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Issue 1, May 2002
May 17, 2002
Vol. 1 Issue 1
Technical Tidbit
Technical Tidbit
Sometimes a responder needs to be able to convert between one unit of measurement for concentration to another unit of measure.  The two most common units found in reference sources are parts per million (ppm) and milligrams per cubic meter (mg/m3).  Most toxic chemicals that have a significant vapor pressure and therefore have the potential of producing a toxic vapor cloud usually have the toxic concentration expressed as ppm.  The toxic concentration is also referred to as the Level of Concern (LOC), i.e., IDLH, ERPG-1, ERPG-2 or ERPG-3.  Many hazardous materials have their LOC expressed as both ppm and mg/m3
There are legitimate reasons for reporting LOCs in mg/m3, such as when dealing with particulates, fumes or chemicals whose molecular weight is uncertain.  For those instances where the LOC is reported as mg/m3, a conversion of the mg/m3 LOC to ppm can be made using the following formula:[1]
                        LOCconcentration in ppm = (LOCconcentration in mg/m3 * 24.45)/(molecular weight)
For those instances where the LOC is reported as ppm, a conversion of the ppm LOC to mg/m3 can be made using the following formula:
                        LOCconcentration in mg/m3 = (LOCconcentration in ppm * molecular weight)/24.45
Isopropoxymethylphosphonyl fluoride, which is commonly known as Sarin, also referred to as GB has the following properties:
  • Molecular Weight: 140
  • Boiling Point: 316F
  • Vapor Pressure @77F: < 0.01 atm
Its IDLH is expressed as 0.2 mg/m3
Using the formula above:
                        LOCconcentration in ppm = (0.2 * 24.45)/(140)
                        LOCconcentration in ppm = 0.035 ppm
Obviously an extremely toxic material when compared to Chlorine with an IDLH of 10 ppm.

This conversion of mg/m3 to ppm or ppm to mg/m3 can only be applied in situations where the concentration is small.



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