Occupational Exposure Limits (OELs, TLVs, PELs, etc.) are usually expressed in mg/m³. Only in instances where the substance exists as a gas or vapour at normal room temperature and pressure will you see OELs expresses in parts per million.

Many practitioners find themselves in situations which require them to convert the OEL readings from ppm to mg/m³. If you find yourself in such a situation, you may use the following formula to do the conversion quickly.

where R is the ideal gas constant; T, the temperature in  kelvins (273.16 + T°C); and P, the pressure in mm Hg.