Health physics concerns with nanoparticles

You are certainly correct in your
inference that radioactive nanoparticles are a potential concern. Health
physicists have dealt with various forms of radioactive nanoparticles for many
years. Some have been generated by common physical processes and have
constituted possible health physics concerns—e.g., dispersal of a radioactive
gas into air and subsequent decay in air to a second solid radioactive species.
Such species are born as free atoms, themselves subnanometer particles, some of
which may remain as such long enough to be inhaled and some of which might
attach to larger stable nanoparticles in the air and also be inhaled.

It has also been demonstrated that
particulates, often several to tens of microns in diameter, composed of certain
alpha-emitting radionuclides dispersed into air and collected on high-efficiency particulate filters, have undergone alpha decay on the filter and
released smaller conglomerates of atoms into the moving airstream (some of
which were nanoparticles) by multiple bond breakage from energy delivered
through the alpha-recoil process, with ultimate penetration of some such
species through the filter medium.

In such instances in the past, particles were generally not specifically identified as nanoparticles, although they
may have met the size definition of what constitutes such particles. Nanoparticles
today generally are defined as falling in the size range from 1 to 100
nanometers.

We should note that the current size
description of a particle as a nanoparticle refers, essentially, to one
dimension. Thus a cylindrical-shaped nanoparticle, such as a carbon nanotube,
may have a diameter of less than 5 nanometers but may have a length millions of
times greater. Other nanoparticles, such as the well-known carbon buckyballs
(buckminsterfullerene) are more uniform in shape. When we currently speak about
nanoparticles, it is these and similar species that we have in mind. There are
numerous articles in the literature and references on the Internet that discuss
such particles and many that refer to various implications of radioactive
nanoparticles. A large number of these articles describe practical applications
of such radioactive particles, such as the use of certain nanoparticles to
transport radioactive atoms to a specific target for purposes of radiation
therapy.

You are correct in your apparent concern
that more attention should be paid to special considerations involving possible
exposure to certain radioactive nanoparticles. For instance, a radioactive
species inhaled into the respiratory tract as a small particulate, possibly
with the radioactive atoms attached to dust or other small particles, may
behave differently from the same radioactive species sequestered within a
carbon buckyball. Buckyballs can be generated by simple combustion of most
carbonaceous materials, and it is very conceivable that a fire involving
radioactive material might yield some airborne buckyballs encasing radioactive
species.

Radioactive nanoparticles have
received a bit more health physics attention in the past couple of years than
had been the case, but the dominant health physics literature is still rather
sparse. For example, doing a check of the abstracts of the 2009 annual meeting
of the Health Physics Society, I found five papers that included nanoparticles
as a major topical item; I found none for the 2007 meeting.

Regarding collection of nanoparticles
by HEPA filters, there is no reason to believe that nanoparticles would be
collected with any less efficiency than other larger particles. In fact, HEPA
systems used for air cleanup typically are penetration-tested with an aerosol
size that is deemed to be the most difficult to collect (usually 0.2 to 0.5
micrometers in diameter). Any sizes smaller than the test size should be
collected more efficiently because of enhanced particle diffusion in the
airstream. Similarly, I would not expect the collection efficiency of high-efficiency filters used in stack sampling and other air-sampling procedures to
be negatively affected by the presence of nanoparticles in the sampled air.

As applications of radioactive
nanoparticles continue to grow, I would expect to see more analytical work to
better define some of the possible changes (from what we practice with more
conventional particles) in protection considerations that might require
implementation. Thanks for a timely question.

George Chabot, PhD, CHP