Scientists often measure very small concentrations in units called parts per million, abbreviated as ppm. For example, 1 ppm of a chemical in water means that, in a million units of water, there would only be one unit of the chemical. Many water pollution standards are written in these units. But ppm can refer to one item in a million of anything the same size.

Try This! Visualizing Activity

You will need a sheet of 8 1/2 x 11-inch computer paper and a pen or pencil.

• Calculate how many square centimeters are in the sheet of paper
• Mark off one square centimeter. Based on your calculation above, how many of these square centimeters would fit on one sheet of paper?
• How many sheets of paper would you need to make your little square be one part per million? How many for one part per billion?
• (After you've done this, you can compare your result with ours here.)

Try This! How big is a drop?

Take an eyedropper that is marked in milliliters and fill it with exactly 2 milliliters of water. Now squeeze the bulb slowly, drop by drop, counting exactly how many drops come out as you bring the level to 1 milliliter. How many did you count? Have 10 different students try this. Do they all get the same answer? If not, how far apart are the numbers? What might account for the differences? What is the average number of drops per milliliter?

Setting up ratios

When Journey North science writer Laura Erickson and a scientist from the Environmental Protection Agency tried the experiment, they each squeezed out 20 drops in a milliliter of water. That's an easy number to work with, so let's use it.

20 drops/milliliter = 20,000 drops/liter =1,000,000 drops / 50 liters.

That means one part per million is the same as one drop of a substance in a million drops, or 50 liters, of water. And because there are 3.78 liters in a gallon, one part per million is also the same as one drop of a substance in about 13.2 gallons of water.

The Journey North science writer's bathtub holds about 60 gallons, or 228 liters, of water (keeping the level safely below the overflow drain hole!) So just 7 or 8 drops of food coloring in a full bathtub of water would bring it to a concentration of one part per million.

Try This! Bathtub Experiment

This is an experiment for students to try at home. First, each student should measure their bathtub with a metric tape measure, from the bottom to just under the overflow drain, across the width, and along the length. Calculate how many liters this is. Fill the tub at bath time, and then add drops of food coloring, one at a time, until the color is barely detectable to your eyes. How many drops did it take? If one drop per 50 liters of water is a part per million, how many parts per million were necessary for the color to show?

Parts per Billion

Some substances, like arsenic, are so poisonous that government standards for them in water are too small to be measured in parts per million. Then scientists use parts per billion. The U.S. government currently allows arsenic in water at 50 ppb. How much is that? If one drop of food coloring in 50 liters of water is a part per million, one drop of food coloring in 50,000 liters of water is a part per billion. How many bathtubs full of water is 50,000 liters?

Arsenic is a solid, so scientists use mass to measure arsenic's concentration in ppb. So for every billion liters of water (each liter of water weighs about 1 kg), one kilogram of arsenic is one part per billion.