We have long advocated that government and industry refrain from using the metric-looking units—typified by the infamous milligrams per cubic meter—when discussing gas concentrations. The obvious reason is that the volume of a gas is affected by temperature and pressure. Thus, a concentration reading in mg/cubic meter is only valid for a particular set of conditions.
Parts-per-million (ppm) or parts-per-billion (ppb) should always be used, to avoid confusion.
Indeed, if you refer to the very useful OSHA table Z-1, you will see the entries for OSHA PELs (Permissible Exposure Limits). There are two columns: one for ppm, and the other for mg/m3. Now, here’s where it gets interesting.
Footnote (d) tells us that the ppm designation is in “Parts of vapor or gas per million parts of contaminated air by volume at 25 degrees C and 760 torr.”
Footnote (e) tells us that the mg/m3 designation is in “Milligrams of substance per cubic meter of air. When entry is in this column only, the value is exact; when listed with a ppm entry, it is approximate.”
However, these footnotes are oddly written. The main difficulty is that the ppm measuring unit is unaffected by temperature or pressure. It is the mg/m3 unit that must have an accompanying specification of the temperature and pressure. Ironically, the provision regarding an entry in which mg/m3 is the only unit mentioned would generally refer to a particulate (a solid) suspended in air, such as the listing for calcium carbonate.
Has OSHA itself become confused?
Some may object to the blanket statement that a ppm concentration is unaffected by temperature and pressure. After all, in our own Knowledge Base, we do post an article entitled “Correction for Atmospheric Pressure Differences on Gas Detection Instruments.” But, this entry tells you what to do if your instrument has been calibrated at one barometric pressure, and will be used at another.
While the parts-per-million, which rigorously is the mole fraction of the analyte in the balance gas does not change with temperature and pressure, the partial pressure will change. Unfortunately, most gas detection instruments do not measure mole fraction, but respond—directly or indirectly— to the partial pressure of the analyte.
In a way, this is related to the example we give in which mg/L and ppm are used interchangeably in water pollution studies, but such interchangeability does not work in air pollution measurements…
A user mistakenly thought that because in AQUEOUS solutions, milligrams per liter is nearly equivalent to parts-per-million—of the dissolved SOLID—the same would be true for gas mixtures. Using this “logic,” he communicated his requirement for monitoring 5 ppm chlorine in air as 5 milligrams per liter. In actuality, 5 mg / liter of chlorine in air works out to be 1696 ppm (at 20 degrees Celsius and 760 mmHg).
This confusion is caused by the classical error of comparing apples and oranges. Or, to put it another way, water is a great deal heavier than air!
Moral of the story: Use ppm or ppb when designating a gas concentration.