This article is from the Amalgam-Related Illness FAQ, by Leif Hedegard.
Dental amalgam consists of approximately 50% (inorganic) mercury (by weight) ( DHHS 1993 page I - 2). Besides mercury the amalgam usually consists of silver, tin, copper and sometimes zinc, palladium, or indium ( DHHS 1993 page I - 2). Mercury is highly toxic. Whether or not mercury or any other specific (toxic) metal will cause toxic and / or immunological reactions depends upon a combination of at least:
Allergic and / or autoimmune reactions to metals, however, do not need to follow the same dose-dependency as that seen in toxic reactions, in fact we know very little about the doses needed in order to give allergic or autoimmune reactions in susceptible humans. Mercury is released from your amalgam-fillings (Bjorkman 1992, Gay 1979, Langworth 1988, Skare 1994, WHO 1991...) and is taken up by your body (Molin 1990, Nylander 1987, Skare 1990, Skare 1994, Weiner 1993, WHO 1991...). However, it seems that somewhere around 5-200 times (see section 7.2) higher levels of absorbed Hg-dose, than those people absorb from amalgam-fillings on a group level, are required to produce adverse health effects (on a group level) in individuals occupationally exposed to inorganic mercury . This dose-gap points strongly in the direction that the majority of the population would not be affected by mercury from their amalgam-fillings. But if there is a minority (for example 1-3%) of the population that is substantially more (non-allergic-) sensitive to mercury / amalgam than the rest of the population there could well be a relation between amalgam-fillings and a multi-symptomatic illness in such a minority, the question is however scientifically unclear. A small minority of the (non-occupationally mercury exposed) amalgam-bearing population, has raised mercury levels from their amalgam fillings (see section 7.3), could (some of) these individuals get symptoms because of raised mercury levels rather than raised sensitivity to mercury?
A small fraction of the population with suspected (non-allergic-) amalgam-related illness as well as apparently healthy people has been reported to have rather high mercury levels in some body-fluids. However, individuals with suspected (non-allergic-) amalgam-related illness have, on a group level, not shown to have significantly higher mercury levels in their blood-plasma (Berglund 1996, Molin 1987,) / whole blood / erythrocytes (Berglund 1996, Molin 1995) / urine (Aronsson 1989, Berglund 1996, Molin 1995,) / intraoral air (Aronsson 1989 , Berglund 1996,Fredin 1988) than healthy people with the same amount of amalgam. So it seems that if there is a (non-allergic-) amalgam-related illness in individuals suspecting they have such an illness, it is, on a group level, based upon increased (non-allergic-) sensitivity towards mercury / amalgam rather than higher mercury levels in these body fluids compared with the general population. This does not exclude that there could be a sub-minority among people with suspected amalgam-related illness that has an illness more because of raised mercury-levels than raised sensitivity to mercury. There have been reports of cases where raised mercury levels from, as it seems, amalgam fillings is highly suspected to be the cause of illness (Barregard 1995, Langworth 1996, Taskinen 1989).
Marcusson (1996), in a double-blinded study, patch-tested (with mercury or placebo) a selected group of patients who had earlier reported symptom-increase in conjunction with drilling out of old amalgam fillings. Marcusson reported that the symptoms increased after patch testing with phenyl mercuric acetate (but not significantly with metallic mercury) compared to placebo. As the calculated mercury uptake from a patch-testing (4-10 ug, Marcusson1996) is about the same as one days uptake of mercury from amalgam (3-17 ug, WHO 1991, a non-allergic systemic reaction to such a patch-test can be seen upon as a sign of a extreme individual non-allergic sensitivity to mercury.
Before jumping to any conclusions, one would have to see the scientific societies response to this article and a reproduction of the study by another research group. Being, as far as I know, the only publicised double-blinded provocation test in this patient-group. I think it has, at least, a value as a stimuli for further research in the area.
Also make sure to read these books: Poison in Your Teeth: Mercury Amalgam (Silver) Fillings...Hazardous to Your Health! and Mercury Detoxification by Tom McGuire