Tutoring college chemistry, you sometimes get asked about parts per million. The tutor explains a concept more often heard in everyday life than in the academic world.

Parts per million, or ppm, is often used to express the concentration of an impurity, or the tolerable concentration of it.

For example, the EPA has set the maximum allowable concentration of chlorine in drinking water as 4 ppm (long-term; variations may be allowed over short intervals).

Parts per million is calculated by mass: specifically,
ppm=(grams substance of interest)/(million grams)

Often you read that ppm is equal to mg/L. The reasoning uses the idea that milli means 0.001 and, at the same time, 1L of water has mass 1000g (or 1 million mg):

mg/L = 0.001g/1000g = 1mg/1000000mg = 1/(1 million)

Depending on how strictly you interpret the definition of ppm, mg/L is a potential oversimplification. Let’s imagine you add 30.0 mg table salt to a litre of water. Now, the total mass has also increased by 30.0 mg. Stricly speaking, your ppm might be seen as

ppm = 30.0/1000030. = 29.999ppm

Well, even to four significant digits (and we only have three, anyway), that’s still 30.00ppm.

Since ppm is normally used to describe very small concentrations, mg/L is a practical definition.

Sources:

www.nesc.wvu.edu

US EPA

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.