust samples have similar peaks in the region of º2 shown, the most
intense peaks of the three major crystalline phases are present. The next two diffraction
patterns show a similar º2 region and compares one of the settled dust samples from the
bridge with a settled dust collected southeast of the WTC (Fig. 7b). The two-settled dust
samples collected from the motor vehicles are both similar to each other as well as being
similar to the settled dust samples from the other locations. In addition, each diffraction
pattern was carefully examined for the most intense peaks of the commercial asbestos
minerals. The d-spacings (related to 2θ) and the intensity of the major peaks diagnostic for
asbestos are shown in Table 4. No peaks consistent with the presence of any of the
commercial asbestos minerals were found in the XRD patterns of the seven bulk samples
collected indicating if any asbestos is present it is < 1 % by mass (Table 5).
Table 3
Results of continuous scan powder X-ray analysis of seven bulk samples collected
in the area of the collapse of the Twin Towers
Sample No.
1
2
6
3
4
5
7
Location
Crystalline phase identified by XRD
Fireproofing steel girder
Autovan on White Street
Dust from truck on Chambers Street
Foot bridge across Westside Highway
Foot bridge across Westside Highway
Foot bridge across Westside Highway
Street dust, Southeast WTC
Calcite, gypsum, quartz
Gypsum, calcite, quartz
Quartz, calcite, gypsum
Gypsum, calcite, quartz
Gypsum, calcite, quartz
Gypsum, calcite, quartz
Calcite, gypsum, quartz
Table 4
The most intense peaks in the continuous scan X-ray diffraction patterns
of the asbestos minerals
Mineral (ICDD Card No.)
D (Å)
0
2
Iv*
Grunerite asbestos (or amosite) (27-1170)
8.22
3.06
10.754
29.159
44 %
100 %
Anthophyllite asbestos (45-1343)
8.26
3.23
3.03
10.71
27.620
29.380
21 %
52 %
100 %
Clinochrysotile asbestos (27-1276)
7.31
3.65
12.107
24.386
71 %
100 %
Riebeckite asbestos (or crocidolite) (27-1415)
8.35
3.102
10.595
28.780
68 %
100 %
Tremolite asbestos
2.69
3.12
33.18
28.62
100 %
84 %
* Iv = Intensity using variable slits with most intense peak being 100 % peak.
Table 5
Results of analysis by continuous scan X-ray diffraction for the most intense peaks
of the commercial asbestos minerals
Sample No.
1 (fireproofing)
2 (settled dust – auto van)
6 (settled dust – truck)
3 (settled dust – bridge 1)
4 (settled dust – bridge 2)
5 (settled dust – bridge 3)
7 (settled dust – southeast)
Continuous scan X-ray diffraction
Amosite
Anthophyllite
Chrysotile
Crocidolite
ND*
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
* None detected.
Analysis of the Settled Dust Samples Polarized Light Microscopy. Each of the bulk
samples was examined using polarized light microscopy (PLM) at magnifications from 50500x on a rotating stage. Immersion oils of various refractive indices (RI) were used to
characterize the RI of the different types of particles present. No asbestos minerals were found
to be present in any of the seven bulk samples examined. Although a trace of elongated nonasbestiform amphibole was found in Sample 3. No asbestos of any fiber type was identified
by PLM (Table 6). Analytical results indicate much less than 0.1% by volume asbestos of any
fiber type is present.
Table 6
Results of analysis by polarized light microscopy for commercial asbestos fiber types
Sample No.
1 (fireproofing)
2 (settled dust – auto van)
6 (settled dust – truck)
3 (settled dust – bridge 1)
4 (settled dust – bridge 2)
5 (settled dust – bridge 3)
7 (settled dust – southeast)
Amosite
ND*
ND
ND
ND
ND
ND
ND
Polarized light microscopy
Anthophyllite
Chrysotile
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Crocidolite
ND
ND
ND
ND
ND
ND
ND
* None detected.
Analysis of Seven Bulk Samples by ATEM. Each grid preparation was examined over a
range of magnifications from 50-20,000x magnifications. The analysis focused on identifying
the various particles present, particularly fibrous minerals such as asbestos and other
particulate material, exposure to which can be associated with human health hazards. We will
report the results for asbestos here. No amphibole asbestos of any type was identified in any
of the seven bulk samples. However, traces of chrysotile asbestos were present in all six
settled dust samples (Table 7). At some time, chrysotile asbestos must have been airborne
prior to becoming settled dust. It is problematic to use the analysis of settled dust to determine
the extent to which these asbestos fibers, when airborne, were respirable and at what
concentration. The bulk sample of fireproofing removed from a piece of structural steel
contained no asbestos of any type. This is consistent with the reports indicating the
fireproofing of the structural steel on the higher floors of the WTC did not contain chrysotile
asbestos (Langer and Morse, 2001, FEMA, 2002). We would estimate the concentration of
chrysotile asbestos in the six settled dust samples to be less than 0.01% by volume.
Table 7
Commercial asbestos fiber types identified by analytical transmission electron microscopy found
in seven bulk samples of dust from the collapse of the Twin Towers
Sample No.
