Parts per million (ppm) is a unit of measurement for very low concentrations by mass (or weight) of one substance dissolved in another substance, called the solvent. Strictly speaking, you can't convert ppm to micrograms per cubic meter, since a cubic meter is a measure of volume, not mass. However, as long as you know the solvent's specific gravity, you can determine how many micrograms are present at any ppm concentration.

Understand what the units of measurement mean. A microgram is one-millionth of a gram. Thus, a concentration of one ppm means there is one microgram of a substance per gram of solvent.

Find the mass of the solvent contained in one cubic meter. To do this, look up the specific gravity of the solvent. Water has a specific gravity of 1.00 gram per cubic centimetre. Since there are one million cubic centimetres in a cubic meter, if the solvent is water, you have exactly one million grams. Suppose, however, you are working with sulphuric acid, which has a specific gravity of 1.85 grams per cubic centimetre. The mass in one cubic meter of sulphuric acid is thus 1.85 multiplied by 1,000,000, or 1.85 million grams.

Calculate the number of micrograms present in one cubic meter if the concentration is one ppm. Since one ppm means 1 microgram per gram, the number of micrograms present will be the same as the number of grams of solvent. Thus, if a substance is dissolved in one cubic meter of sulphuric acid at a concentration of one ppm, there are 1.85 million micrograms present.

Multiply the ppm by the micrograms present at a concentration of one ppm. For instance, suppose the concentration is 25 ppm. A one ppm concentration in sulphuric acid equals 1.85 million micrograms. Multiply 25 by 1.85 million. This equals 46.25 million micrograms in one cubic meter of sulphuric acid.

#### Tip

Although ppm as a unit of measurement is primarily used when working with liquids, you can also use this measurement with solids and even gases. The easy way to do this is to look up the specific gravity of the solid or gas (for example, air has a specific gravity of 0.0012 at normal temperature and pressure) and then follow the steps above.