Fundamentals of Particle Dynamics - Computational Fluid and Particle Dynamics in the Human Respiratory System - page 145

suspended in air for a sufficient amount of time. In the study of inhalation toxicology
and particulate matter, we can categorise particles by their size. Thus we have coarse
particulate matter, PM (2.5-10

μ m), fine PM (<2.5 μ m) ultrafine particles (<0.1 μ m),

and nanoparticles (<100 nm). Particles greater than 5-10 μ m are usually removed

by the upper respiratory system. Particles smaller than 5 μ m, however, can penetrate

deep into the lung and become a health hazard.

6.3.1 Particle Shape and Equivalent Diameters

Particles that are spherical in shape are simply defined by their diameter. For non-
spherical particles, a range of equivalent diameters or descriptive diameters are used.
For excipients and drug products, we can consider an image of a drug particle for
analysis under optical microscopy which produces the particle in its two-dimensional
projection. From this image, the Feret diameter is defined as the maximum distance
from edge to edge of a particle in a pre-defined direction. The Martin diameter is
the length of the line in a pre-defined direction which separates a particle into two
equal portions. The projected area diameter is the diameter of a circle having the
same projected area as the particle in question (Fig. 6.6 ).

Fig. 6.6 Diameter descriptors
using optical microscopy for
a non-spherical particle

Particulate matter and aerosols consist of a large number of particles/droplets.
Different mean diameter descriptors are used to represent an equivalent diameter for
the group of particles that is being analysed. Let us consider three particles having a
diameter of 3, 4, and 5

μ m, respectively. The arithmetic mean is

d i
n

D 10 =

(6.5)