The reason for this breakdown, as well as the different isotopic proportions found in uranium used as fuel, has to do with nuclear physics: U235 can fission, U238 cannot easily fission. "During enrichment of uranium, to turn it into reactor fuel or highly enriched nuclear weapons material the enrichment process increases the amount of U235 to 3 percent in reactor fuel and more than 90 percent in weapons," Kristensen says. "The leftover U238 is referred to as depleted uranium."
Because DU contains much less U235 than natural uranium, it is less of a health threat, in terms of radioactivity, than both natural and fuel-grade uranium.
U238 is not radioactive in and of itself, but naturally decays and transforms to other elements, including lead, over time. "Some of those elements are radioactive, and one of them, radon (a gas), can be problematic because it can be inhaled and emits alpha particles, which, if embedded in the lungs, can cause cancer," Kristensen says. Still, U238 decays slowly—half of the material decays in 4.5 billion years—so the trace elements are miniscule."
WHO notes that the kidneys are most likely to be damaged from depleted uranium's chemical toxicity. Such damage would more likely result from ingestion of food and water containing uranium isotopes and inhalation of uranium-contaminated dust. External gamma exposure is generally not a major concern because uranium emits only a small amount of low-energy gamma radiation, and beta exposure is only of concern for direct handling operations, according to a study (pdf) produced in 2001 by the U.S. Department of Energy's Argonne National Laboratory.
Health questions persist
The health effects resulting from DU exposure depend on the route and magnitude of exposure as well as the metal's characteristics, such as particle size, chemical form and solubility, according to UNEP, which has studied the use of this material in armed conflicts in Kosovo (pdf), Serbia and Montenegro (pdf), and Bosnia and Herzegovina (pdf). The three studies concluded that, whereas radiation can be detected at DU sites, the levels are so low that they do not pose a threat to human health and the environment.
At the same time, however, the studies identified a number of remaining scientific uncertainties that should be further explored. These include the extent to which DU on the ground can filter through the soil and eventually contaminate groundwater, and the possibility that DU dust could later be resuspended in the air by wind or human activity, with the risk that it could be inhaled. These assessments of the Balkan wars were made two-to-seven years after NATO air strikes using DU weapons.
"Although our assessments to date, under conditions prevailing in the Balkans, have concluded that DU contamination does not pose any immediate risks to human health or the environment, the fact remains that depleted uranium is still an issue of great concern for the general public," former UNEP Executive Director Klaus Toepfer said in a statement in 2003.
Given the U.S. military's claims that it is no longer using DU weapons in Afghanistan and a lack of clear evidence that DU poses immediate and severe health risks, Karzai's comments are more likely politically motivated than grounded in science. "Domestically he has to shore-up his constituents by making a show of not toadying to the Americans," Gustafson says. "At the international level, he has to extract the best deal possible from NATO and the Americans. This means putting on the pressure in ways that he can to get his way with them, whilst ensuring they keep supporting him."