Lessons Learned from Unintentional Aerosols - Safety of Nanomaterials along Their Lifecycle - page 99

it less likely that they cross epithelial and endothelial cells, which separate inspired
air from circulating blood.
5.2 TYPES OF NANOPARTICLES
There are many different kinds of nanomaterials. We choose to broadly divide them
into three classes. First, there are nanomaterials that have been around for many
decades, such as colloidal gold and colloidal carbon (India ink). Second, many famil-
iar materials that are small and polydisperse have particles in the nano range. For
example, there is evidence that both diesel particles (Kinsey et al. 2007) and welding
fume (Sowards et al. 2008) contain nanomaterials. These nanofractions appear to
have an unusual ability to cross the air-blood barrier intact, where they are subse-
quently ingested by hepatic macrophages and other components of the mononuclear
phagocyte system that have access to the circulating blood. Finally, there is the class
of new, highly engineered materials such as nanofibers, fullerenes, and quantum
dots, as well as abundant nanomaterials designed for oncology or other pharmaceuti-
cal purposes. These are truly novel, and their potential toxicity is largely unexplored
(see Table 5.1).
Colloids are stable emulsions. For centuries, we have known of the existence of
such formulations. The particles are small and electrically charged enough that,
even though they may have a density greater than 1, they remain in suspension. The
molecular motion of solvent molecules (Brownian motion) is sufficient to keep them
suspended and their charged surfaces (repulsive forces) prevent them from aggre-
gating when particles approach each other. India ink is a stable colloid of carbon
nanoparticles. Majno and Palade used India ink, including some nanoparticles, to
characterize inflammation and to study the anatomic sites responsible for increased
vascular permeability (Majno and Palade 1961). Intravenously injected 198 Au colloids
were used decades ago to treat liver cancer. Brain and Corkery used nanoparticles
of 198 Au to quantify in situ lung macrophage phagocytosis rates (Brain and Corkery
1975). Unlike bare nanoparticles of gold, these nanoparticles (approximately 50 nm)
were coated with collagen. This charged proteinaceous surface made them easily
phagocytized by alveolar macrophages.
Further, there are many small particles, particularly those produced by combus-
tion or condensation, which are respirable. Usually, these polydisperse aerosols con-
tain particles that extend down into the nano range. We will emphasize these in this
chapter.
TABLE 5.1
Classes of Nanomaterials
1. Stable emulsions: colloidal gold, colloidal carbon (India ink).
2. Smallest particles of familiar polydispersed materials: welding fume, smoke, and diesel particles.
All have fractions less than 100 nm.
3. Highly engineered nanoparticles with unusual properties: fullerenes, quantum dots,
nanomedicines, carbon nanotubes.