It might seem harmless at first: a thread of dental floss tossed in the toilet, a contact lens swirling down the drain of the bathroom sink. But even the tiniest items can contaminate waterways.

The small fragments of plastic contact lenses are believed to be contributing to the growing problem of microplastic pollution. Pharmaceuticals, which are also frequently flushed down the drain, have been found in drinking water in some countries, and the consequences are not fully known.