Ultratrace Osmium in Wild Mushrooms: Environmental Occurrence and Chemical-Form-Dependent Molecular and Toxicological Interpretation.

Osmium (Os) is an exceptionally scarce platinum-group element whose occurrence in terrestrial biota remains poorly characterised. This study investigated ultratrace Os occurrence in wild mushroom fruiting bodies collected from urban and rural green spaces in Leicester and Leicestershire, UK, as part of a broader environmental biomonitoring programme. An Os-specific analytical audit reconstructed 157 original analytical records into 121 fruiting-body biological units and evaluated occurrence using study-specific procedural-background and low-end calibration criteria. Under the primary analytical interpretation, 28/121 fruiting bodies (23.1%; exact 95% CI: 16.0-31.7%) produced Os-associated responses distinguishable from the operational background criterion, whereas 93/121 were classified as non-detects. Although no fruiting body reached the study-specific operational LLOQ, the occurrence pattern was informative: 58/59 directly comparable classifications were concordant under a stricter same-batch procedural-blank scenario, while broader sensitivity analyses demonstrated that classification at these ultratrace levels remains dependent on analytical-background definition. The findings provide evidence that Os-associated signals occur in wild fungi at extremely low analytical levels and empirically demonstrate how analytical-background definition constrains interpretation at the boundary between occurrence and quantification. Quantitative environmental, exposure and toxicological characterisation will require dedicated Os-specific methods with lower validated quantitative limits, explicit recovery control and preservation of chemical-speciation information.