Dust and Airborne Mineral Particles

The composition of airborne particulate matter varies predictably with land use, geology, and industrial activity. A residential street in a limestone city has a different dust from a district beside a steelworks, and both differ from a coastal dockyard. Understanding the dominant sources is the first step in interpreting particle assemblages from forensic samples.

• Crustal resuspension: quartz, feldspar, clay minerals, and calcite from local soil and unpaved roads; the mineral proportions mirror the underlying geology.
• Building weathering: calcite from limestone and mortar carbonation, gypsum from sulfation of lime surfaces, concrete dust from construction and demolition.
• Traffic: iron-rich spheroids from brake wear, tyre rubber particles (carbon + Zn + S), catalytic converter platinum-group element particles, diesel soot aggregates.
• Combustion: fly ash spheres from coal or biomass combustion; soot from incomplete combustion of hydrocarbons; the specific fly ash chemistry varies with fuel type.
• Industrial processes: chromium-rich particles near electroplating or cement works, lead particles near battery recycling or smelting, titanium dioxide from pigment manufacture, abrasive particles from metalworking.

Dust is collected from clothing by tape-lift or by scraping into a clean container. From vehicles, dashboard and floor vacuumings are standard. For airborne samples, filter membranes (polycarbonate or mixed ester) are exposed for a timed period and then carbon-coated for electron microscopy. Each particle of interest is imaged at high magnification, and the electron beam is focused on it to generate an EDX spectrum showing elemental composition.

Automated SEM-EDX systems (such as the QEMSCAN and similar platforms) can classify hundreds of particles per hour by comparing each EDX spectrum to a library of known compositions. This is valuable for building a statistical picture of an assemblage: the proportions of quartz, iron oxide, fly ash, metallic particles, and biological material. The resulting particle-count data is then compared between the questioned sample (from clothing or a vehicle) and reference samples from candidate locations.

Asbestos identification in forensic samples follows a regulated protocol because the health and legal consequences of misidentification are severe. The WHO method for ambient air uses phase contrast optical microscopy (PCOM) to count fibres meeting the dimensional criteria (length >5 µm, diameter <3 µm, aspect ratio >3:1), but PCOM cannot distinguish asbestos from manufactured mineral fibres by morphology alone. Electron microscopy with EDX is needed to confirm mineral identity.

In a forensic context, asbestos fibre evidence arises in at least three distinct situations. First, occupational health litigation or manslaughter prosecution: an employer is alleged to have exposed workers without adequate controls, and fibre concentrations in environmental samples and in lung tissue at autopsy are key evidence. Second, illegal dumping or building demolition without proper abatement: soil and dust samples from a site are analysed to establish what was present. Third, building-condition disputes: whether a refurbishment caused fibre release into occupied areas.

The forensic logic of dust as a location indicator runs as follows: if a person was present in a particular environment long enough for dust to settle on their clothing, and if that dust has a particle assemblage distinctive enough to distinguish that environment from all others the person might have visited, then the dust links the person to the location. The strength of that link depends on how distinctive the assemblage is and how well the analyst has sampled background variation.

Cases where dust has provided useful evidence include: a suspect denied being at a demolition site where a body was concealed, but chrysotile asbestos fibres and gypsum particles consistent with the demolished building were found on their jacket; a vehicle alleged never to have been driven near a power station was found to have fly ash particles in the cabin vacuumings with an elemental composition matching a specific coal-fired plant; and a shipping container claimed to have originated from one port had a dust assemblage consistent with a different port's local geology.

Industrial dust prosecutions combine forensic geology with occupational health science. A company is accused of exposing workers to hazardous mineral dust without adequate protection. Lung tissue from a deceased worker is analysed to quantify the retained fibre burden and identify the fibre types: the mineral identity determines which exposure the death is attributed to.

For environmental crime, the question is whether a site has been illegally contaminated with hazardous particulate material. Chromium-contaminated soils from unlicensed tipping, heavy-metal particle deposition from an unlicensed smelter, or dioxin-bearing fly ash from waste incineration all produce particle assemblages detectable by SEM-EDX and bulk chemical analysis. Establishing the geographic extent of contamination requires systematic grid sampling of soil and dust across the affected area, generating a spatial concentration map that can be entered as evidence in an environmental prosecution.

Since the 1980s, asbestos in buildings has been progressively replaced by manufactured mineral fibres (MMF): glass wool, stone wool (rock wool), and ceramic fibres. MMF products are used in roofing, cavity wall insulation, ductwork lining, and fire protection. They are morphologically similar to asbestos under the light microscope, which creates a serious misidentification risk if PCOM is used alone.

SEM-EDX resolves the ambiguity. Glass wool particles are dominated by Si and Ca with little or no Mg or Fe and have a smooth, glassy amorphous texture in the secondary electron image. Stone wool contains more Fe and Al but lacks the crystalline cleavage planes and characteristic EDX spectra of the amphibole asbestos types. Ceramic fibres are high-alumina (Al-Si) without the Na, Fe, or Mg that distinguish amphiboles. Correct identification is critical in litigation: if fibres recovered from a claimant's lung tissue are MMF rather than asbestos, the causation argument for mesothelioma is not supported.