Parts Per Million (ppm) and Parts Per Billion (ppb)

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In water treatment one very common unit of measure is parts per million (ppm) and parts per billion (ppb). What does “one part per million” mean?

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For every one million parts of water, there is one part of the contaminant. Or, that for every kilogram of water, there is 1 milligram of the contaminant.

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An even smaller unit of measure is parts per billion (ppb). A concentration of 5 ppb would mean that for every billion parts of water, there are only 5 parts of the contaminant!

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So, how small is a part per million, anyways?

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One part per million is equivalent to:
 

• One automobile in bumper-to-bumper traffic from
  Cleveland to San Francisco

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• One pancake in a stack four miles high

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• One cent in $10,000​

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If a part per million is really small, does that mean I don't have to worry about these contaminants in my water?

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Not quite! Just because a concentration is small doesn't mean that it is safe for human health. In reality, when contaminants are measured in parts per million, they are relatively abundant in water. For example, 1 part per million of carcinogenic chloroform in a cup of coffee  will contain 8,000,000,000,000,000,000 molecules of chloroform!

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It’s all about Toxicity

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It’s important to remember that every contaminant has a different level of toxicity. For some chemicals, such as lead and arsenic, 1 ppm will be is very dangerous to human health. For other contaminants, like Nitrate, 1 ppm is perfectly healthy. 

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It's also important to remember the difference between acute and chronic toxicity. A contaminant may be acutely toxic when a large quantity causes immediate illness.  Chronic toxicity may occur with exposure to a low dose over a long time period. In water treatment, there are usually low levels of contaminants present, so we are concerned with low doses and chronic toxicity.

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More Units of Concentration in Water Treatment

 

The basic units of measurement we use in water treatment are units of mass and volume. While weight is measured in pounds, mass is measured in grams (g). One kilogram (kg) is equal to 1,000 grams. One gram is equal to 1,000 milligrams (mg) or 1,000,000 micrograms (μg).  One paperclip “weighs” approximately 1 million micrograms.

 

Volume can be measured in gallons, liters, and cubic feet. When we measure contaminants, we usually want to know how much is present in one liter of water. One liter is the size of a typical reusable water bottle. When you buy soda, you can buy 3-Liter bottles.

 

Concentration is a compound unit of measurement, just like price per gallon of gasoline. Concentration tells how how many milligrams are in one liter of water, that is the milligrams per liter (mg/L).

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In water treatment, both milligrams per liter (mg/L) and parts per million (ppm) are used. It may seem confusing that there are so many different units, but its easy to remember the connections between them. The following summarizes it:

 

1 mg/L = 1 ppm

1 μg/L = 1 ppb

 

1 mg/L = 1,000 μg/L

1 ppm = 1,000 ppb