1 (fireproofing)
2 (settled dust – auto van)
6 (settled dust – truck)
3 (settled dust – bridge 1)
4 (settled dust – bridge 2)
5 (settled dust – bridge 3)
7 (settled dust – southeast)
* None detected.
Amosite
ND*
ND
ND
ND
ND
ND
ND
Analytical transmission electron microscopy
Anthophyllite
Chrysotile
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Crocidolite
ND
ND
ND
ND
ND
ND
ND
Fig. 7a. Comparison of the continuous scan X-ray diffraction powder pattern of the two settled
dust samples collected from the surfaces of two vehicles. Below appear the reference patterns
peak positions for the three major cristalline phases – calcite, gipsum and quartz
Fig. 7b. Comparison of the continuous scan X-ray diffraction powder pattern of the one settled
dust samples collected from the Chambers street foot bridge (black line) and the second southeast of the WTC (grey line). Below appear the reference atterns peak positions for the three
major cristalline phases – calcite, gipsum and quartz
The three crystalline phases (quartz, gypsum and calcite) identified by XRD and
PLM analysis were also identified by ATEM. Generally, the gypsum has a non-fibrous
morphology although gypsum fibers are not rare. The other two crystalline phases:
calcite and quartz, almost always occur as non-fibrous particulates.
Results of the analysis of six ambient air samples collected near the WTC post 9/11
by analytical transmission electron microscopy
Airborne particulates were collected on six membrane filters over a three-week period
in October to determine the airborne concentration of asbestos in Lower Manhattan (Table 8).
The samples were collected at various times of the day to obtain a representative
concentration of asbestos fibers in the air as the WTC debris removal program performed
different tasks at night. The sampling strategy involved collecting only area samples (where
the sampling pump was not attached to a person but rather remained in one location during
the entire period of the sampling) at 274 Water Street (Fig. 8). These air samples were
collected outside (also referred to as ambient) to determine if a measurable ongoing increase
in airborne asbestos concentration (above background) in Lower Manhattan could be
associated with the massive asbestos containing dust cloud produced by the collapse of two of
the world’s tallest buildings and the ongoing debris removal. The air was sampled at a flow
rate of 20 liters per minute (LPM) and the duration of sampling varied between samples. But
was sufficient to collect between 4,770 and 6,064 liters of air (see Table 8 for details of
sampling). The results of the analysis of the six filters by ATEM are shown in Table 9 and
summarized below.
Table 8
Description of air monitoring on Water Street near Brooklyn Bridge, New York,
NY starting 27 days after Twin Towers of the World Trade Center collapse
ID No.
(date collected)
Area sample 1
10/08/01
Area sample 2
10/09/01
Area sample 3
10/10/01
Area sample 4
10/21/01
Area sample 5
10/25/01
Area sample 6
10/30/01
Filter pore size
and type of
membrane
0.8 m CME*
Fiber
Duration of
sampling,
min
238.5
0.8 m CME
Fiber
240.1
20
0.8 m CME
Fiber
241.2
20
0.8 m CME
Fiber
287.0
20
0.8 m CME
Fiber
245.3
20
0.8 m CME
Fiber
303.2
20
Type of
sample run
* Area of 25 mm in diameter carboxymethycellulose.
Flow rate
(LPM)
20
Volume
Sampling station
of air sampled,
(date collected)
l
4770
5th floor north
window 10/08/01
4802
5th floor north
window 10/09/01
4824
5th floor north
window 10/10/01
5740
5th floor north
window 10/21/01
4906
5th floor north
window 10/25/01
6064
5th floor north
window 10/30/01
Fig. 8. The highlighted area of Lower Manhattan has 57,511 resident according to the 2000 US
Census and was used in the risk assessment as the general population. Three settled dust samples
were collected in the area of Styvesant high School one from a motor vehicle on Chambers street,
one from and auto van on White Street and one southeast of WTC. The six ambient air samples
were collected at 274 Water Street near the Brooklyn Bridge
Table 9
Results of the examination of six ambient air samples by analytical transmission electron microscopy.
The samples were collected on Water Street near the Brooklyn Bridge in October, 2001
ID No.
Area 1
10/08/01
Area 2
10/09/01
Area 3
10/10/01
Area 4
10/21/01
Area 5
10/25/01
Area 6
10/30/01
4770
75
0.9075
0
0
11,244
Total airborne
asbestos
concentration,
f/ml
< 0.00009
4802
75
0.9075
0
0
11,319
0.00009
4824
75
0.9075
0
0
11,371
< 0.000088
5740
75
0.9075
0
0
13,530
< 0.000074
4906
76
0.9196
0
0
11,718
< 0.000085
6064
75
0.9075
0
0
14,293
< 0.00007
Volume of
air, l
Grid area
No. of fields
examined*,
examined
mm
Asbestos
 5m
< 5m
Volume of air
scanned, ml
(Mean  Sd <0.00008  0.000008)
* For 25 mm filter, assume a collection face with a 11.07 mm diameter and a total surface area of 385 mm2.
