Chemistry / Unit 3 Chem 0420 short lesson, about 20 seconds
Parts per Million and Parts per Billion
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The idea
Parts per million and parts per billion extend mass percentage to very low concentrations, scaled by 10^6 or 10^9.
In practice
A fluoride level of 4 parts per million means 4 g fluoride per million grams of water — too small for percentage.
Where people slip
Reading 4 ppb as 4 ppm overstates the concentration a thousandfold — the two units are never interchangeable.
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