How to convert exposure concentrations and TWA
Air sampling results usually come back in ppm, while safety data sheets and many published limits are written in mg/m³. Converting between the two needs the molecular weight of the substance and a stated temperature and pressure, because the molar volume of a gas changes with both.
The conversion is mg/m³ = ppm × molecular weight ÷ 24.45, where 24.45 liters is the molar volume at 25 °C and 1 atm, the reference state used for OSHA PELs, ACGIH TLVs and NIOSH RELs. Benzene has a molecular weight of 78.11, so 1 ppm equals roughly 3.19 mg/m³.
The 8-hour time weighted average is measured concentration × exposure hours ÷ 8. A 6-hour exposure at a given concentration averages out to three quarters of that value over a full shift. A low TWA does not clear the job on its own: short-term exposure limits and ceiling values apply to brief peaks and are evaluated separately.
OSHA PELs, ACGIH TLVs and NIOSH RELs often differ for the same chemical, and they are revised over time, so look up the current published value before entering a limit. Sampling strategy, analytical method and any control decision should be handled by a certified industrial hygienist.
Frequently asked questions
At 25 C and 1 atm one mole of gas occupies 24.45 liters, so mg/m3 = ppm x molecular weight / 24.45. Benzene, molecular weight 78.11, works out to about 3.19 mg/m3 per ppm.
OSHA PELs and ACGIH TLVs are stated at 25 C and 1 atm, where the molar volume is 24.45 liters. The 22.4 figure applies at 0 C and gives a different answer.
The 8-hour TWA is concentration times exposure hours divided by 8, assuming no exposure the rest of the shift. Short-term limits such as STEL and ceiling values still have to be checked separately.