The fraction of the particles examined in the collected air ranged between all the
particles in 11,244 to 14,293 ml of ambient air. Each of the six air samples was examined at
20,000x magnification using ATEM. Our procedure is the most sensitive method for the
detection of airborne asbestos; the direct-transfer preparation of the air filter causes minimal
changes in size distribution and any asbestos fiber present will be visible under these
conditions. In addition, by collecting a higher volume of air and examining a larger area of the
collection filter, the sensitivity was ~10-fold below what is normally used by the
Environmental Sciences Laboratory to monitor asbestos for the purpose of risk assessment in
the non-occupational environment (Nolan and Langer, 2001). Even when monitoring to the
lower sensitivity not a single asbestos fiber was found in the 73,475 ml of ambient air
examined. The mean volume of ambient air in the six air samples was 12,246 ml.
Estimation of asbestos exposure pre & post 9/11
A risk assessment for asbestos-related disease requires knowledge of the type of
asbestos and the cumulative exposure, which represents the intensity and duration of exposure
and is usually given in fibers per milliliter multiplied by years (f/ml × years). The
Environmental Science Laboratory has sampled the ambient air in NYC since around the time
of the construction of the World Trade Center complex including the period when asbestoscontaining fireproofing was being blown onto the steel structure of the North Tower (WTC1).
Since the early 1970-s, methodologies for air sampling have changed from indirect-transfer
sample preparation, where the size distribution was altered and the results were reported in
mass per unit volume of air, to the direct-transfer method of sample preparations where
airborne size is largely retained and the number of fibers per ml of air are determined (Nolan
and Langer, 2001). The conversion between the two air-monitoring methods has limitations
that we choose to avoid by only using air samples analyzed by direct-transfer for background
comparison with the result obtained for post 9/11.
For establishing background levels of asbestos in the ambient (or outdoor) air in NYC,
we selected air samples collected in 1989, 1992 and 1994 which had been analyzed by the
same methodology used for the post 9/11 air sample. Before 9/11, the concentration of
airborne asbestos in NYC was consistently less than 0.0008 f/ml where all types of asbestos
fibers having lengths equal to or greater than 0.5 m were counted (Table 10). We assume the
background for asbestos in the ambient air in NYC is below this concentration and
representative of the levels in Lower Manhattan prior to 10AM on the morning of September
11th (Fig. 9).
Table 10
Historical air samples collected in New York and Asbesr City (Russian Federation)
to evaluate ambient (or outside) airborne asbestos concentration
and analyzed by analytical transmission electron mocroscopy
Site / date
JFK Airport
Terminal, 12/14/89
Type of
sample
No. of
samples
Volume of
samples, l
Area / ambient
Blank
Area / ambient
2
2
5
2,048 ± 92
0
2,042 ± 19
3
7
–
2,005 ± 38
4
–
Erasmus Hall High
School, Brooklyn,
Blank
NY, 12/28/92
Public School 192Q, Area / ambient
109-89 204th Street,
Queens, NY,
Blank
7/6-7/95
Total asbestos
Average
Airborne
fibers*
volume of air concentration,
f/ml
≥ 5 μm < 5 μm scanned, ml
0
0
1,266 ± 77
< 0.00079
0
0
–
< 8.3 f/mm2
2
7
1,283 ± 12
< 0.00078
(Gru, Ctl)
0
0
–
< 4.1 f/mm2
0
0
1,260 ± 24
< 0.00079
0
0
–
< 4.1 f/mm2
Asbest City,
Sverdlovsk Region,
Russian Federation
Area / ambient
13
900
13
127
387 ± 185
< 0.003
Simbols: Gru – grunerite, Ctl – chrysotile. * Total number of asbestos fibers found in all air samples
Fig. 9. Estimated of the chrysotile asbestos exposure to the general population from the dust
released when the Twin Towers collapsed and while the airborne concentration of asbestos
was elevated. The best estimate of the maximum cumulative chrysotile asbestos exposure to
the general population of Lower Manhattan during the period post 9/11 prior to returning to
background is 0.28 f/ml × year
To our knowledge no air sampling data have been reported for the initial dust cloud on
9/11 and it is doubtful if such a particulate dense aerosol could have been collected and
analyzed to determine the airborne asbestos concentration. However, taking into consideration
the small amount of chrysotile asbestos in the settled dust, we make an estimate of the
maximum concentration of airborne asbestos at 50 f/ml with a length greater than or equal to
5.0 m. This high exposure level is consistent with uncontrolled historical exposures in
chrysotile asbestos mines and mills in Québec where the ore contains a minimum of 2-4 %
chrysotile asbestos (Gibbs & DuToit, 1973). As representative WTC settled dust samples
contain less than 0.01 % asbestos, the visible aerosol although enormously high, was
overwhelmingly made-up of non-asbestos particulate matter and our estimate of airborne
asbestos is most likely higher than actually occurred (Fig. 3).
