Asbestos shares with carbon nanotubes some morphological and physico-chemical features. An asbestos-like behaviour has been recently reported by some authors, though the mechanism of toxicity may be very different. To identify at the atomic level the source of toxicity in asbestos, the effect of progressive iron loading on a synthetic iron-free model nanofibre previously found non-toxic in cellular tests was studied. A set of five synthetic chrysotile nanofibres [(Mg,Fe)3(Si2O5)(OH)4] has been prepared with Fe ranging from 0 to 1.78 wt %. The relationship between fibre-induced free-radical generation and the physico-chemical characteristics of iron active sites was investigated with spin-trapping techniques on an aqueous suspension of the fibres and Mössbauer and EPR spectroscopies on the solids, respectively. The fully iron-free fibre was inert, whereas radical activity arose with even the smallest amount of iron. Surprisingly, such activity decreased upon increasing iron loading. Mössbauer and EPR revealed isolated iron ions in octahedral sites that undergo both axial and rhombic distortion and the occurrence of aggregated iron ions and/or extra-framework clustering. The isolated ions largely prevailed at the lowest loadings. Upon increasing the loading, the amount of isolated iron was reduced and the aggregation increased. A linear relationship between the formation of carbon-centred radicals and the amount of rhombic-distorted isolated iron sites was found. Even the smallest iron contamination imparts radical reactivity, hence toxicity, to any chrysotile outcrop, thereby discouraging the search for non-toxic chrysotile. The use of model solids that only differ in one property at a time appears to be the most successful approach for a molecular understanding of the physico-chemical determinants of toxicity. Such findings could also be useful in the design of safer nanofibres.