It has been common practice in modern dentistry in the US and other countries for decades to use amalgam fillings. If you have metal fillings, chances are extremely high that they contain 50% mercury. Even if the dentist calls them a "silver" filling, while that is technically correct because it contains silver and is silver colored, they are not disclosing that it is not pure silver.

Now after decades of exposure we have a population exhibiting signs of mercury poisoning.
Your dentist may or not be up to date with current scientific data that shows these fillings are unsafe, and may be continuing not only outdated but actually harmful practices that you pay them for! It is important to do your own research to gauge whether you can trust his practice.

The information provided here is the tip of the iceberg, and @teamsteem contributed many more valuable references.