Answer to Question #8974 Submitted to "Ask the Experts"

The use of nanoparticles is becoming more prevalent in industry and research. They may become either activated, radioactively contaminated, or generated from radioactive material/nuclear fuel. There is some evidence that their chemical properties may affect internal absorption rates and could therefore alter the airborne limits in the industrial hygiene area. What, if any, research/evaluation/recommendations have been made in the health physics field, such as reduced airborne radioactivity limits? Additionally, what, if any, effect would the radioactive nanoparticles have on a HEPA (high efficiency particulate air) ventilation system performance and exhaust stack particulate filter sampling?

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