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Unit of measure

Which unit of measure to choose?

To assess the exposure of professionals, it is essential to characterize the particles dispersed in the air (aerosol) or in a liquid (colloid).

To define the exposure limit values requires a choice of a unit of measure that is adequate to express the dose-effect relation. Consider, for example, a volume of air for which the theoretical particle distribution decreases from 104 particles.cm-3 for 10 nm to 200 particles.cm-3 for 100 nm.


Characterization in number, mass, or surface of a sample of the ambient air.

For a same ambient air sample, three representations of the distribution according to the particle size are presented:

  • the first distribution is expressed in number of particles,
  • the second in function of the mass of the particles classical historical unit for the ELV (“Exposure limit value”) / OEL (“Occupational exposure limit”),
  • the third in function of the surface of the particles → the most representative parameter of the reactivity?

The conversion between the various metricsis a source of error. It is preferable to selectthe correct device at the outset.