Nanoparticles are useful, but their small size can also cause problems. They can be inhaled into the lungs, and some may be absorbed through the skin. Because they are so small, they may enter cells and may be carried in the bloodstream to organs around the body. Some studies suggest that certain nanoparticles can damage cells.
Nanoparticles are a fairly new technology, so the long-term effects of being exposed to them are not fully known. Their effects can also differ from the same material in bulk, so it is not safe to assume a substance is harmless just because the bulk material is.
Nanoparticles can also escape into the environment. For example, silver nanoparticles can wash out of fabrics into rivers, where they may harm living things. Scientists carry out tests and research to find out whether a product is harmful, and people then weigh the risks against the benefits before it is used.