After approximately 5 hours, we assume the airborne concentration of chrysotile
asbestos to have decreased by 50-fold to no more than 1 f/ml with a length of 5 m or greater
(Fig. 4). We further assume that over the next 26.4 days, until the first air sample was
collected on October 8th, the airborne concentration of chrysotile asbestos decreased linearly
to a level consistent with background, i.e., effectively zero (Fig. 9). This assumption is
conservative since the decrease is more likely to have been exponential.
Two periods describe the estimated
chrysotile asbestos exposure
Analysis of representative settled dust samples released from the attack on the World
Trade Center found only trace concentrations of chrysotile asbestos to be present. These
analytical results are consistent with the reports describing the uses of asbestos in the WTC.
The first 39 stories of the North Tower (WTC1) were sprayed with chrysotile asbestos
containing fireproofing and this type of asbestos was used in other construction products in
the towers (Langer and Morse, 2001). Based on the analysis of settled dust the exposure
estimate for the general population need only consider exposure to chrysotile asbestos. The
cumulative exposure from the collapse of the first tower until the first air sample was
collected on October 8th is estimated in the following manner:
September 11th from 10AM – 3PM. 5 hours at 50 f/ml with a length greater or equal to
5 m in length is equivalent to 1 day (24 hr) at an exposure of 10.4 f/ml or 3 working days (8
hrs). This initial exposure will be described using a cumulative exposure index (given in f/ml
× years) that can be added to a second time period of lower (but still above background)
asbestos exposure. Three working days at 10.4 f/ml yield a total exposure of 10.4 f/ml x 3
days or 31.2 f/ml x days. The yearly average can be obtained dividing by 250 working
days/year to describe the cumulative exposure on an annual basis that corresponds to 0.12
f/ml x years.
From 3AM on September 11th to October 8th. We are estimating that by mid-afternoon
of September 11th the airborne concentration of chrysotile asbestos had decreased to 1 f/ml
with a length greater or equal to 5 m. Furthermore, we are assuming the airborne chrysotile
asbestos concentration decreased linearly by over 11,000-fold to the experimentally
determined value of < 0.00009 f/ml by October 8th. Additional analysis over the month of
October indicates the concentration of airborne chrysotile asbestos remained consistently
similar to the value determined on October 8th below the sensitivity. The air samples were
collected at different times of the day to determine if excursions were occurring – none were
found.
Assuming the asbestos fiber concentration decreased linearly, after approximately 13
days, the estimated exposure fell to 0.5 f/ml. Such an exposure for 26.4 days is equivalent to
13.2 f/ml × days. Again, these are 24hr days rather than 8hrs working days so we multiply the
exposure by 3 so over the 26.4-day period the cumulative exposure is estimated to be 39.6
f/ml × days. Once the exposure is averaged over a year (divide by 250 days/yr – 0.16 f/ml ×
years and added to the 5hr exposures of 0.12 f/ml × year, making the total cumulative
exposure 0.28 f/ml × years.
The two-time periods of cumulative exposure can be added
0.12 f/ml × years (initial 5 hrs after first tower collapses on 9/11) + 0.16 f/ml × years (next 26.4 days)
To estimate the total cumulative exposure from 9/11 to 10/8 = 0.28 f/ml × years
If you were not exposed to the initial 5hr dust cloud on 9/11, your cumulative chrysotile
asbestos exposure will be 43 % lower (Fig. 9).
Risk assessment for pleural mesothelioma and lung cancer
from cumulative exposure to chrysotile asbestos post 9/11
The risk of developing an asbestos-related mesothelioma depends on the type of
asbestos one is exposed to, the cumulative exposure and the age at which exposure first
occurs (Hodgson and Darnton, 2000). The number of asbestos-related mesothelioma (OM) can
be calculated for a given asbestos exposed population using the following relationship:
OM 
R M  TPOP  E CA
,
100
where OM – no of asbestos-related mesotheliomas observed (the number we wish to
calculate);
RM – risk of mesothelioma expressed as a percentage of the total expected mortality
per f/ml × years of asbestos exposure. The RM used, 0.001, is obtained from
Hodgson & Darnton 2000 (their Table 1) (adjusted to 30 years of age at first
exposure) and over estimates the chrysotile asbestos risk as some exposure to
amphibole asbestos occurred in the cohorts used to determine the value. If 5 % or
more of the exposure was due to the commercial amphibole asbestos minerals –
crocidolite or amosite, the RM value used would be larger;
TPOP – total exposed population for Lower Manhattan is all 57,511 residents estimated
from United States Census 2000 (see Fig. 8 for area included);
ECA – cumulative chrysotile asbestos exposure given is 0.28 f/ml × years (Fig. 9). The
best estimate for the maximum cumulative chrysotile asbestos exposure to the
Lower Manhattan general population prior to the ambient airborne asbestos level
returning to background.
Solving for OM:
OM = 0.16 mesothelioma cases due to 9/11 exposure to chrysotile asbestos.
