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06/24/03

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.

References

This information comes from Michigan State University Extension bulletin E-1641, How Much Is A Part Per Million?

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