Determination of a Diagnostic Signature for World 

Trade Center Dust using Scanning Electron 

Microscopy Point Counting Techniques 

By Gregory P. Meeker, Amy M. Bern*, Heather A. Lowers, and Isabelle K. 

Brownfield 

Any use of trade, product or firm names in this report is for descriptive purposes only and does 

not imply endorsement by the U.S. Government. 

Open-File Report 2005 - 1031 

U.S. Department of the Interior 

U.S. Geological Survey, Denver Federal Center, Denver, CO 80225 

*Current address: U.S. Environmental Protection Agency, National Enforcement 

Investigations Center, Denver Federal Center, Denver, CO 80225 
Introduction 

The tragedy of the September 11, 2001, collapse of the World Trade Center (WTC) 

towers and other buildings in and adjacent to the WTC site produced a dust cloud that 

was visible from space and covered much of lower Manhattan in millimeters to 

centimeters of extremely fine powdered material. This material was inhaled and ingested 

by thousands of people on the day of the event and for several days afterward. In 

addition, thousands of apartments, offices, and public buildings were contaminated by the 

dust through a variety of pathways. The short-term medical effects of this exposure were 

manifested in what became known as the World Trade Center (WTC) cough, documented 

as respiratory and other health problems among many of those who were exposed (Gavett 

and others, 2003; Prezant and others, 2002). Potential long-term medical effects of this 

event may not be known for many years. Several studies have examined various 

components of the dusts generated by the collapse of the WTC (Meeker and others, 2005; 

Plumlee and others, 2005; Badger and others, 2004; Yiin and others, 2004; McGee, 2003; 

Offenberg and others, 2003; Chatfield and Kominsky, 2002; Lioy and others, 2002, 

Millette and others, 2002; Clark and others, 2001). 

Concerns remain about the possible presence of WTC dust in indoor and outdoor 

environments in Lower Manhattan and surrounding areas. Identification of 

contamination from WTC dust more than 3 years after the event is complicated by 

dilution and possible variations in relative abundance of dust components arising from 

factors such as exposure to moisture, distance from the WTC site, and elevation. 

Detection of contaminants of potential concern (COPC) including asbestos, crystalline 

silica, lead, and man-made vitreous fibers (MMVF) originating from WTC dust is 

complicated by possible contributions of these materials from a variety of other unrelated 

sources including construction materials, asbestos-containing insulation, and lead-based 

paint. 

Identification of WTC contamination would be easier if unique components or ratios of 

components were present in WTC dust that could be identified by routine analytical 

techniques. This preliminary report seeks to quantitatively define the fine-particle 

fraction of WTC dust for the purpose of identifying a diagnostic signature. The signature 

could then be used by those working on this and other aspects of the WTC dust 

contamination issue in order to identify low levels of residual WTC dust. The analytical 

procedures in this report are not intended to be used as routine methods for analysis of 

samples; they were developed to obtain an accurate quantitative determination of the 

relative abundances of components in bulk WTC dust. It is anticipated that a final report 

will follow after background samples are collected and analyzed for possible WTC dust 

signature components. 

Sample Preparation 

Samples were collected from outdoor and indoor locations at various distances from the 

WTC site. Samples USGS 4, 6, and 12 were collected from ground level between 

September 16 – 17, 2001, at distances of 0.80, 0.60, and 0.55 km, respectively, from the 
center of the WTC site. These samples were wetted by one rain storm prior to collection. 

Sample USGS 36, collected on September 12, 2001, was obtained inside an apartment on 

the 30th floor of a building 2 blocks (0.40 km) from the WTC site. Details of sample 

collection procedures and locations for the above samples are given in Clark and others 

(2001) and Swayze and others (2005). Sample LM2 is an outdoor sample collected on 

September 16 – 17, 2001, approximately 0.70 km east of the WTC site. Sample L18-2 

was collected indoors on November 19, 2001, from an area adjacent to the WTC site 

(0.25 km west). For further details on samples LM2 and L18-2 see Lioy and others 

(2002). 