Using the estimated cumulative chrysotile asbestos exposure associated with the
collapse of the Twin Towers and the specific asbestos fiber type to which the general
population was exposed the risk assessment indicates less than 1 excess mesothelioma case,
0.16, when the entire 57,511 residential population of Lower Manhattan dies. The background
for mesothelioma has been conservatively estimated to be 1 case per 10,000 deaths. Therefore
5.7 mesotheliomas would be expected in this population the increased risk of asbestos-related
mesothelioma from the 0.28 f/ml × years related to the 9/11 attack could be less than 1/6 of
background or 0.00028 % mortality among the 57,797 population of Lower Manhattan (Fig.
10).
Fig. 10. Percentage of mesothelioma morrtality (adjusted to 30 years of age at first exposure)
for cumulative exposure to crocidolote asbestos and chrysotile asbestos
Risk assessment of asbestos-related lung cancer
for the population of Lower Manhattan exposed by the events of 9/11
For a given cumulative asbestos exposure the risk of developing lung cancer will
increase as a percentage of the lung cancer risk in the population exposed. The most
significant risk factor for developing lung cancer for the general populations of Lower
Manhattan, as most everywhere else, is cigarette smoking. Approximately 80-85 % of
the lung cancer risk is attributed to smoking (Doll & Peto, 1981). We will assume that
on average 8 % of cigarette smokers develop lung cancer while among those that
choose not to smoke only 0.8 % will develop lung cancer (Wilson and Crouch, 2001,
p. 230-231). Furthermore, we will assume that the risk of lung cancer increases
linearly with cumulative asbestos exposure following the relationship:
Obs L  Exp L 
R L  E CA  Exp L
,
100
We wish to calculate the increase in the observed number of lung cancers (ObsL) due to
exposure to chrysotile asbestos.
Where ExpL – expected percentage of lung cancer deaths assumed to be 8 % among smokers;
RL – risk of lung cancer expressed as a percentage of lung cancer deaths per f/ml ×
years of asbestos exposure. The RL used is 0.062 obtained from Hodgson and
Darnton 2000 (their Table 2) and is specific for chrysotile asbestos;
ECA –cumulative chrysotile asbestos exposure given is 0.28 f/ml × years (Fig. 9). The
best estimate of the maximum cumulative chrysotile asbestos exposure to the
general population of Lower Manhattan prior to the ambient airborne asbestos
level returning to background.
ObsL = 8 % + 0.00139 % = 8.00139 %.
If the entire population of Lower Manhattan, 57,511, smoked cigarettes the lung cancer
mortality, after the death of the entire population, would be expected to be 8 % or 4,601 lung
cancer cases. The cumulative chrysotile asbestos exposure of 0.28 f/ml × years would
increase the percentage of lung cancer to 8.00138 % or add approximately 1 additional case to
the 4,601 lung cancer cases among the general population of Lower Manhattan. If the entire
population of Lower Manhattan choose not to smoke and were not exposed to asbestos from
the events of 9/11, 0.8 % or 460 cases of lung cancer would be expected to occur. The 0.28
f/ml × years cumulative chrysotile asbestos exposure would increase the percentage of lung
cancers among the non-smokers to 0.800139 %. The asbestos-related increase in lung cancer
is 10-fold lower among the non-smokers because their risk of lung cancer is 10-fold lower.
Approximately 0.1 of a lung cancer case among 57,511 deaths among non-smokers (Fig. 11).
Thus, if all of the Lower Manhattan residents were non-smokers, the exposure to 0.28 f/ml ×
years of cumulative chrysotile asbestos exposure would cause an increase of approximately
0.1 of a lung cancer case within the 57,511 deceased residents. Thus, if all the Lower
Manhattan residents were non-smokers, the 0.28 f/ml × years of cumulative exposure to
chrysotile asbestos would cause an increase of approximately 0.1 of a lung cancer case within
the 57,511 decreased residents.
Discussion and conclusions
The murderous attacks on the Twin Towers of NYC’s World Trade Center (WTC) and the
subsequent collapse of both towers created a pressure wave, which dispersed an enormous amount of
construction dust containing trace concentrations of chrysotile asbestos into the ambient air of Lower
Manhattan (Fig. 2a-g). Although estimating the airborne concentration of asbestos on and shortly after
9/11, has limitations; it undoubtedly increased the background concentration of asbestos in the
ambient air in NYC, which remained elevated for some period of time (Fig. 3, 5a). A cumulative
exposure was estimated for the duration of the time when the background concentration of asbestos
was elevated using air sampling data post 9/11 and exposure data available in the scientific literature.
Fig. 11. Comparison of the risk of lung cancer for non-smokers
& smokers as a function of exposure to chrysotile asbestos
The potential for the almost 58,000 people living in Lower Manhattan to develop an increased
incidence of asbestos-related cancer from post 9/11 asbestos exposure depends principally on
three factors: asbestos fiber types to which exposure occurred, the cumulative asbestos
exposure (the intensity and duration of exposure usually reported as f/ml × years) and for
mesothelioma their ages when the exposures first occurred. When sufficiently large exposures
occur, greater than those to the general population in Lower Manhattan post 9/11, cigarette
smoking can be an important additional factor in increasing the risk of asbestos-related lung
cancer (Fig. 11). Analysis of the settled dust by XRD, PLM and ATEM indicates that of the
six regulated asbestos fiber types only chrysotile asbestos is present in the settled dust. We
have adequately estimated cumulative chrysotile asbestos exposure related to 9/11 thus we
can undertake a risk assessment for the general population of Lower Manhattan due to the
asbestos released by the events of 9/11. We will describe the preliminary results of our risk
assessment here.
