MUCH MERCURY IS TOO MUCH?
WHAT CAN BE DONE ABOUT IT?
In an effort to preserve the human dentition for more
than one hundred years, dentistry has been able to restore to form
and function the ravages of dental decay. The oral environment is
a harsh one, with phenomenal temperature changes, punishing biting
forces, a myriad of microrganisms and all varieties of acids. All
of this, in a wet environment, twenty-four hours a day for a lifetime.
Because of this, all restorative materials have break down products.
Dental amalgam is made up of 50% mercury and 50% alloy of silver,
tin and copper. After being mixed, placed, and set in the tooth
the mercury is bound physically to the other metals. However, it
is subject to release through chewing, corrosion, abrasion, heat,
and many other stimulating factors. It is this release of mercury
vapor that is of the most concern for several reasons. 1. Mercury
is highly toxic, even in very minute quantities. 2. Mercury toxicity
has a broad range of symptoms mimicking many other conditions and
therefore its diagnosis can be misleading. 3. Mercury easily enters
the brain and in the case of pregnant women will enter the fetus.
But we are exposed to mercury in daily life and reason
would dictate that some level of exposure is very well tolerated.
We are, after all, adapted to an imperfect environment. Therefore,
the question arises: How much mercury is too much?
This is exactly the kind of question that expert
toxicologists at the Agency for Toxic Substance and Disease Registry,
a division of the Public Health Service, are mandated to address.
They have determined Minimum Risk Levels for a variety of toxic
substances including mercury. To quote the ATSDR: "An MRL is an
estimate of the daily human exposure to hazardous substance that
is likely to be without appreciable risk of adverse non-cancer health
effects over a specified duration of exposure. These substance-specific
estimates, which are intended to serve as screening levels, are
used by ATSDR, health assessors, and other responders to identify
contaminants and potential effects that may be of concern at hazardous
waste sites. It is important to note that MRLs are not intended
to define clean up or action levels for ATSDR or other agencies."
"MRLs are intended to serve as a screening tool to help public health
professionals decide where to look more closely. They may also be
viewed as a mechanism to identify those hazardous waste sites that
are not expected to cause adverse health effects. Most MRLs contain
some degree of uncertainty because of the lack of precise toxicological
information on the people who might be most sensitive (E.G., infants,
elderly, and nutritionally or immunologically compromised) to the
effects of hazardous substances. ATSDR uses a conservative (I. E.,
protective) approach to address these uncertainties consistent with
public health principles of prevention. Although human data are
preferred, MRLs often are based on animal studies because relevant
human studies are lacking. In the absence of evidence to the contrary,
ATSDR assumes that humans are more sensitive than animals to the
affects of hazardous substances and that certain persons may be
particularly sensitive, thus the resulting MRL may be as much as
one hundred fold below levels shown to be non-toxic in laboratory
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