"The so-called RBE, the relative biological effectiveness, for alpha particles is about 20 times higher than that of x-rays or gamma rays," says Tom Hei, also of Columbia University, who studies radiation and cancer. Brenner agrees that alpha-radiation is the biggest concern, but adds that its short range also makes it less harmful in some ways: "The alpha particles have to reach sensitive cells to be of any relevance. The distance they can travel in tissue or water or something like that is in tens of microns." In other words, if a person is exposed to alpha-radiation from the outside of the body¿from standing next to a pile of uranium, for example¿the alpha-radiation won't penetrate the skin, if it reaches the skin at all.
So what is important, then, is not so much the amount of radiation involved, but how much enters the body. The relevant unit for the impact of radiation on tissue is the Sievert (Sv), defined as the amount of energy given off in one kilogram of tissue. Another unit to describe the same thing is the Rem (100 Rem is equivalent to 1 Sv).
In a normal setting, a person is exposed to between one and three millisieverts (1Sv = 1,000 mSv) per year. If you were to stand about three feet from 1 kilogram of DU for one year¿the equivalent of about three A-10 shells¿you would be exposed to about one millisievert per year. But the tissues exposed would most likely be skin or fat¿neither of which are among the sensitive cells Brenner mentions above. Indeed, to do real damage, the radiation would have to reach tissue such as bone marrow.
Theoretically that could happen if a soldier got fragments of uranium embedded in his or her body through injury in combat. During Operation Desert Storm, about 30 soldiers were hurt when their tank was hit by "friendly fire" that contained depleted uranium. As a result of the incident, several soldiers were left with DU shrapnel embedded in their bodies. "Then perhaps the DU is right next to bone marrow, for example, so the alpha particles would have enough range to damage the blood cells," Brenner says. The soldiers' health is being closely monitored, but so far there is no evidence of any ill effects.
Ingested or Inhaled?
Of course, there are other ways that depleted uranium can enter the body. When DU projectiles hit a target, they partly burn up, creating uranium dust particles, or aerosols. Unlike southern Iraq, Kosovo and Yugoslavia are agricultural regions, and some observers have raised the concern that uranium dust particles might enter the food chain through crops.
According to the AC-Laboratorium Spiez, an independent laboratory that tests soil samples for the United Nations and other organizations, only about 17 percent of the DU particles found after a DU explosion are easily soluble, and might thus find their way into foods. Of those, only 2 to 5 percent are actually taken into the blood stream through the digestive system, making it a negligible source of radiation. "That would be the smallest possible source of exposure," says Brenner. "Because, again, the alpha particles would then be within some stuff, within liquid or whatever and it wouldn¿t have enough range to get out."
Apart from ingesting the aerosols, they can also be inhaled¿a potentially more harmful path. "When you inhale some of these particles¿for instance, in the case of radon, which is a decay product of uranium¿these particles give off alpha-radiation, which could cause lung cancer," Hei says. The correlation between radon and an increased risk of lung cancer was first discovered in uranium miners, who inhaled large quantities. As many as 75 percent of them got lung cancer. Radon gas also rises naturally from the soil, especially in regions with high granite concentrations such as the New York/New Jersey area.