The generation of reactive oxygen species and other radicals, catalyzed by iron ions at the fiber surface, is thought to play an important role in asbestos-induced cytotoxicity and genotoxicity, but a direct confirmation of this statement needs the availability of asbestos samples differing only for their iron content, without the interference of other physicochemical features. Synthetic stoichiometric chrysotile nanofibers, devoid of iron or any other contaminant, did not exert genotoxic and cytotoxic effects nor elicited oxidative stress in a murine alveolar macrophage cell line; on the contrary, the same nanofibers, loaded with 0.57% and 0.94% (w/w) iron, induced DNA strand breaks, lipoperoxidation, inhibition of redox metabolism and alterations of cell integrity, similarly to natural chrysotile. On the other hand, the incubation with ferric nitrilotriacetate, a cell-permeating iron complex, even if it caused an intracellular overloading of iron very similar to that elicited by iron-loaded synthetic chrysotile and by natural chrysotile, did not exert any of these effects. This suggests that chrysotile is not toxic by acting simply as a carrier of iron into the cell, but rather that the redox activity of iron is potentiated when organized at the fibers surface into specific crystallographic sites having coordination states able to activate free radical generation. Synthetic chrysotile fibers may be proposed as a standard reference sample and model solids for experimental studies on asbestos fibers aiming to clarify the mechanisms of its toxicity and to synthesize new fibers devoid of pathogenic effects.