<<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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