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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.

What is the relevant distribution?

231_distribution_surface

Number and volume distributions of the same aerosol.

231_distribution_volume

Number and surface distributions of the same aerosol.

The distribution in volume, representative of the distribution in mass, of these particles is significantly different from the distribution in numbers. To establish thresholds in mg/cm³ may mask the impact of the smallest particles that may be the most dangerous.

Surface distribution?

If one thinks that the total surface available for the particles is a representative reactivity parameter, we will prefer to characterize the aerosol by its distribution on the surface whose shape can be observed in our example.