Sample preparation methods follow a modified version of the approach outlined in Bern 

and others (2005). Representative aliquots of WTC bulk dust samples were dry sieved 

through a 150 μm (100 mesh) ultrasonic sieve. A 0.2 g aliquot of the sieved sample was 

suspended in 125 mL of isopropanol alcohol. Using an Eppendorf pipette with 1 mm 

diameter tip opening, six 20 μL drops of the suspension were added to a Millipore filter 

apparatus with several milliters of alcohol in the funnel and a carbon coated 25 mm 

cellulose filter having a 0.4 μm pore size. The amount of sample was adjusted to yield 

coverage of approximately 2 – 4 percent on the filter. Coverage greater than about 10 

percent causes particles to overlap, which can cause analytical errors. The filter was 

placed on a scanning electron microscope (SEM) stub using carbon adhesive tab. After 

drying, the stub was carbon coated using a carbon evaporator prior to analysis in the 

SEM. 

The above method worked well for all components except MMVF. For some sample 

preparations, the abundance of MMVF particles was significantly reduced on the 

analytical filters relative to their abundance in bulk dust as observed by optical 

microscopy. The reason for fiber loss is unknown but may result from charging of the 

glass fibers by the electron beam during analysis in the SEM. To more accurately 

determine MMVF abundances, new aliquots of the samples were prepared by pipetting 

the suspension directly onto conductive carbon tape and analyzed in the SEM as 

described below. 

Analytical Methods 

Analyses were performed using a JEOL 5800LV electron microscope equipped with an 

Oxford ISIS energy dispersive x-ray spectrometer (EDS) and analysis system. Typical 

analytical conditions were 15 KeV accelerating voltage, 0.5-5 nA beam current, and zerodegree tilt. Data were processed using standardless quantitative analysis and compared to 

values obtained for BIR-1G glass (Meeker and others, 1998). Precision and accuracy 

vary for each element. Elemental values depend on particle size and shape but generally 

did not exceed 10 percent relative error for accuracy and 5 percent relative error for 

precision for Si, Al, Mg, Ca, and Fe, and 20 percent relative error for accuracy and 

10 percent relative error for precision for Na, K, Ti, and Mn (Bern and others, 2005; 

Meeker and others, 2003). 
For the filter samples, area-percentage coverage of total sample was determined using 

binary representations of backscattered electron images. Area fraction of individual 

particles was determined by direct measurement using digital images. The chemistry of 

each particle equal to or larger than 3 μm in the 500X magnification field of view (FOV) 

was determined and binned according to particle type. This process was also performed 

at 2,000 times magnification for all particles less than 3 μm. Twenty randomly selected 

fields of view at each magnification were analyzed for each sample. The number of 

particles counted on each sample ranged from 900 to over 3,000 depending on the density 

of filter coverage. The results for each magnification were normalized to equal area and 

combined to quantify particle abundances. 

Particles were binned by type based on extensive analysis of WTC dust by multiple 

analytical techniques (Meeker and others, 2005, and references therein). Typical particle 

fields are shown in Figure 1. Particle types used in this study are listed in Table 1. 

The area percentage of MMVF was determined separately for each sample by analyzing 

one field at 100 times magnification on a separate aliquot of sample prepared as 

described above. In addition, five fields on two samples (WTC 4 and WTC 6) were 

analyzed for all particles. These results were compared to the results obtained by the 

filter method. Particle size (length x width) distributions for the major components in 

each sample (< 150 μm size fraction) were also determined. 

