Calculation and application of the selenium toxicity coefficient for potential ecological risk assessment

A peer-reviewed national-scale study determined a selenium toxicity coefficient of 2 using Håkanson’s risk-index principles and applied it to 1,232 surface-soil sampling sites across China. The analysis classified 69.9% of sites as low to moderate ecological risk, 24.4% as considerable risk and 5.7% as high or very high risk. The highest-risk locations were concentrated in localized areas associated with selenium-rich strata rather than being distributed uniformly nationwide.

The coefficient was calculated through a three-step process. First, relative abundance numbers for selenium and eight other metals were derived from their abundance in igneous rock, soil, fresh water, terrestrial plants and terrestrial animals. For each medium, the most abundant element was assigned 1 and the others proportional values. The highest value for each element was then excluded, and the remaining four values were summed to limit the influence of an extreme value.

Second, the average abundance number for each element was multiplied by a sink factor reflecting its relative sedimentation tendency. These calculations produced initial toxicity coefficients spanning from 57 to 353,728. Third, the researchers standardized the values relative to zinc, the element with the lowest initial coefficient, and took the square root of each ratio. This step produced a selenium coefficient of 1.9, which was rounded to 2. Comparison with published coefficients for the other eight elements showed small numerical differences but generally consistent relative ordering and multiplicative relationships.

For the national application, researchers systematically collected studies of soil selenium concentrations in China published from 1980 through 2024. Eligible studies sampled the surface layer at depths of 0 to 20 centimetres, used precise instrumental methods such as atomic fluorescence spectrometry or inductively coupled plasma mass spectrometry, and reported means and standard deviations. The compilation contained measurements from 1,232 sampling sites. Although digestion protocols differed among the included studies, the authors considered this heterogeneity inherent to the meta-analysis and stated that it did not compromise their primary objective of calculating the coefficient.

The assessment used 0.21 milligrams per kilogram as the national geochemical background concentration for selenium in Chinese surface soils. The single-element potential ecological risk factor incorporated measured concentration, background concentration and the toxicity coefficient. Risk categories were defined as low below 2, moderate from 2 to below 4, considerable from 4 to below 8, high from 8 to below 16 and very high at 16 or above. A separate geoaccumulation index, also based on the 0.21 milligrams-per-kilogram background, was used with a correction factor of 1.5 to compare contamination classes.

The compiled selenium concentrations ranged from 0.007 to 25.42 milligrams per kilogram, with a mean of approximately 0.35 milligrams per kilogram. Kriging interpolation showed generally higher concentrations in southeastern China and lower concentrations in northwestern China, although the map was used to visualize broad spatial patterns rather than calculate an area-weighted national estimate. Under the GB/T 44971–2024 soil selenium grading standard cited by the researchers, about 36.4% of the sampling sites were classified as selenium-rich or higher, while approximately 2.67% exceeded the 3 milligrams-per-kilogram toxicity threshold.

Elevated risk occurred mainly in Enshi in Hubei Province, Ziyang in Shaanxi Province, Kaiyang in Guizhou Province and Chao’an in Guangdong Province. The researchers associated these localized anomalies with selenium-enriched geological strata or hydrothermal influence. One Chao’an site had a potential ecological risk factor of 140.71, placing it in the very-high-risk category. They cautioned that this value describes potential risk from total soil selenium and does not demonstrate actual biological toxic effects; the study cited limited selenium uptake by tea plants and no reported selenosis incidents at that location.

The potential ecological risk factor and geoaccumulation index showed a quadratic weighted Kappa coefficient of 0.729, indicating substantial agreement between their ordered classifications. An exponential regression was also significant at p less than 0.001, but the authors cautioned that this correlation partly arose because both indices use measured soil selenium concentration and therefore did not independently validate the coefficient. They further noted that, for a single-element risk classification, the selenium coefficient mathematically cancels out and does not change the category assigned to an individual site.

Accordingly, the scientific value of the coefficient of 2 lies in providing selenium’s relative toxicity weight for multi-element calculations and establishing a consistent parameter for screening comparisons. The study was a literature-based ecological risk assessment rather than a direct measurement of selenium toxicity, plant uptake or organism-level effects. It did not account for selenium speciation, pH, redox conditions, organic matter or bioavailability, and all percentages refer to the 1,232 sampling sites rather than China’s total land area. The authors describe the result as a preliminary screening benchmark for prioritizing site-specific investigation, not as a definitive basis for land management or environmental standard revision.

Subject of Research: National screening assessment of potential ecological risk from selenium in Chinese surface soils

Article Title: Calculation and application of the selenium toxicity coefficient for potential ecological risk assessment

Article References: Wen, X., & Huang, Y. (2026). Calculation and application of the selenium toxicity coefficient for potential ecological risk assessment. PLOS One, 21(10), e0359642. https://doi.org/10.1371/journal.pone.0359642

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0359642

Keywords: selenium, soil contamination, ecological risk, toxicity coefficient, Håkanson risk index, geoaccumulation index, China, environmental screening, Calculation, application, toxicity, coefficient