Contaminated City Soils Put Both Children and Birds at Risk, Study Finds

The ground beneath our cities carries a hidden record of industrial ambition, and in San Luis Potosí, Mexico, that record is turning out to be hazardous reading. A new study published in the journal Environmental Geochemistry and Health has mapped potentially toxic elements across the soils of this rapidly urbanizing Mexican metropolitan area and calculated, with unusual rigor, the health risks those elements pose to two very different sets of residents: young children and urban birds. Using a technique known as Integrated Probabilistic Environmental Risk Assessment, or IPERA, the research team led by Donaji J. González-Mille and colleagues at the Universidad Autónoma de San Luis Potosí combined rapid chemical screening, spatial mapping, and Monte Carlo-style probabilistic modeling to move beyond the familiar single-number risk estimates that have dominated urban soil studies. Their conclusion is stark: soils across the metropolitan area are contaminated, and in every zone examined, the accumulated hazard index for birds exceeded the critical value of one, signaling likely non-carcinogenic health effects in ecological receptors, while children in the most polluted districts face hazard quotients above safety thresholds as well.

The methodological backbone of the study is Energy Dispersive X-ray Fluorescence Spectroscopy, commonly abbreviated ED-XRF. This technology bombards soil samples with X-rays, causing the atoms within them to emit fluorescent radiation at wavelengths characteristic of each element. Because it requires minimal sample preparation and delivers rapid, multi-element results, ED-XRF has become an increasingly popular tool for environmental forensics, allowing researchers to analyze large numbers of samples and build detailed contamination maps rather than relying on sparse point measurements. In this study, the technique revealed that manganese, zinc, and lead occurred at the highest concentrations in the soils of the San Luis Potosí metropolitan area, with arsenic also present at levels of toxicological concern. Crucially, the team documented marked spatial variation: contamination was not uniformly smeared across the city but concentrated in identifiable hotspots, a pattern that reflects decades of industrial activity, mining legacies, traffic emissions, and the uneven geography of urban development.

The single most contaminated zone identified was the Morales-Aviación area, which registered a Metal Pollution Index of 116.68, by far the highest value in the study area. This finding is consistent with San Luis Potosí’s long and well-documented history as a mining and metallurgical center. Previous research from the same region has traced arsenic and lead exposure in children living near a copper smelter, documented lead-rich suspended particles in the city’s air, and characterized smelter slag hazards from nearby mining districts. Soil acts as a long-term sink for such pollutants: once metals are deposited, whether from smelter stacks, tailings dust, or vehicle exhaust, they can persist for generations, resuspending into the air with wind and traffic or transferring directly to anything that touches the ground. For a city whose population continues to expand into formerly industrial land, the spatial maps produced by this study are effectively a hazard atlas for future planning.

To translate contamination into risk, the researchers calculated Hazard Quotients for individual elements, comparing estimated doses against toxicological reference values derived from agencies such as the U.S. Environmental Protection Agency and Health Canada. The results revealed a striking asymmetry between the two receptors. For birds, the greatest single-element risk came from lead, with a Hazard Quotient of 9.35, a value more than nine times the threshold at which adverse effects are expected. For children, the dominant driver of risk was arsenic, at a Hazard Quotient of 1.33, modest by comparison but still above the value of one that defines the boundary of acceptable exposure. Lead, a potent neurotoxicant with no known safe blood level in humans, and arsenic, a carcinogen linked to skin lesions, cardiovascular disease, and cancers, are precisely the elements that public health authorities rank highest on substance priority lists, underscoring why even moderate exceedances warrant attention.

When the researchers summed individual Hazard Quotients into a cumulative Hazard Index, the picture became even more sobering. Hazard Index values for birds exceeded one in every zone of the metropolitan area, and across the board they were higher than the corresponding values estimated for children. This pattern reflects the biology and behavior of urban birds: they ingest soil and grit deliberately as part of their digestive process, forage intensively within small home ranges, and consume earthworms and insects that bioaccumulate metals from the soil. The study’s avian risk model draws on wildlife exposure factor handbooks and ecological soil screening levels, treating resident passerine birds as ecological receptors. The choice is deliberate and scientifically well-motivated, because birds, and house sparrows in particular, have repeatedly been shown to serve as sentinels of urban metal pollution. Recent work has linked soil lead gradients to elevated blood lead and reduced hemoglobin in urban sparrows, sublethal lead exposure to altered ecologically relevant behaviors, and lead contamination to reduced nesting success in songbirds.