Results 

Component analysis for the six WTC bulk samples is summarized in Table 1 and Figures 

2 - 7. All of the samples show three primary components – gypsum, phases compatible 

with concrete, and MMVF. The additional particle types shown in Table 1 were found in 

most samples. The data demonstrate that the most consistent particle-type abundance 

ratios occur within the MMVF, i.e., slag wool, rock wool, and soda-lime glass. In all 

samples, slag wool is the dominant MMVF component while rock wool and soda-lime 

glass fibers occur in all samples at similar relative abundances below approximately 10 to 

less than 1 percent total MMVF (Table 1). One exception to this observation was 

identified in a single field counted at 100 times magnification on sample L18-2. In this 

field, a single large soda-lime glass fiber and a single large rock wool fiber were found; 

these two fibers significantly affected MMVF relative particle abundances. If these two 

fibers are not included, the relative MMVF abundances for this sample are similar to 

those for the other samples. A second field on this sample was counted at 100 times 

magnification; the resulting data were consistent with the other samples (Table 2). In all 

samples, the relative abundances of rock wool and soda-lime glass fibers are based on a 

small number of fibers; thus, the statistical significance of reported proportions of these 

fiber types is correspondingly low. 
2 

Table 1. Range in area percent of major and minor components for all samples. 

Particle Type Comment Percent Range, 

Outdoor 

Percent Range, 

Indoor 

Gypsum Includes all Ca sulfate particles 26.3 – 53.3 63.3 – 63.7 

Concrete All phases compatible with 

hydrated cement 

19.3 – 30.8 14.0 – 21.0 

MMVF* Total 20.3 – 40.6 9.5 – 19.2 

Slag wool Based on table 2, field 2 91.7 – 98.1 89.5 – 93.3 

Rock wool Based on table 2, field 2 0 – 6.6 5.2 – 5.8 

 Soda-lime glass Based on table 2, field 2 0 – 6.0 0.9 – 5.3 

Chrysotile Bundles and single fibers 0.4 – 1.8 0 – 0.1 

Silica Primarily crystalline 0.8 – 3.4 0.4 – 0.7 

Ti-rich Primarily Ti and Ti oxide 0 – 0.1 0 – 0.6 

Zn-rich Primarily Zn and Zn oxide 0.2 – 0.4 0.1 – 0.6 

Pb-rich Primarily Pb and Pb oxide N.D. 0 – 0.03 

Fe-rich Primarily Fe and Fe oxide 0.2 – 1.3 0.1 – 1.1 

Other Identified but not binned 2.6 – 5.9 1.4 – 2.6 

Unidentified Could not be classified based on 

bulk chemistry 

0.2 – 1.4 0 – 0.1 

*Man-made vitreous fibers (MMVF) 

All samples also contain gypsum and concrete phases. In the outdoor samples, these 

components, along with total MMVF, vary in relative abundance. This variation is likely 

related to samples having been exposed to moisture and precipitation, which caused 

varying amounts of gypsum dissolution prior to sample collection. The two indoor 

samples, unaffected by precipitation, have much less variable compositions. 

By far, the most abundant nonfibrous particles in all samples are gypsum and concrete. 

Particle size distributions for these components (Figs. 8 and 9) suggest relationships to 

distance and elevation. Percent frequency is compared to area and maximum diameter, 

as measured on the SEM. The majority of these nonfibrous particles in each sample have 

similar particle area distributions with the majority of particles in the range from 0.3 to 3 

μm . Sample L18-2, collected adjacent to the WTC site, is characterized by a somewhat 

higher concentration of particles in the 3 to 300 μm2

 size range. Particles in samples 

USGS 4 and 6 fall at slightly higher values of total area, between 1 and 300 μm2, than in 

the other outdoor samples. The effect of particle-size distribution as a function of distance 

is most clearly seen in Figure 9 where samples L18-2 and USGS 36 clearly deviate from 

the other samples with respect to size distribution. Sample L18-2, the closest sample to 

the WTC site, shows a higher abundance of larger diameter particles. Sample USGS 36, 

collected on the 30th floor of a building, shows a higher abundance of smaller diameter 

particles. MMVF diameters for all samples combined are given in Table 3. The 

distributions of MMVF diameters display no clear relationship to distance from the WTC 

