decay does not typically occur under white fillings. they stick to teeth very well, whereas metal fillings do not, and a lot more tooth structure has to be drilled away. metal fillings galvanize and pit over time and cavities get under the fillings. the ADA recommends replacement every 8 yrs because of this. your dentist is very misinformed. as far as the mercury ones not being harmful, i suggest your dentist watch the smoking teeth video, Click Here.