Silver-colored (amalgam) fillings continually release tiny amounts of mercury into the mouth. People who have them have more mercury in their saliva and urine. Choose composite fillings instead, which are mercury free.
If you have silver-colored fillings, consider having them replaced. Choose a dentist who protects patients from the particles and vapors released when they're drilled out. And don't do it when you are pregnant, planning a pregnancy, or breastfeeding.