site. 
Table 2. Results from sample L18-2 

Component Comment Area Percentage 

Slag wool Field 1, all fibers counted 52.1 

Rock wool Field 1, all fibers counted 9.6 

Soda-lime glass Field 1, all fibers counted 38.3 

Slag wool Field 1, two large fibers removed 86.5 

Rock wool Field 1, two large fibers removed 2.9 

Soda-lime glass Field 1, two large fibers removed 10.7 

Slag wool Field 2, all fibers counted 89.5 

Rock wool Field 2, all fibers counted 5.2 

Soda-lime glass Field 2, all fibers counted 5.3 

Table 3. Diameter data for man-made vitreous fibers 

 All Samples Combined 

Rock wool Slag wool Soda-lime glass 

Minimum 0.2 0.1 0.1 

Maximum 15.6 21.0 13.0 

Average 3.8 4.7 4.0 

Conclusions 

Six bulk WTC dust samples, collected from locations in different directions, elevations, 

and from outdoor and indoor environments show relatively consistent abundance ratios of 

major and minor components. For the purposes of identification of WTC dust, these 

abundance ratios appear to be within the necessary limits of variability. Furthermore, the 

critical dust components can be identified easily and quickly using routine SEM and xray microanalysis techniques. 

Data presented here suggest that the presence and relative abundance of the three MMVF 

components – slag wool, rock wool, and soda-lime glass – along with the presence of 

concrete particles and gypsum could be used as a primary diagnostic signature for WTC 

dust. Secondary signature components could include FeOx, ZnOx, silica, and chrysotile. 
An analysis strategy for routine samples could evolve using rapid scans of settled dust by 

SEM to look for the presence of MMVF. If found, these fibers could then be analyzed 

using EDS to determine fiber compositions. If the majority of fibers (> 85 percent) 

detected were of slag wool composition, or if slag wool was found at a predetermined 

critical concentration, the sample would then be searched for gypsum and concrete 

particles along with the other two MMVF components. Further confirmation of the 

presence of WTC dust could then be reached by looking for secondary components in the 

approximate abundances found in this study. Alternatively, if slag wool, gypsum, and 

concrete were present, the sample could then be analyzed for contaminants of potential 

concern such as asbestos, lead, and potentially problematic organic compounds. 

Because the dust component ratios are shown here to be relatively constant from sample 

to sample, it should be possible for health workers to establish conservative health-based 

criteria for COPC relative to the abundance of slag wool. If slag wool fibers are not 

found in settled dust samples above a predetermined critical level, it is unlikely that 

COPC derived from the WTC could be present at significant levels in the samples. 

A successful application of these data to the WTC dust contamination problem in specific 

environments will depend on the degree to which WTC dust components are found in 

typical background samples. For example, MMVF are a major component of some 

acoustical ceiling tiles. Indoor environments with these tiles would be more likely to 

contain MMVF in settled dust than environments with other ceiling materials. Recently 

remodeled buildings are more likely to contain settled dust with gypsum and even 

concrete. In difficult cases, the size distribution data presented here might prove useful in 

distinguishing WTC source materials from similar materials from other sources. Dilution 

effects and any variations that might occur at greater distances from the WTC site must 

also be considered. 

Acknowledgements 

The authors wish to thank Dr. Paul Lioy for providing two of the samples used in this 

study. This report has been greatly improved by reviews and discussions with Ed du 

Bray and Todd Hoefen. This work was funded by the U.S. Geological Survey. 

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gypsum slag wool 

Figure 1. Secondary electron image of a typical field at 500 times magnification (left). The same 

image shown in binary backscatter mode is on the right. EDS spectra are shown for gypsum and 

slag wool. 
0% 

ile 

Fe 

Pb 

USGS 4 

Area percentage 

32.6% 

0.4% 

40.6% 

19.3% 

1.1% 

0.8% 

0.2% 

3.6% 

1.4% 

Gypsum Chrysot MMVF Unidentified 

Concrete Phases Quartz Ti+Zn 

Other 

Figure 2. Relative abundances of dust components for outdoor sample USGS 4, collected 0.80 

km south of the World Trade Center site. Components are shown in clockwise order as listed 

