Because most people have the same type of fillings in there mouth, it doean't seem to be logical that one person can be poisoned by the mercury in the fillings and the rest of us sre no sffected.
Here is a quote from the EPA: "people are most likely to be exposed to metallic mercury from mercury released from dental fillings. The smount of mercury released from dental fillings is generally not considered to be high enough to cause adverse effects."
Except in occupational settings where elemental mercury is used, most of the health risk due to methylmercury exposure is from fish consumption.
EPA has set a limit of 2 parts per billion for mercury in drinking water nationwide.