Lessons Learned from Unintentional Aerosols - Safety of Nanomaterials along Their Lifecycle - page 98

when appropriate technologies have been used to search for it. The paradigm for
evaluating these incidental aerosols is common to risk characterization for both
larger sizes of polydisperse distributions or of sophisticated, highly engineered nano-
materials. In all cases, we use the paradigm of hazard identification, dose-response
characterization, exposure assessment, cost-benefit analysis, and ultimately, risk
management. Also common is the importance of characterizing the particles in
terms of chemical form, size, shape, surface area, number, density, extent of agglom-
eration, porosity, charge, reactivity, solubility, durability, crystalline structure, and
other important and relevant physical and chemical properties.
Thus, although highly engineered nanomaterials are new and have unique and
unexpected attributes, humans have always been exposed to nanoparticles as the
smallest size fraction of polydisperse aerosols. This can be the result of wind, crush-
ing, grinding, and/or combustion. Since Prometheus discovered fire, humans using
fire have been exposed to nanoparticles. This is an important topic, because even
though they are novel, highly engineered nanomaterials lead to exposures for rela-
tively few individuals. Quantum dots and fullerenes, or even nanomedicines, account
for relatively few human exposures. Even commodity carbon nanotubes or metal
oxides rarely expose the public. In contrast, unintentional aerosols, like air pollution,
indoor cooking, and natural processes, lead to huge numbers of exposures. Everyone
on the planet is exposed to unintentional aerosols.
We point out that most essential questions, such as where do particles deposit, are
they cleared, do particles dissolve, and do constituents of particles reach other parts
of the body, and especially what biological responses are likely, are common to both
nanoparticles, and larger particles. We are concerned with the same organs, such as
the respiratory tract, the GI system, and skin. The laws of aerosol physics controlling
particle deposition, such as gravity, diffusion, and inertial impaction, are the same.
Many of the tools used in the laboratory are similar and independent of particle size.
Especially, the repertoire of biologic responses is similar. Mechanisms, such as the
importance of reactive oxygen species, are shared between nanoparticles and their
larger and more familiar cousins.
Finally, it is also the case that many nanoparticles that meet the bright-line of size
as individual particles are usually present in the form of agglomerates. They may be
nano in size initially, but when they are collected, shipped, and then reused, com-
plete dispersion of the particles is unlikely and often impossible. Thus, the public and
workers exposed to nanomaterials are most likely to breathe aggregates, which are
larger than the 100 nm bright-line.
It is important to understand the deposition, clearance, and translocation of unin-
tentional nanoparticles. There is no reason to think that the physical and biological
mechanisms that pertain to them are unique to unintentional aerosols. As detailed
elsewhere in this topic, there are considerable data on these topics in relation to
nanoparticles. These principles apply equally well to unintentional aerosols as they
do to highly engineered aerosols. Whether they are uniformly small or simply part of
the tail of the size distribution of polydisperse aerosols, they behave similarly. Like
all nanoparticles, unintentional aerosols may have an increased potential for translo-
cation from the air to the blood. The percent of particles that cross the air-blood bar-
rier remains small, but can be significant. Uptake by microphages generally makes