Around 30 years ago my dentist told me that mercury vapour is constantly released from these fillings and a dental study had found that after removal some old fillings had much less than the original 50% mercury. It was found that most of it was lost by slowly vapourising. The study also found that the level of vapour in the mouth was well above the safety limits set for industrial workplaces! To see if it was true my dentist got hold of a metering device for mercury vapour and simply put the device inside a patient's open mouth and the needle went off the scale (his words). That patient only had one filling! That was enough for me - I had all mine replaced.
Just did some googling and came a cross an article which says something similar.
Apparently there is a conspiracy theory that the dentistry industry has been covering up to avoid crippling lawsuits. I didn't bother looking into it further since I no longer have mine.
I should add that although I had around 4 or 5 fillings at the time I didn't notice any difference over the years after replacement. However some the effects can be quite subtle so who knows.
