The nanoparticles have a propensity to form aggregates (stable liaisons) and then agglomerates that are much less stable, up to the dimensions of several microns. The agglomerates dimension can condition their deposition mode in the respiratory system. These agglomerates although of greater dimensions, keep however a high specific surface because of their nano-structure. Once deposited in the respiratory tract, their stability (the trend to break up the agglomerate) certainly plays a role in their toxicity. This is a point which has to be clarified.