It seems innocent enough. Your dentist is giving you a new filling. You get some of those little metal slivers in your mouth and he tells you take a swig of water. Rinse and spit. No problem, right? Unfortunately, each one of those slivers is about half mercury. Multiply that simple routine millions of times, and you have the reason dentists have become the leading source of mercury pollution in our country's wastewater. Each sliver of mercury down the drain ends up in wastewater, then the nearest river or bay, and ultimately in the fish we eat. In fact, at …