# Mass median diameter

Fifty percent of the mass of material in the aerosol is associated with particles of diameter greater than the MMD. For a log-normal distribution with geometric standard deviation [math]\sigma_g[/math] the MMD can be calculated from the count median diameter (CMD):

[math] MMD = CMD + 3 \exp((\ln \sigma_g )^2) [/math]

If the material is of uniform speciﬁc activity the MMD will be equal to the activity median diameter (AMD). Fifty percent of the activity in the aerosol is associated with particles of diameter greater than the AMD. The relationship of particle diameter to particle aerodynamic diameter and hence of the AMD and CMD to the activity median aerodynamic diameter of an aerosol formed from the particles depends on the density and shape of the particles and on how the diameters were measured.

ICRP Publication 130, 2015

**Return to Glossary**