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Background Attack Aftermath Evidence Misinformation Analysis Memorial

World Trade Center Dust

Settled Dust Aerosol From the Pulverized Towers

North Tower dust cloud
Within one minute of the North Tower's collapse, the mammoth cloud of thick dust engulfed most of the southern end of Manhattan. See other photographs of the dust clouds.

When the South Tower and then the North Tower exploded they each produced vast clouds of dust that first so attenuated the sunlight on the streets below that witnesses described a pitch-black environment. They then settled, covering virtually every surface with fine, penetrating dust. The dust permeated the insides of buildings, and contaminated air conditioning systems, necessitating lengthy decontamination procedures.

The dust was present everywhere around Ground Zero, but settled in thicker accumulations in the downwind directions south and east of the complex. The dust settled to a depth of 3 inches in locations as far as six blocks from the World Trade Center. 1  

Apparently the dust contained a significant proportion of the towers' constituents, such as their concrete, glass, and gypsum. Photographs of Ground Zero show piles of shattered steel and aluminum cladding, but show virtually no signs of the tens of thousands of tons of concrete that constituted the 4-inch-thick floor slabs of each of the towers' 110 floors. This observation, combined with the documented extent of the dust, suggests that the vast majority of that concrete was pulverized into fine dust.


The Lioy et. al. Dust Study

A score of scientists conducted detailed analysis of selected dust samples in order to determine the exposure of nearby residents and workers to toxic contaminants. 2   The study performed inorganic and organic analysis on dust samples collected from three locations. The inorganic analysis identified radionuclides, ions, and asbestos; and the organic analysis identified numerous dangerous types of compounds, including: polycyclic aromatic hydrocarbons (PAHs); polychlorinated biphenyls, dibenzodioxins, and dibenzofurans; phthalate esters; and brominated diphenyl ethers. Unfortunately for those affected, the study's discovery of carcinogenic materials was not disclosed until a summary of the findings were published in EHP in November of 2001. By that time, the worst exposures had already occurred.

The study was based entirely on dust samples collected from just three locations:

  • A building on Cortlandt Street, one block east of the WTC.
  • An automobile on Cherry Street, about 0.7 km east of the WTC.
  • An automobile on Market Street, about 0.7 km east of the WTC.

All three locations were sheltered, rain on September 15 having washed away dust in exposed locations. The study used three principal methods to determine physical and chemical characteristics of the dust.

  • Microscopic examinations
  • Particle size fractionation using gravimetric sieving and aerodynamic separation
  • Inorganic and organic chemical analysis

Whereas the study was undertaken to assess the risks of the dust to human health, we examine the results with a different objective: to see what evidence they provide about how the buildings were destroyed. We asked the following questions:

  1. What fraction of the dust was concrete, and what was the average particle size?
  2. What levels of radioisotopes were measured and were those levels consistent with known sources in the buildings and crashed aircraft?
  3. What levels of sulfur were measured, given the evidence of steel corrosion through sulfidation documented in the limited metallurgical examination of the FEMA Report's Appendix C ?
  4. Were explosive residues detected?

The dust samples from the study, taken from sheltered areas, were light and fluffy. They were apparently not representative of the thick fallout that settled on the streets.

Unfortunately, despite the detail of the study, it fails to provide satisfying answers to these questions. The report does not quantify what proportion of the dust was pulverized concrete, though it does note that the samples were very alkaline. It notes that significant portions of the dust were glass fibers and cellulose fibers, and that the samples were fluffy. Thus, it appears that the samples were not representative of the dust that settled in open areas in the immediate aftermath of the attack. Being from sheltered areas, their samples were likely biased greatly in favor of lightweight fibrous debris, which would tend to stay aloft much longer than powdered concrete.

In regard to radioisotopes, the report is rather cryptic. It states:

We analyzed approximately 50 peaks based on statistical significance (counting/lack of interferences). These included thorium, uranium, actinium series, and primordial radionuclides

However, it fails to quantify how the levels of these radionuclides compare to background levels.

