Concentrations of chemicals in air are typically measured in units of the mass of chemical (milligrams, micrograms, nanograms, or picograms) per volume of air (cubic meter or cubic feet). However, concentrations may also be expressed as parts per million (ppm) or parts per billion (ppb) by using a conversion factor. The conversion factor is based on the molecular weight of the chemical and is different for each chemical. Also, atmospheric temperature and pressure affect the calculation.
Typically, conversions for chemicals in air are made assuming a pressure of 1 atmosphere and a temperature of 25 degrees Celsius. For these conditions, the equation to convert from concentration in parts per million to concentration in milligrams per cubic meter (mg/m³) is as follows:
Concentration (mg/m³) = 0.0409 x concentration (ppm) x molecular weight
To convert from mg/m³ to ppm, the equation is as follows:
Concentration (ppm) = 24.45 x concentration (mg/m³) ÷ molecular weight
The same equations may be used to convert micrograms per cubic meter (μg/m³) to parts per billion (ppb) and viceversa:
Concentration (μg/m³) = 0.0409 x concentration (ppb) x molecular weight
Here is an example. The average molecular weight of TVOC is 100 g/mole. If the concentration of TVOC in air is 1 ppm, convert to the units of mg/m³ by multiplying 0.0409 x 1 x 100 = 4.09 mg/m³.
Read more about “Understanding Units of Measurement”;