When Metals and Metalloids Become Organic: What Two Decades of Speciation Have Taught Us About Environmental Risk

Environmental risk assessment of metals and metalloids is still dominated by total-element concentrations, although microorganisms, plants, animals and abiotic reactions continuously transform elements into chemical species with very different mobility, bioavailability and toxicity. Two decades ago, coordinated work on organometal(loid) compounds demonstrated a surprisingly broad chemical space of methylated, alkylated and volatile species in gases, waters, soils, sediments and biological systems. This review revisits the concepts that emerged from that work in light of advances in environmental microbiology, toxicology, speciation analysis and regulation. Several ideas have proven robust: Biomethylation can mobilise elements. The maximum formation of hazardous species need not coincide with the maximum total contamination, and the human body and microbiome can act as additional speciation reactors. Other ideas require revision. Most importantly, methylation is neither inherently detoxifying nor inherently toxifying. Depending on element, oxidation state, molecular structure and biological context it can promote volatilisation, bioaccumulation, reactive metabolites, elimination or sequestration. We argue that the central challenge is now to connect environmental occurrence to absorbed dose and target-tissue speciation. Arsenic provides the clearest example of species-resolved analytical chemistry influencing risk assessment; comparable frameworks remain underdeveloped for most other metal(loid)s. Environmental risk assessment should therefore consider chemical speciation where it materially changes exposure or toxicity while recognising that mixture effects, exposure duration and interindividual variability remain incompletely resolved.