Gas concentrations are usually expressed in terms of parts per million, or mg/m3. Being able to convert from one unit to another is important since some regulations, industrial hygienists etc. prefer one format over the other. However, converting from one to other is not as simple as converting temperature from degrees Fahrenheit to degrees centigrade because ppm and mg/m3 are not strictly the same thing, mg/m3 is an actual concentration whereas ppm is a relative concentration.
If all this unit conversion sounds difficult, it need not be. ChemDAQ has produced a free unit conversion spreadsheet tool, available for download from the ChemDAQ website (at the bottom of the page). This tool allows simple conversion from ppm to mg/m3 and back again and even provides the molecular weights for many common gases and vapors found in healthcare. If we have missed any compounds that would be useful, please let us know and we will add them to the list.
A more detailed explanation of the difference between ppm and mg/m3 is provided below for those who are interested.
More Detailed Explanation
A concentration is defined as the amount of something per unit volume, so if the amount of a gas is expressed by its weight in mg, then the amount of gas per cubic meter (mg/m3) is the concentration. For a given gas concentration, the mass of gas per unit volume will be proportional to the molecular weight. Thus 1 liter of air has a mass (weighs) about 1.1 g, but 1 liter of 100% chlorine would weigh about 2.8g. The concentration unit of mg/m3 is the mass of the gas per cubic meter.
A part per million, as the name implies, is a relative concentration. If ethylene oxide - air mixture contains 1 ppm ethylene oxide, then for every million air molecules there is one ethylene oxide molecule. Parts per million are particularly useful when dealing with compressed gases, which may be why we use it in gas detection so much. A 2000 psi compressed gas cylinder of 10 ppm ethylene oxide has a much higher internal concentration in terms of mg/m3 than the test gas coming out of the regulator at 1 psi, but the relative concentration of EtO to air, i.e. 10 ppm is the same.
Therefore the relative amount of gas (in ppm) must be multiplied by the molecular weight, as well as changing the units to calculate the weight. In addition to adjusting the units, to convert from ppm to mg/m3, it is necessary to specify a temperature (usually 25 oC) and a pressure (usually 1 atm.).
If the conversion of gas concentrations from ppm to mg/m3 or vice versa sounds involved, use the ChemDAQ gas concentration conversion tool, . The tool is written as an Excel spreadsheet. Enter the concentration (as ppm or mg/m3) in the appropriate box, enter the molecular weight from the handy table shown on the right hand side and the result will be calculated. The converter uses 1 atmosphere pressure and 25oC as a default, but other values can be entered if needed.
Please let us know if you have any comments about the converter, good or bad. If there are any other gases or vapors you would like to see added to the table or other functionality added to the converter, please leave a comment to let us know.