OELs for metals, salts and other compounds that do not form vapours at room temperature and pressure are expressed in mg/m 3 only. However, some OELs may be expressed in units such as fibres/cc (e.g., for asbestos). If the solvent is water, we can assume the density at standard temperature and pressure is 1.0 g/mL. Partial pressure is extremely important in predicting the movement of gases. This is why exposure limits are usually expressed in mg/m 3. 1) Unless specified otherwise, ppm usually refers to ppm by weight: 2.89 ppm 2.89 g per 1,000,000 g (or any other weight unit, I have just chosen g for convenience) 2) Now you need to know the density of the solvent to convert the volume to mass. Partial pressure is the pressure that a gas in a mixture of gases would exert if it occupied the same volume as the original mixture at the same temperature. NOTE: Occupational exposure limits (OELs, TLVs, PELs, etc.) can be expressed in parts per million (ppm) only if the substance exists as a gas or vapour at normal room temperature and pressure.
Also, atmospheric temperature and pressure affect the calculation. The conversion factor is based on the molecular weight of the chemical and isĭifferent for each chemical. However, concentrations may also be expressed as parts per million (ppm) or parts per billion (ppb) by using a conversion factor. If you have been experiencing issues with this conversion after reading this, you will have a better understanding.Ĭoncentrations 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).
This article will briefly explain how to convert from ppm to mg/m3.