The purpose of this post is to provide a reference for the various studies of the WTC dust, with specific attention to anything relevant to supposed evidence of thermite or explosives (like iron microspheres, and flakes of red paint). It's not light reading. I might summarize at some point. 2001 Oct 12 - Chatfield, E. J. and Kominsky, J. R., 2001 Characterization of Particulate Found in Apartments after Destruction of the World Trade Center https://web.archive.org/web/20030811121623/http://eqm.com:80/news/wtc-report.pdf No discussion of iron or spheres. Lists iron elemental level at 0.86%, 0.41% and 1.2%. Samples taken 18 Dec 2001. 2001 Nov 27 - Roger N. Clark, Robert O. Green, Gregg A. Swayze, Greg Meeker, et al (USGS) Environmental Studies of the World Trade Center area after the September 11, 2001 attack. https://pubs.usgs.gov/of/2001/ofr-01-0429/ Somewhat higher iron percentages. 1.41 to 4.13 2002 - Paul J Lioy, Clifford Weisel, James R Millette, et al 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 110(7):703-14 DOI:10.1289/ehp.02110703 Very brief mention of iron: "In addition to the elements quantified by ICP/MS analyses, the SEM dispersive X-ray analyses showed large signals for iron and calcium, which are major components of construction materials. Similar observations were found for silicon, which is consistent with the glass fragments and fiberglass found in each sample." 2002 - Millette, J. R., Boltin, R., Few, P., & Turner, W. (2003). Microscopical Studies of World Trade Center Disaster Dust Particles. Microscopy Today, 11(5), 32–37. doi:10.1017/s1551929500053220 https://www.researchgate.net/public...of_World_Trade_Center_Disaster_Dust_Particles (Originally published MICROSCOPE, Vol 50:1, 29-35 (2002) Does not mention iron, but does mention "Metal Flakes" and "Paint Particles" 2003 April, EPA World Trade Center Background Study Report https://archive.epa.gov/wtc/web/pdf/background_study_report.pdf Focus on toxic substances. Does not mention iron or spheres. 2003 June, EPA Chemical Analysis of World Trade Center Fine Particulate Matter for Use in Toxicologic Assessment https://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.files/fileID/13030 Values of iron are 0.36% to 1.51%. Much lower than the control samples which had 3% 2003 - R.J. Lee investigation of 130 Liberty Street (Samples after May 2002, published Dec 2003) Various reports. Most famously for "Composition and Morphology", which has a figure of 5.87% next to "Fe Sphere" under "Mean of Composition" in a table described as "Statistical P-values for the comparison of TP-01 dust and dust in Background Buildings." (which seems wrong, as these are not p-values). The values in Lee are hard to interpret. What do they actually mean? Why is the combined Class A particle types 35.58? Other studies say mineral wool alone is over 50% of the composition. 2004-2005 - The EPA Expert technical review Panel. https://web.archive.org/web/20080306030000/http://www.epa.gov/wtc/panel/meeting-20040331.html Multiple meetings, with a variety of presentations and discussion on developing a signature for WTC dust. They did reference the Lee study a few times, but not in any great detail. I could find no mention of iron spheres, but I've been unable to do a comprehensive review, as there's a few hours of audio, and many pages of reports. This is from the final report, and is the only substantive mention I could find: https://web.archive.org/web/2008030...tc/panel/pdfs/Final_Report_August_17_2005.pdf 2005 - Heather A. Lowers and Gregory P. Meeker Particle Atlas of World Trade Center Dust, USGS "The primary metal and metal-oxide phases in WTC dust are Fe-rich and Zn-rich particles (Meeker and others, 2005b). " Discusses glass spheres, but not iron spheres. Contains four spectra of iron particles, two of which are spheres. All have lots of oxygen. https://pubs.usgs.gov/of/2005/1165/508OF05-1165.html#heading08 2005 - Gregory P. Meeker, Amy M. Bern*, Heather A. Lowers, and Isabelle K. Brownfield Determination of a Diagnostic Signature for World Trade Center Dust using Scanning Electron Microscopy Point Counting Techniques Does not mention spheres. Gives similar elemental compsition of Iron-rich to before "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. " 2005 Peer Review of EPA report Which does not really seem to tally with: 2005 Nov 3, Meeker, Lioy, Lippman Comments on WTC Signature Study and Peer Review http://www1.ae911truth.org/documents/EPA_dust_SubGroupComments_110305_iron_spheres.pdf A very brief passing mention of "iron spheres" as a possible signature component. Probably based on the Lee report. Note it's lumped in with "trace elements", so the presence there does not indicate a lot of them.