Health Effects of Ambient Ultrafine Particles - Aerosols: Measurement, Dosimetry, and Health Effects - page 210

Those particles that penetrate to the thorax are also very eficiently deposited. The deposition
of UFPs in the human tracheobronchial airways was measured in a hollow airway cast for particle
sizes between 40 and 200 nm by Cohen et al. [22]. The data indicated higher deposition for the
smaller diameter particles, again conirming diffusion as the primary deposition mechanism.
Additionally, this work demonstrated that deposition in the tracheobronchial airways was higher
than predicted by diffusional deposition in a tube, assuming a fully developed or parabolic low
proile. Subsequent studies showed that the deposition of iodine vapors with a diffusion coeficient
of 0.08 cm 2 s -1 , used as a surrogate for an approximately 1.8 nm particle, agreed with the theoretical
prediction of diffusional deposition [23]. The same research group also showed that the charge on
UFPs enhances deposition in the airway replica [24,25], that is, charged particles and particles in
charge equilibrium have higher deposition eficiency in the tracheal region as compared to neutral
particles of the same size. The ratio of deposition eficiency for charge-equilibrium and neutral
particles was 1.6 and 2.7 for 20 and 125 nm particles, respectively [26]. This is important because
most ultraine ambient particles carry one, or a few charges.
The deposition in airway casts for 1.75, 10, and 40 nm particles was measured at low rates
corresponding to respiratory minute volumes at rest and during moderate exercise [27]. Replicate
casts of the upper tracheobronchial airways of 3, 16, and 23 year old humans were used, including
the larynx, trachea, and bronchial airways down to generations 5-8. The deposition of the 1.75 nm
particle was substantially higher than that of the 10 and 40 nm particles. The dependence of particle
deposition on the low rate was relatively weak, and deposition eficiencies were only slightly higher
at the lower low rates. The deposition models for diffusion from parabolic low underestimated
aerosol deposition, whereas the diffusion deposition predicted for plug low overestimated the tra-
cheobronchial deposition. This is in agreement with the earlier studies [22].
8.5 TOXICOLOGY
Submicrometer particles with demonstrated health effects include diesel exhaust, radon progeny,
cigarette smoke, metal fumes, acidic aerosols, and trace metals. When generated, these primary
aerosol particles are ultraine. Additionally, biofragments such as endotoxin extend into this size
range. Table 8.2 shows the toxic effects demonstrated when UFPs are inhaled.
A substantial body of experimental data on the toxicity of ambient particles has developed in
recent years as an indirect result of The Clean Air Act enacted by the United States in 1970. This
act established the Environmental Protection Agency and mandated the setting of Primary Ambient
Air Quality Standards that would protect the public against adverse health effects of ubiquitous
pollutants (such as ambient PM) with an adequate margin of safety. The Clean Air Act Amendments
of 1977 then required that the air quality standards be reviewed at 5 year intervals and revised as
necessary.
Evidence has accumulated that implicates UFPs as a cause of the adverse effects of exposure to
ambient PM, but the mechanisms are as yet unclear. Mechanisms that have been proposed for the
induction of lung injury include irritant signaling [28], acid effects [29], and inlammation [30]. It
is clear that UFPs generally exhibit greater toxic potency than larger particles of the same material
[31]. Table 8.3 provides a summary of experiments that compared responses to UFPs with response
to larger diameter particles of the same material.
Further research into the effects of UFPs has been stimulated by the introduction of engineered
manufactured nanoparticles into the economy, inevitably spreading them into the environment and
biosphere as a whole [32]. A working deinition of engineered UFPs is given by Warheit (2010), as
those with diameters of roughly 100 nm that exhibit a property that is uniquely different from that of
the bulk counterpart. Given the relatively similar size of cellular components, the unique properties
can have substantial implications for biological response.
Other suggestions for the increased potency are that (1) they are biologically more reactive, (2)
there is a much higher number and surface area for the same total particle mass, and (3) they deposit