Rhenium in Wild Mushrooms: Environmental Occurrence and Censoring-Aware Quantification

Rhenium (Re) is a scarce technology-critical transition metal used in high-temperature superalloys, catalysts and other advanced materials, yet its occurrence in terrestrial biota and potential relevance to environmental exposure remains poorly characterised. This study investigated total Re in wild mushroom fruiting bodies collected from selected green spaces in Leicester and Leicestershire, United Kingdom, using inductively coupled plasma mass spectrometry (ICP-MS) and censoring-aware statistical analysis. The analytical dataset comprised 157 records. After exclusion of four unmatched tissue-only records, 153 analytical portions were reconstructed into 121 fruiting-body-level biological units, avoiding pseudoreplication from separately analysed cap and stem tissues. The analysis incorporated procedural digestion blanks, sample-specific dry-weight analytical limits and interval censoring. Under the primary analytical scenario, 73/121 units (60.3%) were non-detects, 21 (17.4%) were detected below the operational lower limit of quantification (LLOQ), seven (5.8%) were partially quantified and 20 (16.5%) were fully quantified. Overall, 48/121 units (39.7%) contained at least one Re signal above the operational detection threshold, whereas 27/121 (22.3%) contained at least one quantitatively valid component. Fully quantified concentrations ranged from 3.91 to 163.70 ng g^-1 dry weight. The complete dataset comprised 73 left-censored, 28 interval-censored and 20 exact observations. An interval-censored lognormal model estimated a population median of 1.055 ng g^-1 dry weight (95% bootstrap confidence interval: 0.672-1.571), with all 1000 bootstrap refits converging successfully. Classification was identical for all 59 units directly comparable under the primary and stricter same-batch procedural-background definitions. These findings extend the limited evidence for Re occurrence in wild fungal fruiting bodies and should be interpreted as environmental-occurrence data rather than as evidence of either adverse risk or safety. Because total elemental Re does not resolve chemical form, gastrointestinal bioaccessibility or internal dose, the measured concentrations do not support quantitative estimates of dietary exposure or human-health risk.