Nanoparticles are structures between 1 and 100 nanometres across, and they are now used in sunscreens, cosmetics, sports equipment, fabrics and medicines. Because they are so small, they behave differently from the same material in bulk, and scientists are still finding out how they affect living things.
One worry is that nanoparticles can enter the body through the skin or the lungs. They are small enough to pass into cells, and some may travel round the body and cause harm. Their very large surface area to volume ratio also makes them more reactive, so some may be toxic. The long-term effects are not yet fully known.
Another worry is the environment. Silver nanoparticles, used in some socks and clothes to kill bacteria, can wash out into waste water. There they may kill useful bacteria and damage the environment. Because of these risks, new nanoparticle products need careful testing before they can be shown to be safe, and the benefits of each use have to be weighed against the possible risks.