Alloy and amalgam dis-integration question

I am aware that brass screws in a marine environment
will de-zinc leaving a weak pinkish 'ghost'
of the original brass - presumably a porous
form of copper. I could well believe that
the density of this remnant material may be
half the original density of the brass.
Could the same marked density reduction either
due to galvanic action or vaporisation occur
to the mercury component of dental amalgam ?
Since starting the thread on sci.med.dentistry
copied below I did a bit more exploration.
I suspected that filing the filling material may
have smeared/disguised any porosity.
I had another smaller piece from the same filling
and placed in the jaws of some end-snips inside
a polythene bag and sheared into two pieces.
This showed light-grey , powdery surfaces which when lightly
filed showed the silvery colour as before.
Viewing under 30x magnification,
the grey surfaces, then probably a
porous granular material with random tiny black patches,
presumably of silver oxide.
s.m.d thread repeated here
Allowable level of mercury ingestion ?
Twice in one week I've read media articles about
mercury / amalgam fillings. Plenty of anecdotes
but nothing quantitive.
I've an odd bit of filling that dropped out six months
ago and decided to do a back-of-envelope calculation.
Filling now decidedly black on surface but filing
a flat shows silvery metal and no apparent porosity
under x30 microscope.
Weight 0.58 gm +-0.01gm
Volume 0.12cc +-0.02cc from displaced water in a
2mL hypodermic syringe barrel.
so Old filling density 0.58/0.12 = 4.8gm/cc
Amalgam filling composition
52% Hg, and 48% of Zn,Cu and Ag
densities : 13.6,6.9,8.8,10.5
not knowing composition in more detail
have taken average of Zn,Cu and Ag
so new Amalgam density of 11.3
Loss over say 20 years (assumed, so a big unknown ,
somewhere between 2 years and 40 years )
=(11.3 - 4.8 ) * (0.58 / 4.8 ) * 0.52 = 0.41
assuming all metals are lost equally, unlikely but I've
no info to quantify better.
0.41 gm over 20 years
so 20 mg per year
If only mercury is totally lost, and in the limit, only
leaving the other metals intact then a figure of
30mg per year of ingested mercury
No allowance for direct wear, off the filling surface, also
ingested. And only one such filling considered.
What is the allowable figure for yearly ingestion of Mercury ?