Construction of the Twin Towers occurred at a time when imports of chrysotile asbestos
from Canada exceeded 500,000 t per year and it was commonly used in the fabrication of
building materials and some of these products were used in the construction of the WTC
(Ross and Virta, 2001). The fireproofing of the Twin Towers’ structural steel was originally
intended to be a sprayed on chrysotile asbestos product (Table 1). Although the lower floors
of the North Tower (WTC1) were fireproofed with this product, the use of asbestos for this
application was abandoned prior to completing half of the first tower due to public health
concerns (Langer and Morse, 2001). Exposures to the general population from the over spray
of the asbestos fireproofing and other construction processes by which asbestos was released
into the ambient air was drawing the attention of NYC officials, who were carefully
monitoring these asbestos spray applications (Reitze et al, 1972). Although spray application
of asbestos fireproofing was not yet banned in NYC, the contractors voluntarily choose to
change to asbestos-free fireproofing prior to completing the first tower. However, chrysotile
asbestos containing building products found additional uses besides fireproofing in the
construction of the WTC (Langer and Morse, 2001). It is estimated that 320,000 commercial
buildings in the U.S. contain friable asbestos (Nolan and Langer, 2001).
Based on our knowledge of the use of chrysotile asbestos in the Twin Towers finding
trace levels of the mineral in the settled dust from their collapse was to be expected. Do to the
limited use of this fibrous minerals in the construction of the towers, it seems reasonable to
assume that a higher concentration, such as 1 % or more reported by others, may occur, but
are not representative of the settled dust in general and therefore not relevant to the dust
cloud. The uniformity of composition among the six settled dust samples analyzed indicate
that mixing occurred tending the dust toward a fairly uniform composition. The chrysotile
asbestos content was at trace levels (Table 7).
The extent of the settled dust post 9/11 and the potential for additional asbestos
release from the demolition and removal of the WTC debris suggested we should
collect ambient air samples to determine, if and for how long, background
concentrations of airborne asbestos were increased in Lower Manhattan post 9/11. Our
air-sampling program began 27 days after the collapse of the Twin Towers. The airsampling site was near the WTC site and selected to be representative of ambient nonoccupational exposure in Lower Manhattan (Fig. 9). To gather the best exposure data
possible, the sampling program was designed to be sensitive enough to detect very
small concentrations of airborne asbestos (considered the low end of background) and
identify the asbestos fiber type(s) present (ASTM, 2001). Air samples were collected
on six days over a three-week period in October; no asbestos of any fiber type was
identified (Table 9). The sensitivity for asbestos was below 1 fiber in 12,246 ml or a
thousand-fold below OSHA’s occupational asbestos standard of 0.1 f/ml (OSHA,
1994) (see Fig. 12 for comparison of post 9/11 with other standards).
The asbestos fiber concentrations in the ambient air in Lower Manhattan one month
post 9/11 are consistently below the sensitivity of the historical air samples used to establish
the background in NYC. As post 9/11 air samples that were designed to a sensitivity an order
magnitude lower than those of the historical air samples collected pre 9/11, airborne asbestos
concentrations pre and post 9/11 may actually be identical. These levels of asbestos exposure
are at the lower range of ambient airborne asbestos reported by WHO (1986):
“Based on surveys conducted in 1986, fiber concentrations (fibers  5 m in length) in
outdoor air, measured in Austria, Canada, Germany, South Africa and the USA, were between
0.0001 and about 0.01 f/ml, levels in most samples being less than 0.002 f/ml. Means or
medians were between 0.00005 and 0.02 f/ml based on more recent determinations in Canada,
Italy, Japan, Slovak Republic, Switzerland, United Kingdom, and US”.
One month post 9/11, ambient airborne asbestos concentrations in Lower Manhattan
were at the lower end of what the WHO reports as background for ambient air. The positive
controls collected in Asbest City were at the high end of the range for ambient air at 0.003
f/ml, greater than or equal to 5 m in length (Fig. 12) or 10-fold higher if chrysotile of all
fiber lengths is considered. These air samples indicate the validity of the methodology and
only required analyzing all the particles in 387 ml (in the WTC air samples all the particles in
a 30-fold greater volume of air were examined, Table 9). In Asbest City where the cumulative
chrysotile asbestos exposure to the general population is order of magnitude higher, than that
of the general population of Lower Manhattan due to the events of 9/11, an increased
incidence of asbestos-related cancer has not been convincingly demonstrated (Shcherbakov et
al, 2001).
The post 9/11 air monitoring results reveal a similar pattern to those reported for the
asbestos in the ambient air after the 1995 earthquake in Kobe, Japan where no long-term
increase in the background levels of airborne asbestos were also found (Higashi et al., 2001).
