Tiny plastic particles measuring about 100 nanometres (millionths of a millimetre) are used in many products, for example, to encapsulate dye or aromatic substances or as additives to shampoos and cosmetics. Many of them land directly in sewage as soon as the products are used. Together with other plastics, for example from tyre rubber in road runoff, they end up in the water-treatment plants. But until now, it has not been possible to measure them there. Unlike larger particles (microplastics), they cannot simply be sieved out and weighed or counted. In addition, it was unclear, except in models, how much nanoplastic was retained in water-treatment plants and how much entered the environment.