What distinguishes this study from much of the prior literature is its probabilistic architecture rather than its analytical chemistry. Traditional deterministic risk assessments plug single, often worst-case, values into exposure equations, producing a single risk figure that conceals the underlying uncertainty. The integrated probabilistic approach used here instead treats key exposure parameters, such as soil ingestion rates, contaminant concentrations, and body weights, as distributions, and then runs repeated simulations to generate a probability distribution of risk. The result is a far more informative output: decision-makers can see not just whether the average child or bird exceeds a threshold, but what proportion of the exposed population is likely to do so, and with what confidence. Applied across spatially explicit zones, this allows environmental authorities to prioritize surveillance and remediation where risk distributions cluster most heavily, rather than committing limited resources uniformly across a whole city.

The findings arrive amid a growing international literature on the health consequences of urban metal exposure in children. Early-life exposure to mixtures of lead, arsenic, and manganese has been associated with deficits in intellectual functioning, kidney injury biomarkers, altered growth trajectories, and neurodevelopmental harm, with recent reviews emphasizing that co-exposure to multiple metals can produce additive or synergistic toxicity through shared neural signaling pathways. Because children play on the ground, exhibit frequent hand-to-mouth behavior, and absorb metals more efficiently than adults, soil contamination translates into disproportionate risk for the youngest members of an urban population. In San Luis Potosí specifically, earlier biomonitoring studies have documented arsenic and cadmium exposure in children near the smelter complex and urinary biomarkers of metal exposure in surrounding communities, giving the new soil maps a sobering historical resonance: the contamination documented today is the legacy of exposures that have been measured in children’s bodies for more than three decades.

The implications extend well beyond one Mexican city. Rapid urbanization across Latin America, Asia, and Africa is converting industrial and mining land into residential neighborhoods, often without systematic soil screening, meaning that millions of children and countless urban wildlife populations may be living on unrecognized contamination. The study’s authors argue that their spatially explicit IPERA framework offers a replicable template for such settings: combine rapid field-deployable XRF screening with geostatistical mapping, layer on probabilistic risk modeling for both human and ecological receptors, and use the resulting risk surfaces to target environmental surveillance and risk-reduction interventions. Such integrated approaches also echo a broader movement in environmental health toward assessing human and ecological risks within a single analytical framework, recognizing that children and birds in the same neighborhood are exposed to the same soil, dust, and resuspended particles.

For San Luis Potosí, the practical priorities suggested by the data are clear. The Morales-Aviación zone, with its pollution index of 116.68, stands out as the obvious first target for detailed site investigation, soil remediation, and exposure reduction measures, while the consistently elevated avian Hazard Indices argue for treating ecological protection as an integral part of urban environmental policy rather than an afterthought. The authors caution that their assessment rests on modeled exposure scenarios and established risk coefficients rather than direct measurements of metal burdens in children or birds, and datasets from the study are available from the corresponding authors on reasonable request for further validation work. Nevertheless, the convergence of independent lines of evidence, from historical biomonitoring to the new soil maps, points in one direction. The soils of this metropolitan area are contaminated, the risks to children and birds are quantifiable and geographically uneven, and the tools now exist to identify exactly where intervention will deliver the greatest health benefit for both the human and non-human residents of the modern industrial city.

Subject of Research: Probabilistic assessment of toxic metal contamination in urban soils and its health risks to children and birds

Article Title: Integrated probabilistic risk assessment of potentially toxic elements in urban soils: implications for children’s and birds’ health

Article References: González-Mille, D. J., Pérez-Vázquez, F. J., Chapa-Vargas, L., Costilla-Salazar, R., Torres-Dosal, A., Rayo-Reyes, P. P., Fernández-Macías, J. C., & Ilizaliturri-Hernández, C. A. (2026). Integrated probabilistic risk assessment of potentially toxic elements in urban soils: implications for children’s and birds’ health. Environmental Geochemistry and Health, 48(15), Article 588. https://doi.org/10.1007/s10653-026-03478-0

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03478-0

Keywords: urban soil contamination, potentially toxic elements, heavy metals, risk assessment, lead, arsenic, children's health, birds, house sparrow, ED-XRF, Monte Carlo simulation, San Luis Potosí