The report makes several mentions of sulfur, indicating that many glass and cement particles in a fibrous form contained sulfur. It also mentions that the three samples contained around 40,000 ng/g of sulfides. Interestingly, it states that X-ray analyses showed large signals for iron, but fails to quantify the proportion of iron, omitting it from the table giving concentrations of elements.

Finally, the scientists did not broach the issue of whether the dust showed evidence of explosive residues. Their report does not appear to have sufficient detail to use it as a basis for drawing any conclusions about the question of explosives. All their disclosures of the dust composition are partial, addressing questions about the levels of heavy metals and toxic hydrocarbons, but failing to provide even complete compositional analysis of elements.


The USGS Study

A study by the US Geological Survey (USGS) provides some analytical detail missing from the above study. It lists the elemental composition of a number of dust samples collected from outdoor and indoor locations. 3   The following table summarizes the percentages by weight of the more abundant elements in samples for which sulfur was measured.

sample element: percentage by weight
Si Ca Mg S Fe Al
WTC 01-02 21.20 15.01 3.11 1.33 4.13 4.13
WTC 01-03 26.30 9.58 2.23 0.87 2.16 2.75
WTC 01-14 15.30 17.65 2.83 4.32 1.87 2.86
WTC 01-15 13.60 18.58 2.64 5.40 1.87 2.59
WTC 01-16 17.00 13.36 1.79 3.68 1.92 2.27
WTC 01-21 12.80 18.94 2.68 5.10 1.49 2.73
WTC 01-22 17.00 16.80 2.77 3.70 2.78 2.78
WTC 01-25 13.20 20.37 3.29 4.03 1.33 3.28
WTC 01-27 15.20 19.51 3.04 4.29 1.72 3.05
WTC 01-28 13.80 19.65 2.83 4.56 1.80 2.95

Gypsum, the primary constituent of most wallboard, has the formula CaSO4-2(H20), and is 18.62% sulfur and 23.38% calcium by weight. 4   Gypsum is also about 5% of portland cement, the binder used in most concrete and constituting. Since aggregate constitutes between 70% and 80% of most concrete, gypsum might account for 1% to 1.5% of the concrete in the towers. 5  


The DELTA Group Study

A group of scientists at UC Davis, the DELTA Group (Detection and Evaluation of Long-range Transport of Aerosols), performed what they described as the "most thorough analysis yet" of Ground Zero dust and smoke in May of 2002. The study was based on the analysis of air samples from a rooftop air monitor about one mile north-northeast of Ground Zero. The group noted that very fine particles were found at levels not previously found in ambient air, and that the particles included metals such as vanadium. They also noted that particles of concrete and glass were coated with combustion products. 6  

They identified high levels of coarse particles, above, which included powdered concrete and glass with a coating of combustion products, in size range of 5 to 12 micrometers diameter. Very fine particles were found at levels not previously seen in ambient air samples.

Studies by Ryan, Jones, et. al.

By 2008, if not earlier, the most most detailed and comprehensive analysis of World Trade Center dust had been published by a group of independent researchers including physicist Steven P. Jones and chemist Kevin Ryan. Combining data from earlier published studies with results from experiments on dust samples they had obtained, these scientists found both the apparent products of aluminothermic pyrotechnics and unexploded fragments of such pyrotechnics. These researchers also examined published data on the emissions of gases from Gound Zero, and supplied straightforward hypotheses, consistent with the view that energetic materials were used to destroy the complex, for observations that were highly anomalous within the context of the government-sanctioned collapse theory.


1. The Horror, The Horror, planetary.org, 9/12/01 [cached]
2. Characterization of the Dust/Smoke Aerosol that Settled East of the World Trade Center (WTC) in Lower Manhattan after the Collapse of the WTC 11 September 2001, Environmental Health Perspectives (EHP), 7/2002 [cached]
3. Chemical Compositions of the WTC Dusts and Girder Coating Material, USGS.gov, 11/30/01 [cached]
4. Gypsum Mineral Data, WebMineral.com, [cached]
5. Scientific Principles: Production of Portland Cement, [cached]
6. Trade Center air laden with very fine particles, DELTA scientists find, calag.ucop.edu, [cached]

page last modified: 2013-03-21