Thank you, Almond. Can you give us some examples from your experince or from the literature, how abundant are such particles after some fires or in urban dust etc. (some rough numbers)?
Unfortunately, it's not such a simple matter as reporting a few percentages. The methods I use for analyzing dust, dirt, ash and debris focuses on individual analysis of particles in the SEM using X-ray EDS. I can estimate relative abundances within a statistical class (based on multivariate agglomerative nesting of the derived compositional data) on a particle basis. But if you hand the same sample to a wet chemist for processing, she'll come up with a completely different relative abundance (by weight) for iron.
We then run into the biggest problem of all: I'm aware of no studies which place even broad statistical estimates on those values. Baseline studies are useful for determining if you require some other process to explain an excess of iron microspheres. My typical baseline for an urban dust (a randomly sampled control taken from an unperturbed surface) is about 0.1% to 0.5%. This is easy enough to test, simply go to an urban center and collect some dust samples. Generally, researchers sample off of car windshields, storefront windows, door knobs and other flat surfaces upon which dust is likely to settle.
The baseline, however, doesn't account for any of the variables that could produce many more of said spheres. Two burning buildings may produce completely different ash clouds depending on the nature of the building materials and the fires themselves. Frankly, it's a wildly uncontrollable experiment.
If I were given a sample that contained 5% iron microspheres, I would look for another source of the iron microspheres beyond simple urban contamination. My typical abundance for iron rich microspheres in fly ash is anywhere from 0.3% to 4%, depending on the type of ash, the nature of the coal power plant, and whether or not it was magnetically separated out. In unprocessed ashes, I've found abundances from 0.1% to 10%, the latter coming from a waste industrial furnace.
That leads me to another point, when looking for another source, I would be completely satisfied if someone told me that two of the tallest skyscrapers in Manhattan were on fire for hours and then collapsed, spewing dust and ash all over downtown. "Yep," I would say, "That explains it." Pigments, inks, toner cartridges, colorants, dyes, paints, when they combust, they will leave behind the iron microspheres that were originally there, but the iron microspheres now make up a significant portion of the ash. We then have heat, abrasion, and the collapse mechanics, all of which are capable of liberating or producing more of these spheres. These spheres, by the way, are produced at temperatures far below the melting point of bulk iron.
