<<Disclaimer: Verify this information before applying it to your situation.>>
ppm = 1 of anything per 1,000,000 (one million) "parts" of
anything else.
An example might help illustrate the "parts" idea:
If you divide a pie equally into 10 pieces, then each
piece would be a part per ten; for example, one-tenth of
the total pie. If, instead, you cut this pie into a
million pieces, then each piece would be very small and
would represent a millionth of the total pie or one part
per million of the original pie.
So, parts per million is a measure of concentration, the
amount of one material in a larger amount of another
material.
For our purposes, ppm = the weight of gliadin (from
gluten) in a certain weight of food.
Units of "ppm" are used in soil chemistry to mean 1 mg of
something per Kg (kilogram) of soil. A milligram is a
thousandth of a gram and a gram is a thousandth of a
kilogram. Thus, a milligram is a thousandth of a
thousandth, or a millionth of a kilogram, one part per
million.
So it follows that 1 mg of gliadin per Kg food would be 1
ppm.
So you have a pizza that may be contaminated with gluten
at the concentration of 200 ppm. If you could cut your
pizza into a million pieces, 200 of the pieces would give
you the weight of the gliadin in your pizza.
Obviously, we'd much rather the level of gluten
contamination be closer to 1 ppm.
But because it is hard to visualize exactly what 1 ppm is
we use comparisons. One part per million is equal to:
one penny in $10,000
one minute in two years
one dime in a one-mile-high stack of pennies
3 inches on top of mount everest
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If you're interested:
In solution chemistry the "ppm" is equal to 1 mg
(milligram) dissolved into 1 L (liter) water. Why 1 liter?
As it turns out, 1 L water weighs one kilogram (as before
1 mg/Kg - 1 ppm). So a ppm of sodium chloride solution
would be 1 mg sodium chloride in 1 L water. If the
drinking water in your city is fluoridated at the water
treatment plant, it is probably added at 1 ppm.
Dr. Megan Tichy
Lecturer, Texas A&M University
3255 TAMU, Department of Chemistry
College Station, TX 77843-3255
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