The Kobe earthquake was a magnitude 7.2 on the Richter Scale, which is 5-fold greater in
magnitude than that caused by the collapse of each 500,000 t WTC Tower and involved the
loss of 5,500 lives and left 200,000 people homeless. It is noteworthy that with the loss of life
and the extent of the property damaged at Kobi exceeded that of 9/11 in NYC.
Air sampling in Lower Manhattan has established that ambient airborne chrysotile
asbestos level in NYC had returned to background by October 8th. Therefore the period of
elevated airborne concentrations of chrysotile could not be more than 27 days. To estimate the
cumulative asbestos exposure, we divided twenty-seven days into two exposure periods of
differing intensity and duration. The first time period was the initial five hours after the
collapse of the first tower where airborne asbestos concentrations were estimated to be no
more than 50 f/ml  5 m in length (Fig. 9). Extremely high exposures of this type are
associated with the mining and milling of chrysotile asbestos with a minimum use of control
technology to reduce exposure (Gibbs & DuToit, 1973). In the mining environment the
concentration of asbestos in the ore is 200-400-fold higher than the 0.01 % chrysotile asbestos
found in the WTC dust. Even allowing for the extremely high concentration of airborne dust
after the collapse of the towers it is doubtful a dust containing such a small amounts of
asbestos could generate an aerosol, which exceed the levels of the mining environment – an
intensity of 50 f/ml of asbestos. Within five hours the dense dust cloud had cleared
sufficiently for the skyline of Lower Manhattan to be visible again from seven miles away on
the Brooklyn College campus (Fig. 4). For the second time period we estimated the airborne
concentration of chrysotile asbestos had decreased 50-fold to 1 f/ml by the time the skyline
was again visible, further decreasing linearly to background over the next 26.4 days. The
mean exposure during that time is estimated to be 0.5 f/ml  5 m in length (Fig. 9).
Fig. 12. Comparison of asbestos exposures from the collapse of WTC complex with historical, permissible
and background asbestos exposures. Note the United States Environmental Protection Agency (EPA) does
not determine the actual airborne concentration of asbestos but only reports the number of structures per unit
area of the collection filter (EPA f/ml value given below is estimated). EPA does not define structure as any
of the six regulated types of asbestos therefore asbestos fiber type is not known. The Environmental Science
Laboratory (ESL) determined the historical airborne asbestos concentration in NYC to be indistinguishable
from those one month post 9/11
Over the 27 days when the airborne chrysotile asbestos concentration exceed
background we estimate a cumulative exposure of 0.28 f/ml × years to have occurred based
on the settled dust analysis it is predominantly, if not exclusively, a chrysotile asbestos only
exposure. Approximately 43 % of the cumulative exposure is from the initial dust cloud. If
one was fortunate enough to avoid this five hour exposure, the estimated cumulative exposure
would be reduced to 0.16 f/ml × years. The estimated cumulative asbestos exposure is
conservative in that it most likely greater than what actually occurred consistent with our goal
of estimating the upper limit of the asbestos-related cancer risk.
The United States Environmental Protection Agency (EPA) and NYC’s Department of
Health US EPA 2002 report on 4,946 air samples collected between September 11, 2001 and
January 22, 2002. Twenty-seven of these samples exceeded the asbestos standard developed
by EPA for AHERA of 70 structures per mm 2, for re-occupying school buildings after
asbestos abatement. Twenty-seven of the thirty-one air samples that exceeded the AHERA
standard occurred prior to September 30, 2001. These data are consistent with our conclusion
that by October 8th the airborne concentration of asbestos in Lower Manhattan had returned to
background.
Among the 6.4 % of the air samples (27 of 442 air samples collected in September,
2001) above the AHERA standard for re-entering schools post asbestos abatement the EPA
does not report – the type, airborne concentration or size distribution of the asbestos present –
all of which are needed for a modern risk assessment. The EPA’s air monitoring results are
expressed as structures per mm2 of the air-monitoring filter. The term structure does not
specify the type of asbestos found on the filter. The actual volume of air sampled is not
reported, but the AHERA method recommends 1,200 to 1,800 liters of air be sampled. We
can estimate the f/ml asbestos exposure by assuming a 25 mm diameter air-monitoring filter
was used by EPA, which has an effective collection area of 385 mm2, and then 70 structures
per mm2 would correspond to 26,950 structures per filter. If 1,200 liters (or 1,200,000 ml) of
air were pumped across the filter to collect the structures than the airborne concentration of
structure would be 0.022 f/ml (Fig. 12). If 1,800 liters of air were filtered the value would fall
to 0.015 f/ml. EPA does not specify by how much each of these 27 samples exceeded the 70
structure/mm2 standard but in the worst case it appears 93.6 % were below 0.022 f/ml. Again,
these EPA/NYC air samples tend to support our view that our estimated average exposure of
0.5 f/ml (for the airborne chrysotile concentration during the 26.4 days post 9/11) is above the
actual intensity of the asbestos exposure over that period of time.
The 70 structures/mm2 standard used by EPA/NYC to monitor asbestos in the ambient
air of Lower Manhattan is an asbestos standard for indoor air (specifically for re-occupying
schools after asbestos abatement) and the value was determined to reflect the number of
asbestos fibers on the air filter statistically identical to background (Federal Register, 1987).
