Here’s one more reason to dread your dentist: Many dental offices flush old fillings down the drain, washing the mercury inside them into the nation’s waterways. That makes dentists the single largest discharger of the toxic metal, according to a national study entitled “Dentist the Menace?” and published by a collection of health and environmental groups. All told, dentists use about 40 tons of mercury per year to make silver fillings, a practice they’ve been engaged in for some 150 years. While the mercury might not do damage in people’s mouths, it spends many more years in the natural environment, where it eventually breaks down and can cause nerve and brain damage if ingested. It is possible to use white plastic composites instead of mercury, but until recently, the dangers of washed-away mercury fillings had gone unremarked.