You need to ideally discuss your concerns with your dentist. The filling can be re-done or patched up. This is entirely your call. To patch it up, you can expect it to chip off again- and having it re-done may be the best option- but still not guaranteed to NOT chip again. Make sense? If you have concerns regarding the mercury vapour, then discuss this with the dentist. If you have a new amalagma filling, then not much of the amalgam has to be removed in order for more to be added. In order to do a composite filling, then ALL the amalgam has to be removed. It can also be patched up with a materila called glass ionomer that will repair the filling.  If the tooth is heavily filled, then it is always advised to have a crown for protection. Hope this has helped.