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I asked for a simple explanation for those of us who are not mathmatically inclined of ppm.  Here are some that I think are easy to understand.

about 1.4 cups of whatever in to an average size 60x30 ft swimming pool.  

DN
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Cecilia-
I asked my sister who works in a lab to put it in lay terms and she wrote:

"This is a way of expressing very dilute concentrations of substances. Just as per cent means out of a hundred, so parts per million or ppm means out of a million. Usually describes the concentration of something in water or soil. One ppm is equivalent to 1 milligram of something per liter of water (mg/l) or 1 milligram of something per kilogram soil (mg/kg)." -Marcia Booth, U.C. Davis    

Thank you for bringing the question. It makes me do some thinking.
-Randy in San Francisco, CA
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Think of it as dividing a volume of something into a million different particles. When something is listed as having 200 ppm, that is 200/1000000 or equal to 0.0002 which is a very small amount of the whole regardless if it is a teaspoonful or one cup.  Now we truly can begin to see how sensitive our bodies are. 
Betty

Cecilia, TX

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