Table 1– Definitions of commonly used terms that describe an aerosol
TermAbbreviationDefinition
Labelled dose or nominal dose#The mass of drug that is available within the aerosol generator per actuation. This is the dose that is metered.
Total emitted dose or delivered dose#TEDThe mass of drug emitted per actuation that is actually available for inhalation at the mouth.
Fine-particle doseFPDThe mass of particles <5 μm in size within the total emitted dose.
Fine-particle fractionFPFThe fine particle dose divided by the total emitted dose.
Aerodynamic equivalent diameterdaeThe diameter of a fictitious sphere of unit density (1 g·cm−3) that has the same gravitational (settling) velocity in the same gas as the actual particle.
Mass median aerodynamic diameterdae,μm or MMADThe MMAD divides the aerosol size distribution in half. It is the diameter at which 50% of the particles of an aerosol by mass are larger and 50% are smaller.
Geometric standard deviationσg or GSDThe GSD measures the dispersion of particle diameter and is defined as the ratio of the median diameter to the diameter at ±1 sd (σ) from the median diameter. In a cumulative distribution plot of the aerodynamic diameter and mass of particles, the GSD is calculated as the ratio of the median diameter to the diameter at 15.9% of the probability scale, or the ratio of the diameter at 84.1% on the probability scale to the median diameter. Aerosols with a GSD ≥1.22 are considered polydisperse. Most therapeutic aerosols are polydisperse and have GSDs in the range of 2–3.
  • #: lung deposition can be presented as a percentage of the nominal or emitted dose. Note that these two parameters are not the same.