The research, published in Environmental Geochemistry and Health by a team from Fire University, the Central Laboratory for Radiological Protection and Warsaw University of Technology in Poland, addressed a long-standing blind spot in the assessment of naturally occurring radioactive materials, known as NORM. Most previous studies have treated coal and its combustion residues as independent sample sets, comparing average ash concentrations from power plants with average coal concentrations from mines. That approach obscures the crucial question of whether combustion itself acts as a predictable amplifier of radiological risk. The Polish team instead burned nine hard coals and measured the radioactivity of each ash against its own parent coal, establishing a direct quantitative link between the two.

The nine coals were deliberately chosen for geological diversity. Samples came from the Bowen Basin in Queensland, Australia; the Central Appalachian coalfield in Virginia, United States; the Karaganda and Ekibastuz basins in Kazakhstan; the Moatize Basin in Mozambique; the Cerrejón Formation in Colombia; and two Polish sources, including the Carboniferous Upper Silesian Coal Basin. Together they span depositional ages from the Carboniferous to the Paleogene and represent markedly different tectonic and sedimentary settings. All were collected from shipments transhipped at the Port of Gdansk using a random representative sampling procedure compliant with the ISO 18283:2008 standard, and each was treated as an independent observation of a distinct geological origin.

Measurement relied on gamma-ray spectrometry using a MAZAR analyser coupled to a sodium iodide scintillation probe housed in lead shielding. The activity concentrations of radium-226 and thorium-232 were determined indirectly through their secular equilibrium daughter products, bismuth-214 at 1764 kiloelectronvolts and thallium-208 at 2610 kiloelectronvolts, while potassium-40 was measured directly from its 1460 kiloelectronvolt photopeak. Each crushed and sieved sample was sealed in a 1.7-litre Marinelli vessel and stored for four weeks to allow radioactive equilibrium to be reached before being measured nine times over 18,000-second counting intervals. Combined relative uncertainties typically fell within 10 to 15 percent for radium and thorium and 10 to 12 percent for potassium, with minimum detectable activities of a few becquerels per kilogram.

The combustion stage took place in a Kolton UNIX 20 solid-fuel boiler, a domestic bottom-feed unit operating under natural draught in a fixed-bed grate configuration, with active-zone temperatures typically between 700 and 1050 degrees Celsius. Each coal was burned to completion, the boiler cooled, and the bottom ash carefully collected. The authors acknowledge that domestic grate firing differs from industrial pulverised-coal or fluidised-bed systems in temperature profile, residence time and ash fractionation, but the fundamental mechanism is the same: the organic fraction of coal is oxidised to gas, while mineral-bound radionuclides are retained and concentrated in the solid residue. Notably, fly ash separated in industrial plants often carries even higher radionuclide concentrations than bottom ash, meaning the enrichment factors reported here may be conservative.

In their raw state, all nine coals proved radiologically unremarkable. Activity concentrations of radium-226, thorium-232 and potassium-40 sat below global average values for hard coal and for the Earth’s crust, with the lowest readings in the Colombian sample and the highest radium and thorium values in the Mozambican coal. The Polish Upper Silesian sample contained the most potassium-40. None of the coals exceeded any of the standard radiological screening indices: radium equivalent activity ranged from 19.3 to 57.7 becquerels per kilogram, far below the 370 becquerel per kilogram safety threshold, external hazard indices stayed well under unity, and gamma dose rates of 1.74 to 25.90 nanogray per hour remained below both the global average of 54 nanogray per hour and the Polish average of 47.4. As fuels, these coals posed negligible radiological concern.

Combustion changed the picture dramatically. Enrichment factors relating ash activity to coal activity ranged from approximately 3.8 to 9.9, clustering around a median of about six for every radionuclide and every derived hazard index. A paired Wilcoxon signed-rank test confirmed the increase was statistically robust, with a very large effect size of r = 0.889 and complete directional consistency: in all nine coal-ash pairs, every measured parameter rose. The mechanism is essentially a mass-balance effect. When the organic matter burns away, the ash yield drops to an estimated 10 to 27 percent of the original coal mass, so the same absolute quantity of radionuclides is packed into a much smaller mass. Earlier studies had reported enrichment factors of two to five; the tighter, higher and more uniform amplification observed here likely reflects the paired experimental design, which excludes the confounding effects of fuel blending and heterogeneous ash streams in large power stations.

The radiological consequences of this amplification were visible in every index calculated for the ash. Radium equivalent activity in the ashes ranged from 149 to 418 becquerels per kilogram, external hazard indices from 0.46 to 1.13, gamma dose rates from 68 to 185 nanogray per hour, annual effective doses from 0.018 to 0.226 millisieverts per year, and the radioactivity concentration index from 0.52 to 1.46. The most radioactive ash, derived from the Mozambican coal, exceeded regulatory thresholds across multiple parameters. Critically, the external gamma dose rate surpassed the recommended reference level of 54 nanogray per hour in all nine ash samples, even though most other indices remained within limits. The authors stress that the study assessed the potential for environmental impact based on the elevated activity of the ash itself, not on direct observation of radionuclide migration into soil, water or air.

Why does this matter beyond the laboratory? Coal ash is one of the most heavily reused industrial by-products on Earth, finding its way into concrete, road base layers, land reclamation schemes and even, occasionally, agricultural applications mixed with sewage sludge. Each of these uses creates a potential pathway for concentrated NORM to disperse into soil, groundwater and the built environment. The difference between a two- or three-fold enrichment and a six-fold one is far from trivial when millions of tonnes of residue are involved, and the authors argue that risk assessments based on average literature ash values may systematically underestimate exposure if coal-specific transformation factors are ignored. Under the Euratom Basic Safety Standards, which set activity concentration reference levels for building materials, some combustion residues may fall within regulatory scope when used in construction.

The study’s conclusions come with honest caveats. Nine coal-ash pairs, while sufficient for the non-parametric statistical framework applied, cannot capture the full global variability of coal composition, and a single prepared sample per pair means the reported uncertainties reflect counting statistics rather than within-material heterogeneity. The domestic boiler used cannot reproduce industrial combustion conditions exactly. Yet the central message stands with unusual statistical clarity: combustion acts as a proportional and predictable amplifier of natural radioactivity, so the radiological properties of an ash can be inferred from its parent coal. As Poland and other nations continue importing coals from geologically diverse basins while promoting circular-economy reuse of combustion residues, the authors call for continuous monitoring of both domestic and imported coal, and for transformation processes to be built into environmental risk assessment frameworks. A material that enters the furnace radiologically harmless may not leave it that way.

Subject of Research: Radionuclide enrichment and radiological hazard transformation during coal combustion

Article Title: Radiological characteristics of coal-to-ash transformation: a paired coal-ash assessment of natural radionuclides

Article References: Łukaszek-Chmielewska, A., Rachwał, M., Rakowska, J., Piotrowska, B., Isajenko, K., & Szyłak-Szydłowski, M. (2026). Radiological characteristics of coal-to-ash transformation: a paired coal-ash assessment of natural radionuclides. Environmental Geochemistry and Health, 48(15), Article 612. https://doi.org/10.1007/s10653-026-03509-w

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03509-w

Keywords: coal ash, NORM, radionuclides, gamma-ray spectrometry, radiological hazard, TENORM, bottom ash, coal combustion, environmental radioactivity, radium-226, thorium-232, potassium-40