Too little or too much :

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Too little or too much ? Trace elements and
dose response theory
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Back to Basics Elements
Remember that all matter is made of elements An
element is a substance in its simplest form (an
element cannot be separated into simpler
substances) For example, the mineral calcite can
be broken down to calcium, carbon and oxygen, but
these constituent elements cant be broken down
any further (at least at the atomic level) There
are over 100 elements, of which 92 occur in
nature An atoms of a particular element has a
unique number of protons and electrons (number of
neutrons can vary, hence the designation of
isotopes)
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Abundance of Elements
Different elements occur in different
concentrations in the Earth system We can
classify elements on the basis of concentration
in nature. For uncommon elements (minor and
trace), we usually cite abundance in ppm (parts
per million). I part per million would be the
equivalent of 1 milligram per 1 kilogram. Major
elements 10,000ppm (or about 1 by volume)
Minor elements 1,000 - 10,000ppm Trace
elements 4
Abundance of Elements
Major and minor elements comprise most matter on
Earth (including us) and therefore are not much
of an issue with respect to toxicity (our bodies
have evolved in the constant presence of these).
Increasing abundance
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Abundance of Elements
The presence of major and minor elements in our
bodies reflects 1. Their natural abundance in
the Earths crust 2. Availability of these
elements to organisms (e.g. aluminum cannot be
dissolved easily). 3. How our bodies process
these elements (e.g. unwanted elements are
discarded and wanted elements are retained)
Increasing abundance
So what might be regarded as a major element in
rocks might be regarded as a minor, or trace,
element in the human body
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Concentration of Elements in the Body
Certain tissues of the body accumulate certain
elements in higher concentrations, partly due to
the role of tissues in the functioning of the
human body. For example, fluorine (as
fluoride) is incorporated into bones and teeth,
so health consequences of variations in
concentrations are most likely to be noticed in
these tissues. Also, strontium can substitute
for calcium in bones, so would be expected to
affect bone health. Likewise, the thyroid gland
uses iodine to make hormones, so variations in
iodine concentration would be expected to be most
apparent in the thyroid gland.
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Also, different organs of our bodies are affected
by different substances. Here are a few
pollutants that find their way into our bodies
and the organs affected.
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Elements in the Earth and in Our Bodies
Trace elements are somewhat of a different
matter. We have evolved to cope with extremely
small quantities of these (and to use some for
biological processes). Butslight variations in
the abundance of trace elements in our bodies can
have profound effects on health. To assess the
effects of trace element concentrations on human
health, we talk about dose response. Dose
response can be depicted in different ways (as we
will see), but basically refers to the health
response to different doses of a particular
substance.
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Importance of Element Concentrations
Certain concentrations of elements in our bodies
are essential to normal functioning A common
misconception is that elements such fluorine,
known to be beneficial to dental health in
certain concentrations, should be more beneficial
in high quantities. But this is not so- too
little is bad, but too much can be
worse. Deficiency or excess of various elements
in our bodies is largely controlled by geological
factors- whether they be in the direct vicinity
of an individual or in the place where food or
water are derived (if imported).
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Universally Harmful Trace Elements
Some trace elements are not beneficial to life
processes in any concentration. These
include Arsenic negative effects on
functioning of heart Cadmium negative effects
on kidneys and skeletal growth Lead negative
effects on kidneys, fertility, nervous system and
heart Mercury negative effects on nervous
system and fertility Aluminum negative effects
on nervous system and skeleton
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Universally Harmful Trace Elements
At extremely low concentrations, these elements
are benign (no effect) At slightly elevated
concentrations (above a threshold), these
elements cause harm But at no levels are these
beneficial
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Essential Trace Elements
Essential trace elements are those elements that
are essential for biological processes in low
concentrations. These include Chromium
plays part in metabolism of sugar Cobalt part
of vitamin B-12 Copper involved in metabolism,
production of hemoglobin Fluorine gives
strength to bones and teeth Iodine used in
production of thyroid hormones Iron essential
component of hemoglobin Manganese involved in
bone growth and metabolism Selenium reduces
aging action of free radicals Zinc important
for body growth, and plays part in immune
system Note these are but a few examples of how
these elements are used (many have several more
functions than indicated)
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Optimal dose
This is a schematic view of how different doses
of a particular element can affect human
health Very low doses can produce adverse health
effects Very high doses can also produce adverse
health effects The key is somewhere in the middle
(the Goldilocks principle)
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Fluoride Dose Response
Heres a real example. A deficiency in fluorine
(as fluoride) hampers bone growth and makes teeth
more prone to decay Within a range of 1 to 5 ppm,
fluoride is beneficial Above 5 ppm, bones and
teeth become brittle, then become abnormally
formed
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Selected elements and their concentration in
rocks (in ppm)

• Interactions of water and plants (and animals
  that eat them) with rocks (and soils developed
  from them) dictate our intake of these elements
• Concentrations of these elements are non-uniform,
  and vary widely among rock types
• Knowledge of rock types in a particular area can
  help pinpoint potential health problems
  associated with concentrations of particular
  elements

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Geographic distribution of rocks
The distribution of rocks types is also
non-uniform, thus health issues related to water
and organism interaction with rocks (and soil
derived from them) are different according to the
geographic distribution of rock types
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Water chemistry
On top of this, water chemistry varies
Remember, for example, that acidic water tends
to dissolved metals more readily than basic
water. In turn, water acidity can be linked to
pollution derived from industrial and residential
areas (largely due to fossil fuel burning).

• Sudbury
• - Site of copper/nickel mines
• Landscape remains quite barren even though
  sulphur dioxide emissions have been substantially
  reduced since the 1960s (water also remains very
  acidic)
• Area underlain largely by mafic igneous rocks
  and metamorphic rocks
• effect of smelting on wildlife obvious, but what
  about humans ?

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Water movement
Plus, water interacts with rocks both at, and
below, the Earths surface (in vapour and liquid
form), and at different rates in different parts
of the Earth.
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Accumulation in Organisms
On top of this, some elements, such as mercury,
can combine (complex) with other substances, such
as organic molecules, allowing them to accumulate
in sediments They can also accumulate upward
through the food chain, thereby increasing levels
of toxic element ingestion by humans
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Can we do it ?
So can we even begin to link trace element
concentrations and human health ? Yes- but it
requires a broad view (this is where geologists
come in handy- note that all the major processes
are geologically based). In the next couple
lectures, we will look at some case studies of
the links between trace element concentration and
human health.
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END OF LECTURE