**6. Nanoparticle elimination**

The use of nanoparticles in humans raises great doubts about their safety and their elimination capacity. If the removal is very fast, the nanoparticles will not reside long enough to fulfill their function. On the contrary, if the retention is very high, the concentration of nanoparticles can increase to the point of becoming harmful. Consequently, a relevant question in the use of nanoparticles in humans is how these biological systems can eliminate nanoparticles once their functions have been fulfilled. The properties of nanoparticles that affect their removal are mainly based on size, shape, composition, charge, and surface chemistry. These aspects will be briefly discussed within the two main elimination routes, (i) renal and (ii) hepatic elimination, to obtain a global vision of the process.
