Intraspecies Variability

James Stanley Smith, Jr., Ph.D. jssmith at oak-creek.net
Wed May 19 08:26:24 EST 1999

You are correct to assume that a small percentage of the populace may be more
sensitive to mercury intoxication than the general populace, however, even
this small population's sensitivity is limited.  US EPA and other human health
organizations typically use an uncertainty factor (UF) of 10 (no information)
or 3 (limited information) to account for variability in human sensitivity.
This UF is in addition to UFs that are typically used to account for a limited
data base (10), interspecies extrapolation (10), extrapolation from subchronic
to chronic study results (10), and extrapolation from a low observed adverse
effect level (LOAEL) to a no observed adverse effect level (NOAEL).  When
appropriate, these UFs are multiplied together (UF product) and used to lower
the lowest amount of mercury shown to have an effect in the most sensitive
animal species (LOAEL) or to lower the NOAEL, defined as the dose below a
LOAEL at which no adverse effect occurs.  As you can see there can be a lot of
additional uncertainty taken into account in the derivation of the level of a
compound that is deemed "safe" and thought to be unlikely to cause adverse
effects over a lifetime of exposure.

For elemental mercury such as that used in amalgam, the US EPA
<http://www.epa.gov/ngispgm3/iris/subst/0370.htm> uses a UF product of 30; 10
was used for the protection of sensitive human subpopulations (including
concern for acrodynia) and 3 was used for the lack of a sufficent data base,
particularly developmental and reproductive studies.

Current scientific studies suggest that an UF of 10 is adequate and in fact
very likely to be overprotective of the most sensitive human population.
Consequently, there is little concern for adverse health effects developing in
humans from chronic exposure to elemental mercury vapor that does not exceed
0.3 micrograms per cubic meter air (ug/m3).

Finally, I do not want to suggest absolutely that some people are not affected
by chronic exposure to lower concentrations of elemental mercury vapor, but I
am not aware of any scientific evidence in support of their health claims.

arie at no.mail.thanks.nl wrote:

> I wonder if this would apply to mercury.
> It's often said that only a small portion of the population (2 - 3 %)
> suffers from detrimental health effects due to the chronical low dose
> intoxication from mercury leaking from amalgam fillings. This could
> mean that the other part has much more resistance.
> arie
> On Mon, 17 May 1999 11:52:46 -0400, toddh at brownvm.brown.edu (Todd)
> wrote:
> >We often use a 10-fold uncertainty factor when considering the
> >intraspecies variability in reactions to a particular chemical.  I am
> >currently interested in looking at cases where that 10-fold factor may
> >understate the true variability in the population.  I would be greatly
> >appreciative if somebody could either give me examples or point me in the
> >direction of a good resource to find examples where this may be the case.
> >Human variation in reactions to Prozac would be one example.  Again, I'd
> >appreciate any help that you could lend.  Thank you very much!
> >
> >Todd


James S. Smith, Jr., Ph.D.
President & Toxicologist

Toxicology & Risk Assessment Consulting
     RR 3  Box 246B
     Route 22
     Gorham, Maine  04038-9428

     Voice      : 207.929.6375
     Facsimile: 207.929.6474
     E-mail     : jssmith at oak-creek.net
     WWW    : http://www.oak-creek.net

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