The EPA relied on a statistical argument claiming that 70 structures/mm2 is statistically
equivalent to background levels of asbestos contaminating the air filters and then simply
rationalized that background was safe. For comparison, the asbestos contamination measured
on our filters were approximately 70-fold lower than the AHERA standard. To our
knowledge, the AHERA standard is the only asbestos standard in the world where the actual
concentration of asbestos in the air is not determined (WHO, 1989). It was not based on any
relationship between cumulative asbestos exposure to the various fiber types and mortality
from asbestos-related cancers (Hodgson & Darnton, 2000).
The NYC Department of Environmental Protection notes that neither the US EPA nor
NYC has an asbestos standard for outdoor air so they rely on an indoor standard for asbestos
of 0.01 f/ml, which seems derivative of the 70 structures/mm2 (NYC, Department of
Environmental Protection, 2002) to interpret the results from ambient air monitor in Lower
Manhattan post 9/11. Air monitoring data indicate that airborne concentration of asbestos
inside and outside of buildings is identical (Nolan and Langer, 2001). Generally, asbestos
standards are not specific to either indoor or outdoor (WHO, 1989). It seems reasonable to
assume that the indoor dust is similar in chrysotile asbestos content to that of the dust cloud
characterized as containing trace concentrations of chrysotile asbestos. The EPA has offered
to clean private residences of the dust from 9/11 (Fig. 13). For the purpose of risk assessment
the EPA/NYC air sampling data has four major limitations. First the exposure response
relationships for asbestos-related cancers are indexed to the number of asbestos fibers ≥ 5 m
while the 70 structures/mm2 standard is indexed to fiber ≥ 0.5 m. Secondly, the asbestos
fiber types vary in potency particularly for mesothelioma where crocidolite asbestos is 500fold more potent than chrysotile asbestos therefore it is important to know what fiber type (or
types) to which people are being exposed (Fig. 10). Thirdly, without regard to fiber length,
the NYC/EPA indoor standard is within 10-fold of the OSHA asbestos standard of 0.1f/ml.
Our own experience with asbestos air sampling (indoor and outside) indicates that 0.01 f/ml is
too high a sensitivity (Nolan and Langer, 2001). Fourth and most importantly the actual
concentration of airborne asbestos was not determined.
Fig. 13. Dusty clothes in a retail store near WTC site post 9/11. Concerns about
the risk of asbestos-related cancer from the trace level of chrysotile asbestos in
the indoor continue to be raised in the media
Assuming that any exposure to asbestos (no matter how slight) increases one’s risk of
developing asbestos-related cancer is often referred to as the linear no-threshold model. It is
commonly used by regulatory agencies throughout the world to provide worst-case scenarios
for exposure to carcinogens. Opinions of medical and scientific experts largely view this
approach as the most protective of human health. It is not known for certain that these slight
exposures carry any cancer risk as they are far below the cumulative exposures known to
cause cancer and associated with increases too small ever to be actually observed.
However, high exposure to the various asbestos fiber types will cause asbestos-related
cancer. Historically, these high exposures could result in 20 % excess cancer mortality among
workers with substantial cumulative exposure to the asbestos minerals (Hodgson & Darnton,
2000). The pattern of asbestos-related cancer is complex and has been studied in considerable
detail (Nolan et al., 2001). This analysis makes two fundamental assumptions about the
carcinogenicity of chrysotile asbestos. Firstly, it is assumed, following Hodgson and Darnton,
2000, that chrysotile is less carcinogenic than the commercial asbestos minerals – amosite or
crocidolite. Secondly, it is assumed that there is a linear dose-response at low dose. Our
approach is to interpolate the increase risk of asbestos-related cancer from high cumulative
exposures, with known risk for the asbestos-related cancers (mesothelioma and lung), to the
very low exposure assuming the risk of cancer to be linear. For a cumulative asbestos
exposure of 0.28 f/ml × year, the entire 57,511 residents of Lower Manhattan may experience
1 mesothelioma case then would be otherwise expected (Fig. 9) and if the entire population
smoked cigarettes approximately one lung cancer case (Fig. 10). If the exposure was to
crocidolite asbestos, the mesothelioma risk would be almost 500-fold higher (Fig. 10). If no
one smoked the risk of lung cancer would be 10-fold lower. As of 1993 approximately 25 %
of all persons over 18 years of age smoke and approximately 86 % of the residents in Lower
Manhattan are over 18 years of age so the actual increased lung cancer incidence would be
about ¼ of a case (Zang and Wynder, 1998). At this very low cumulative asbestos exposure
the synergy with smoking is not a significant factor in the calculation.
Acknowledgements
We acknowledge support from a Higher Education Advanced Technology grant from
the State of New York to support the Environmental Sciences Laboratory, Brooklyn College
of The City University of New York and the International Environmental Research
Foundation of New York City. We thank Drs. B. Price and A.M. Langer for very helpful
suggestions that led to improvements of this paper.
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