| 09 Apr 2026
Soil contamination and soil-mediated human health risks associated with household coal combustion in residential areas of Zavkhan Province, Mongolia
Abstract. Soil contamination by heavy metals represents a growing environmental and public health concern in cold–dry rural settlements where coal-based household heating remains dominant. This study investigates how coal combustion alters soil element dynamics and associated human health risks by applying a process-oriented, integrated soil system assessment in a residential area of Uliastai city, western Mongolia.
Surface soils (0–10 cm) from 38 sites were analyzed using ICP-OES and ICP-MS to determine major and trace element concentrations. Multivariate statistical analysis (principal component analysis, PCA) was combined with contamination indices (enrichment factor and geo-accumulation index) and human health risk assessment to explicitly link contamination sources, transport pathways, soil retention processes, and potential human exposure.
Results reveal a clear separation between anthropogenically influenced metals (As, Pb, Cd, Zn, and Cu) and elements predominantly controlled by geogenic background conditions (Cr, Co, and Ni). Very high to extreme enrichment and geo-accumulation levels for As, Pb, Zn, Cd, and Cu indicate substantial anthropogenic alteration of surface soil metal pools. Comparison of soil, coal, and ash compositions identifies coal combustion ash as the primary source of metal enrichment, acting as a concentrated reservoir that is redistributed to soils via atmospheric deposition and surface processes. Human health risk assessment shows that the most enriched metals, particularly As and Pb, dominate both non-carcinogenic and carcinogenic risks, with inhalation and ingestion pathways contributing most strongly to potential exposure.
The findings demonstrate that soil contamination in Uliastai reflects systemic changes in soil functioning driven by household energy practices rather than isolated concentration exceedances. By integrating source identification, contamination intensity, and health risk within a unified soil system framework, this study provides mechanistic insight into soil–human interactions and offers a transferable approach for assessing soil impacts in coal-dependent rural environments.
Thank you for the opportunity to review this paper. As my expertise is primarily in human health risk assessment, my comments will primarily focus on those aspects of the paper. However, as a general comment, I feel the paper would be much improved by the inclusion of analytical results for representative samples of coal ash and other potential contaminating material from Uliastai. The paper infers that metals observed at higher relative levels were due to anthropogenic sources - this conclusion would be reinforced if those sources had also been analysed.
The human health risk assessment component of the manuscript has several serious issues and I consider that the authors should consider redrafting the manuscript without this section and focussing solely on the soil contamination issues. Some (but not all) of the issues with the risk assessment are:
- The authors have used outdated versions of several key references. Specifically, the 1997 version of the USEPA exposure factors handbook has been used, rather than the 2011 edition, and the 2007 USEPA draft risk assessment of coal combustion wastes has been used, rather than the final risk assessment published in 2014.
- The authors elected to determine exposure through three exposure routes: ingestion, dermal and inhalation. The equations for these exposure routes are presented in equations (3), (4) and (5). Equation (4) dermal exposure contains a term FE, which the authors describe as the dermal exposure ratio, with a value of 0.61. However, in the USEPA equation this term is PC, the chemical-specific dermal permeability constant, which for metals has a value of 0.001. This different will obviously have a large impact on estimates of dermal exposure.
- Equation (5) for inhalation exposure, contains a term PEF, standing for particle emission factor. This factor does not appear in the USEPA RAGS part F guidance on inhalation exposure and it is not further explained by the authors.
- The authors list reference doses in Table 3, mainly references to USEPA 2007. As noted previously, USEPA 2007 has been replaced by USEPA 2014. Neither reference contains dermal reference doses and it uncertain where those in Table 3 have been derived from. The inhalation reference doses are actually reference concentrations (RFCs) and consequently have different units to those identified in the table. Finally, some of the values presented in Table 3 differ from those in the source reference.
- Equations (6) and (7) show the derivation of HIs and HQs. HIs are conventional used to combine estimates of risk across different exposure routes. Table 5 (line 330) states that the contents of the table are HIs but provides separate estimates for each exposure route, presumably HQs.
- In comparing Tables 5 and 6, ingestion is shown as the major route of exposure for non-cancer risks and inhalation as the major route for cancer risk. This is not possible, as the same exposure estimates should be used for both cancer and non-cancer risk assessments.
- Lines 350-351. The authors determine cancer risk separately for children and adults. This approach is incorrect, as the cancer potency factors refer to exposure over a lifetime. A lifestage weighted exposure could have been used or the adult exposure, as a satisfactory surrogate for lifetime exposure.
- The authors make no mention of the exposure of the population to metals through other sources of exposure. In most cases, dietary exposure will be the major source of exposure, with drinking-water also contributing in some cases. Without an indication of the magnitude of exposure from these sources, the risk estimates from soil only will underestimate the human health risk. These sources of exposure are likely to also be impact by the contamination sources mentioned in the paper.