Lessons Learned from Unintentional Aerosols - Safety of Nanomaterials along Their Lifecycle - page 100

5.3 CHARACTERIZING UNINTENTIONAL AEROSOLS

5.3.1 s ize -s eleCtive s amPlers
We need more research specifically designed to look at nanoparticles from these
familiar polydisperse sources. This requires size-selective devices, which collect and
characterize particles smaller than 100 nm. Second, there is a larger literature, for
example, on PM 2.5 and PM 10 , in which the epidemiologic studies for humans or inha-
lation toxicology papers dealing with studies for animals include nanoparticles but
the major mass component is larger in size.
It is important to remember that mass is proportional to the cube of the linear
dimension. Thus, the same mass of a given material has widely different numbers
of particles depending on their size. If you have identical masses of 10 micron,
1 micron, 0.1 micron, and 0.01 micron particles, each order of magnitude is associ-
ated with a 1000-fold increase in the number of particles needed to produce the
same weight. Thus, you need to have a million 100-nm particles to equal the mass
of a single 10-micron particle. If the particles are only 10 nm in size, then a billion
particles will be needed to have the same mass as a single 10-micron particle.

5.3.2 s eleCting u nintentional a erosols from P olydisPerse P artiCles
A problem with studying the biologic responses to unintentional nanoparticles is the
lack of samples of characterized reagents. By definition, we need to deal with a poly-
disperse aerosol and select only the ultrafine particles (UFPs) (those smaller than
0.1 μm). A key challenge is to create methods for selecting these particles and using
them to expose humans and animals to “real world” ambient UFPs. A number of size
selective devices exist to collect UFPs on filtered media. The challenge is then to
remove them quantitatively and without altering the particles. A far better approach
is to select the UFPs aerodynamically and to keep them suspended for subsequent
inhalation exposures.
Demokritou et al. have developed an ultrafine particle concentrator. It has been used
extensively to conduct both human and animal inhalation studies of concentrated ambi-
ent UFPs, typically urban air, which is dominated by both mobile and stationary sources
(Demokritou et al. 2002a; Demokritou et al. 2002b; Gupta et al. 2004). The design first
selects UFPs by their aerodynamic diameter and then grows them to a supermicron
size by using them as nuclei for growth in high-humidity environments. These larger
particles can then be concentrated with a series of virtual impactors. Finally, the con-
centrated aerosol can be returned to its initial ambient size distribution using a thermal
“reshaping” process. The device operates under a wide range of ambient air tempera-
tures and relative humidities. Particle losses in the system were about 10%.
Scientists at the United States Environmental Protection Agency (EPA), as well as
at universities such as the University of Rochester and the University of Toronto, have
used this system successfully. This device represents an evolution of an earlier device
that characterizes ambient particles in the fine range (0.1-2.5 μm) (Demokritou et al.
2002a). This device is widely known as the Harvard Ambient Particle Concentrator.
An excellent review of how to measure fine, coarse, and UFPs has been prepared by
Sarnat et al. (Sarnat et al. 2003; Demokritou et al. 2003).