The Significance of Ultrafine Particles

The Significance of Ultrafine Particles

Reference: Ultrafine Particles in the Urban Air: To the Respiratory Tract - And Beyond? “Perspectives, Editorial” by Günter Oberdorster and Mark J. Utell. Environmental Health Perspectives, Vol 110, No 8, Aug 2002.

Ultrafine particles (UFP) were overlooked by the majority of investigators as a significant factor in respiratory tract health and health of other organs until 1994 when Oberdorster and Utell introduced the hypothesis that UFP could cause toxicity to the human respiratory tract.

These small particles (less than about 0.1 um in aerodynamic diameter) were previously considered to dissipate rapidly by aggregation. Particles less than 0.01 um may be termed “nanoparticles”. Neither of these small particles was thought to have toxic potential before the 1990’s. UFP can be naturally produced. However UFP are continuously generated from a variety of sources, particularly internal combustion engines using gasoline, diesel fuel, and compressed natural gas. All of these emit large numbers of UFP. Although UFP contribute little mass to PM₁₀ or PM_2.5, the numbers of UFP are relatively large as is their total surface area when observing an average urban air particle size distribution.

The authors suggest that regulatory agencies should be aware of all potential health effects of UFP when setting standards for vehicle emissions. Currently vehicle emissions are regulated by mass output. The less mass produced, the better the vehicle fits the standard and the safer it is considered. These standards favor UFP production. Implications of this result are discussed in this editorial.

Early studies in which humans inhaled small particles showed that there is a high deposition of UFP (0.007-0.1 um range) in the respiratory tract as predicted by ICRP 1994¹. When inhaled by exercising or asthmatic humans, there was more deposition in the human respiratory tract than under control experimental conditions. The conclusions of these studies indicated that UFP are not phagocytosed by AM (alveolar macrophages)^*. UFP were found to be translocated to organs other than the lung in rodents exposed to ¹³C particles. Both animals and humans exhibit mild cardiovascular effects after ultrafine carbon exposure. Experimental nimals also show mild pulmonary inflammation after ultrafine carbon exposure.

Further coordinated research is needed by toxicologists, epidemiologists, physicians, engineers and scientists in the fields of UFP generation, sources, characteristics, behavior in ambient air and when inhaled. Only then can a conclusion can be made regarding the danger inherent to humans from respiratory exposure to UFP of different sizes and concentrations. In less than a decade, it is obvious from this editorial that UFP may have effects on more than the respiratory system.

For example the authors question if the CNS could be a target of UFP? There does appear to be a transport of protein macromolecules from lung to blood via the alveolar-capillary barrier. In addition there is evidence that carbon UFP migrate from the rat olfactory bulb to the CNS. More evidence indicating that the lung can offer foreign particle access to the brain also exists.

Finally the authors point out that there is still much skepticism in the scientific community about UFP and their importance.

1 – ICRP, 1994. Human Respiratory Tract Model for Radiological Protection. A Report of the Task Group of the International Commission of Radiological Protection. ICRP Publication 66 (Smith, H, ed). Ann ICRP 24(1-3).

Editorial Note: This is one review of UFPs presented in this edition of the Inhalation.net newsletter. Another review* discusses effects of UFPs on the cytoskeleton of macrophages. This could help to explain how alveolar macrophage malfunction might occur after UFP exposure. There is another review in this issue of the newsletter of a study of the direct transport of a metal via the olfactory route to the brain. Although minimal transport was found, the study itself indicates a general interest in these small particles by the scientific community.**

It should also be emphasized that it was not that long ago that 5 um was considered to be the critical size for particle deposition and potential damage in the respiratory tract. It would seem likely that UFP were overlooked due in great part to lack of much of the technology currently available to study not only the particles but their effects. For example in the second edition of Casarett and Doull's "The Basic Science of Poison" published in 1980, it states regarding particles less than 0.5 um that "deposition in this size range is largely determined by diffusion characteristics of the particles. The size properties of these very fine aerosol particles can be determined by using diffusion batteries" (page 261). More recently the concepts of PM₁₀ and PM_2.5 have been incorporated into studies of air pollution and acceptable regulatory limits. It is not impossible to envision the concept of a UFP particle limit also being incorporated into regulations as the authors of the above editorial might suggest.

*Ultrafine Particles Cause Cytoskeletal Dysfunctions in Macrophages. Mőller, W., Hofer, T., Siesenis, A., Karg, E., and Heyder. J. Toxicol. Appl. Pharmacol. 182, 197-207. 2002.

**Brenneman, K. A., Wong, B. A., Buccellato, M. A., Costa, E. R., Gross, E. A., and Dorman, D. C. (2000). Direct Olfactory Transport of Inhaled Manganese (⁵⁴MnCl₂) to the Rat Brain: Toxicokinetic Investigations in a Unilateral Nasal Occlusion Model. Toxicol. Appl. Pharmacol. 169, 238-248. (Reviewed by A. Weiss, DABT)

By: Susan G. Shami, ScD

Science Editor