Nano-technology has given us a lot of new and exotic materials that are helping us fight tooth-decay such as silver nano-particles and other such materials. They are so good at fighting bacteria and preventing their recurrence that they were allowed to enter the market for public consumption without exhaustive studies into their overall effects.
One problem with silver nano-particles is that they are indeed good at fighting bacteria, too good. It turns out that such a small quantity that you may send down the drain is so deadly to bacteria it does not allow anything to survive(mostly to rivers, streams and eventually the ocean). Entering the eco-system, it upsets the balance of life that we know is ever so linked to each other, from the smallest creatures up the food chain. The result, dead rivers and streams which may have been a result of medical products such as silver nano-particles void of life, void of bacteria and other marine life that began when we brushed our teeth but may last a lifetime.