Measuring Exposure in Units

The two main units used for measuring airborne concentrations are:

• Parts per million (ppm)
• Milligrams per cubic metre of air (mg/m power of 3, or mg m power of ‐3)

The gaseous state (vapours and gases) is measured in ppm and refers to the number of parts of vapour or gas of a substance in a million parts of air by volume, measured at a standard temperature and pressure (usually 25°C and 760 mm Hg, respectively). Particulate matter in dusts, fumes, etc. is measured in mg/m3, which refers to the milligrams of the substance per cubic metre of air.

One further unit of measurement is used in relation to fibres (such as asbestos). Concentrations of fibres are expressed in:

• Fibres Per Millilitre of air (fibres ml ­power of ‐1)

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