There is a lot of controversy about the
toxic effects significance of the various
types of mercury people are exposed to:

vapor, inorganic, organic(methyl) mercury.
ADA and some at Gov't agencies and other
have argued that methyl mercury
is much more toxic than other forms, and mercury
from fish thus a more important
problem than vapor from fillings. However the
evidence seems contrary to
that.

Vapor may be the biggest problem even for
equal exposures,in addition to
the fact it is well documented that mercury from
fillings is the number one source in
people.

Another interesting finding is evidence that
indicates that mercury vapor is 10 times more
toxic to the fetal brain than methyl mercury.
Richardson(paper for Swedish Scientific
Panel) has estimated that about 20% of the
population suffers a subclinical
impairment of kidney or CNS function related
to amalgam mercury.


Some references from my anotated
bibliography on this are the following:

Mercury vapor is transmitted rapidly thoughout
the body and like methyl mercury has significant 
toxic effects at much lower levels than other
inorganic mercury forms(38,265,281,287).
Exposure to mercury vapor or methyl mercury
causes rapid transmital through the placenta to 
the fetus, and much more exposure and damage to
fetus than for maternal exposure to inorganic 
mercury(265,281,287,38).

Elemental mercury vapor is more rapidly
transmitted throughout the body
than most other forms of mercury and has more
toxic effects on the CNS and
other parts of the body according to the World
Health Organization and other
studies(38,183,265,282,287). Mercury vapor
rapidly crosses the blood-brain
barrier, and placenta of pregnant women
(38,85,113,146,162,262, 265,
281,287,etc) Developmental learning and
behavioral effects have been found
>from mercury vapor at much lower levels than
for exposure to methyl
mercury(287).

(38) Ziff S. and Ziff M. Infertility and Birth
Defects: Is Mercury from Dental Fillings a Hidden Cause?,
Bio-Probe, Inc. ISBN:
0-941011-03-8.1987
(85) J.A.Weiner et al,"The relationship
between mercury concentration in human organs and 
predictor variables",138(1-3):101-115,1993;  & "An
estimation of the uptake of mercury from amalgam fillings", 
Sci Total Environ,v168,n3, p255-265,
1995.
(113) M.J.Vimy et al, Maternal-fetal
distribution of mercury released from
amalgam fillings", Am J Physiol
258:R939-R945,1990.     See also (238)
(146) T.Colborn(Ed.),Chemically Induced
Atlerations in Functional
Development,
Princeton Scientific Press,1992 &
Developmental Effects of Endocrine-
Disrupting Chemicals",Eniron Heath
Perspectives, V 101, No.5, Oct 1993.
(162) N.K.Mottet et al, "Health Risks from
Increases in Methylmercury

Exposure",vol63:133-140,1985.
(262) L.W.Chang, "Neurotoxic effects of
mercury", Environ. Res.,1977,
14:329-(265)M.R.Greenwood et al, "Transfer of
metallic mercury into the fetus",
Experientia, 28:1455-1456, 1972.
(281) T.W. Clarkson et al, "Transport of
elemental mercury into fetal
tissues", Biol. Neonate. 21:239-244, 1972.
(287) M.C. Newland et al,"Behavioral
consequences of in utero exposure to
mercury vapor", Toxicology & Applied
Pharmacology, 1996, 139: 374-386;
& K.Warfvinge et al, "Mercury distribution in
neonatal cortical areas ...after exposure to 
mercury vapor",Environmental Research, 1994,
67:196-208.