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]
 
                        LOC_{concentration in ppm} = (LOC_{concentration in mg/m}3 * 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:
 
                        LOC_{concentration in mg/m}3 = (LOC_{concentration in ppm} * molecular weight)/24.45
 
AN EXAMPLE  
Isopropoxymethylphosphonyl fluoride, which is commonly known as Sarin, also referred to as GB has the following properties:

• Molecular Weight: 140
• Boiling Point: 316°F
• Vapor Pressure @77°F: < 0.01 atm

 
Its IDLH is expressed as 0.2 mg/m³
 
Using the formula above:
 
                        LOC_{concentration in ppm} = (0.2 * 24.45)/(140)
                        LOC_{concentration in ppm} = 0.035 ppm
 
Obviously an extremely toxic material when compared to Chlorine with an IDLH of 10 ppm.
 
 

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