http://www.environmentalhealthnews.org/ehs/newscience/inhaled-uranium-enters-brain-through-the-nose/

Uranium travels nerves from nose to brain.

Jul 31, 2009

Tournier, BB, S Frelon, E Tourlonias, L Agez, O Delissen, I Dublineau, F Paquet, and F Petitot. 2009. Role of the olfactory receptor neurons in the direct transport of inhaled uranium to the rat brain. Toxicology Letters doi:10.1016/j.toxlet.2009.05.022.
Synopsis by Paul Eubig, DVM

Radioactive uranium that is inhaled by soldiers on the battlefield and by workers in factories may bypass the brain's protective barrier by following nerves from the nose directly to the brain.

Nerves can act as a unique conduit, carrying inhaled uranium from the nose directly to the brain, finds a study with rats. Once in the brain, the uranium may affect task and decision-related types of thinking.

This study provides yet another example of how some substances can use the olfactory system – bypassing the brain's protective blood barrier – to go directly to the brain. Titanium nanoparticles and the metals manganese, nickel, and thallium have been shown to reach the brian using the same route.

Military personnel and people who work in uranium processing plants are exposed to the weak radioactive element via wounds or by breathing. Exposure may affect brain function; cognitive skills are lowered in soldiers who carry uranium-laced shrapnel.

Uranium has various industrial and military uses. A form of uranium called depleted uranium is very dense and is used in armor-piercing ammunition and military vehicle armor.

Battlefield exposure can occur through wounds – such as with some US military personnel who were injured during the Gulf War. These exposures can be higher than with civilians who work with the element. A study of Gulf War veterans who have uranium shrapnel in their bodies showed that they perform more poorly on general brain cognitive tests of performance efficiency and accuracy.

Uranium can also be inhaled. Soldiers in vehicles hit by uranium rounds and workers in uranium-processing facilities can breathe it in.

The researchers – taking advantage of the fact that uranium can exist in different forms, or isotopes – used rats to compare how the element travels through the body if it is inhaled or injected into the blood.  The animals breathed in one isotope at levels similar to those encountered on a battlefield where depleted uranium weapons are used. They were also injected with a different isotope. Researchers compared the levels of the two isotopes in different regions of the brain.

The inhaled isotope accumulated at 2 to 3 times higher levels than the injected isotope in the olfactory (smell) paths from the nose to the brain and in the frontal cortex and hypothalamus of the brain. This is concerning because the front part of the brain controls executive function, which is the broad ability to gather information, make decisions and initiate action.

The scientists then chemically damaged the olfactory nerves in the nose. The rats with the damaged nerves had three times less uranium in the olfactory system than the rats with intact olfactory nerves.

These finding suggests that inhaled uranium can travel directly from the nose along the olfactory nerves to the front of the brain. The olfactory pathway, then, plays an important role in inhaled uranium reaching the brain.

It is not known from this study if soldiers and civilian workers that breathe uranium could be at an even higher risk for cognitive effects or if inhaled uranium may affect brain function in similar ways as when it is carried through the blood. It is also unclear if these findings would hold true for the human brain since the rat brain is much more developed for smelling than the human brain.

Assessing these possible risks and determining if people's relatively underdeveloped sense of smell could protect the brain would require further studies of people exposed to uranium through inhalation.