Parts Per Million
The health effects of any toxic substance ar related
to the amount of exposure, also known as the dose. The
greater the dose, the more severe the effects. Since some
chemicals can cause toxicity at very low doses, it is
important to be able to understand how these very small
amounts are described. lt is especially important to
understand how these very low levels compare to one
another and what they represent when compared to amounts
of more familiar substances.
Part per million (ppm), part per billion (ppb), and
par per trillion (ppt) are the most commonly used terms
to describe very small amounts of contaminants in our
environment. They are expressed as concentrations rather
than amount so we can easily compare a variety of
different environmental situations.
An example might help you to understand the "part
per.. idea. if you divide a pie equally into 10 pieces,
each piece would be a part-per-ten; i.e., one-tenth of
the total pie. If you cut this pie into a million pieces,
each piece would be very small and would represent a
millionth, or part per million, of the original pie. If
you cut each of the million minute pieces into a thousand
little pieces, each of these new pieces would be a part
per billion of the original pie.
To give you an idea of how little this would be, a
pinch of salt in 10 tons of potato chips is also a part
per billion. Thus, a part per billion of the pie would be
invisible to you.
In this example, the pieces of the pie were made up
of the same material as the whole. However, if there was
a contaminant in the pie at a level of a part per
billion, one of these final invisible pieces of pie would
be made up of the contaminant and the other 999,999,999
pieces would be pure pie. Similarly, one part per billion
of an impurity in water represents a tiny fraction of the
total amount of water. One part per billion is the
equivalent of one drop of impurity in 500 barrels of
water.
Since a trillion is 1,000 times larger than a
billion, part per trillion (ppt) is 1,000 times smaller
(one drop of impurity in 500,000 barrels of water).
ln order to appreciate how these quantities can be used
in a real situation, look at the following example.
We'll use the "part per..." terminology to compare the
relative importance of PCBs in Great Lakes fish vs. PCBs
in Great Lakes drinking water; i.e., which source might
contribute most to PCB exposure of humans living in the
Great Lakes region. The maximum level of PCBs legally
allowed in fish sold in interstate commerce is 2 ppm.
Although there are no legally established levels for PCBs
in drinking water, measurements have shown that the
average PCB content of the Great Lakes drinking water is
4 ppt.
Since we know that a part per trillion is a million
times less than a part per million, we can see that the
maximum allowable concentration of PCBs in fish is about
a million times higher than the level of PCBs in drinking
water. However, people generally consume a lot more water
than fish about 100 times as much water as fish, on the
average. By comparing amounts of fish and water consumed
and PCB level in each, it is clear that fish can be a
much greater source of PCBs than drinking water. We can
also see that the total amount of PCBs consumed is most
dependent on the amount of fish eaten and how
contaminated it is. Thus, the best way to reduce human
exposure to PCBs is to reduce the levels in fish and
reduce human consumption of fish with the highest
contaminant levels.