Activity median thermodynamic diameter

A value of aerodynamic diameter such that 50% of the activity in the aerosol is associated with particles of thermodynamic diameter (dth) greater than the AMTD. The AMTD is used when deposition depends principally on diffusion, typically when the AMAD is less than about 0.3 μm. (Adapted from ICRP Publication 130, 2015)

ICRP Glossary entry - May 2019