Mechanisms of Particle Deposition - Aerosols: Measurement, Dosimetry, and Health Effects - page 52

3.2.5 H ygroscoPicity
In human airways, particle deposition may depend on the physicochemical (hygroscopic) proper-
ties of inhaled substances. Hygroscopicity may be deined as a particle's propensity to absorb water
from a warm, humid environment, thereby changing its diameter and density. The degree and rate
of hygroscopic growth are inluenced by many factors, including chemical composition, tempera-
ture, relative humidity, initial particle size, and duration of exposure. 17
As the relative humidity within the lungs may approach saturation (99.5%), 18 hygroscopic effects
can substantially affect inhaled aerosols. As particle size and density are important factors in
determining deposition, hygroscopic growth is a critical concern in the risk assessment of inhaled
aerosols (inhalation toxicology) and the development of inhaled pharmaceutics (aerosol therapy).
Hygroscopic growth has been shown to occur in many aerosols, including a number of environmen-
tal pollutants and pharmacological agents. 19,20
Theoretical models are useful in characterizing properties (i.e., growth rate, equilibrium size,
aerodynamic diameter) of hygroscopic particles under different in vivo environmental conditions.
Various theoretical models of particle hygroscopicity have been developed for cigarette smoke, 21
aqueous droplets, 22 and soluble particles. 23
Martonen 24 derived equations for the density and aerodynamic diameter of a particle in different
lung airway generations, provided the particle growth rate, r g , is known. The density ρ i of a particle
entering airway generation i can be calculated as

3

⎡
⎢

d 0 ⎤

⎥

ρ = ⎣ ρ − ρ ⎦ + ρ (3.16)

i 0 H O H O

d i

2 2

where
ρ 0 is the initial particle density
d 0 is the initial particle diameter
d i is the particle diameter at the entrance to airway generation i , given by

⎛ ⎞

j −

∑

= i 1

L

j

d = d + r ⎜ ⎟ (3.17)

i 0 g

V j

⎝ ⎠

j = 0

where
L j is the length of airway j
V j is the mean velocity of the particle in airway j

The aerodynamic diameter of the particle in airway i is then given by

(3.18)

d

, = ρ i

C d

ae i c i

A particle's size and density may change while traveling through the respiratory system (Figure 3.1).
It may not be prudent, therefore, to expect the deposition pattern of inhaled aerosols to be deter-
mined by the pre-inspired sizes and densities of its constituent particles. The dynamic process
of hygroscopic growth must be accounted for during a breathing cycle. It is also interesting to
conjecture that evaporation may occur when aerosols from the warm, moist, deep lung are cooled
during expiration.
Both experimental and computational studies have been performed to determine deposition
patterns for inhaled hygroscopic particles in human airway networks. Experimental studies have