below each pie chart. Percentage of each component is given. 
0% 

Fe 

Pb 

USGS 6 

Area percentage 

53.3% 

0.7% 

20.3% 

20.3% 

1.0% 

1.3% 

0.3% 

2.6% 

0.2% 

Gypsum Chrysotile MMVF Unidentified 

Concrete Phases Quartz Ti+Zn 

Other 

Figure 3. Relative abundances of dust components for outdoor sample USGS 6, collected 0.60 

km south of the World Trade Center site. Components are shown in clockwise order as listed 

below each pie chart. Percentage of each component is given. 
0% 

Fe 

Pb 

USGS 12 

Area percentage 

26.3% 

1.8% 

32.4% 

30.8% 

3.4% 

0.9% 

0.3% 

3.8% 

0.2% 

Gypsum Chrysotile MMVF Unidentified 

Concrete Phases Quartz Ti+Zn 

Other 

Figure 4. Relative abundances of dust components for outdoor sample USGS 12, collected 0.55 

km south of the World Trade Center site. Components are shown in clockwise order as listed 

below each pie chart. Percentage of each component is given. 
0% 

i

Fe 

Pb 

LM-2 

Area percentage 

48.8% 

0.7% 

20.8% 

21.8% 

0.8% 

0.2% 

0.4% 

5.9% 

0.6% 

Gypsum Chrysotile MMVF Unident fied 

Concrete Phases Quartz Ti+Zn 

Other 

Figure 5. Relative abundances of dust components for outdoor sample LM-2, collected 0.70 km 

south of the World Trade Center site. Components are shown in clockwise order as listed below 

each pie chart. Percentage of each component is given. 
ile i

Fe 

Pb 

USGS 36 

Area percentage 

63.3% 

0.1% 

9.5% 

21.0% 

0.4% 

1.1% 

1.3% 

2.6% 

0.8% 

<0.1% 

Gypsum Chrysot MMVF Unidentif ed 

Concrete Phases Quartz Ti+Zn 

Other 

Figure 6. Relative abundances of dust components for indoor sample USGS 36, collected from a 

30th floor apartment, 0.40 km south of the World Trade Center site. Components are shown in 

clockwise order as listed below each pie chart. Percentage of each component is given. 
Fe 

Pb 

L18-2 

Area percentage 

63.7% 

0.0% 

19.2% 

14.0% 

0.7% 

0.1% 

0.1% 

1.4% 

0.000% 

0.9% 

Gypsum Chrysotile MMVF Unidentified 

Concrete Phases Quartz Ti+Zn 

Other 

Figure 7. Relative abundances of dust components for indoor sample L18-2, collected on the 

west side of the World Trade Center site. Components are shown in clockwise order as listed 

below each pie chart. Percentage of each component is given. 
0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

(

Percent Frequency 

All Phases 

0.1 0.3 1.0 3.0 10.0 30.0 100.0 300.0 

Area square micrometers) 

USGS-4, 0.80 km*, outdoor 

USGS-12, 0.55 km*, outdoor USGS-6, 0.60 km*, outdoor 

L-Market-2, 0.70 km*, outdoor 

L-18-2, 0.25 km*, indoor 

USGS-36, 0.40 km*, indoor 

Figure 8. Particle size data for each sample presented as area (length x average width) vs. 

percentage frequency. *All distances are approximate kilometers from the center of WTC plaza. 
A

0 

10 

20 

30 

40 

50 

60 

0.1 1.0 

Percent Frequency 

ll Phases 

0.3 3.0 10.0 30.0 100.0 300.0 

Length (micrometers) 

USGS-36, 0.40 km*, indoor 

L-Market-2, 0.70 km*, outdoor 

USGS-12, 0.55 km*, outdoor 

L-18-2, 0.25 km*, indoor 

USGS-4, 0.80 km*, outdoor 

USGS-6, 0.60 km*, outdoor 

Figure 9. Particle size data for each sample presented as length vs. percentage frequency. *All 

distances are approximate kilometers from the center of WTC plaza.