Parts per million (ppm)

For airborne gases and vapors, ppm is by volume, so it describes how many molecules of the chemical there are among a million molecules of air. It is a convenient unit for the small amounts that matter for health. A larger unit, percent by volume, is used for high concentrations such as the Lower explosive limit (LEL), and one percent equals ten thousand ppm.

Dusts, fumes and mists are not measured in ppm, because they are particles and not a gas. They are expressed as milligrams per cubic meter of air (mg/m3). Some limits are given in both units, and conversion between ppm and mg/m3 depends on the molecular weight of the chemical and on temperature and pressure. The conversion is not the same for every substance, so use the figures given in the limit list.

Be careful with ppm in other contexts. In water, soil and food, ppm usually means a mass ratio, such as milligrams per liter or per kilogram, which is a different basis from volume in air. Mixing them up can lead to wrong conclusions.

Instruments such as gas detectors and colorimetric tubes read in ppm, and the exposure limits on the SDS for vapors will often be in ppm. Make sure instrument units, the limit units and the averaging time all agree before comparing.