President Obama has called for the withdrawal of 33,000 U.S. troops from Afghanistan over the next year and the remaining 68,000 by the end of 2014, but questions linger regarding what the troops are leaving behind after more than nine years of combat. In particular, Afghanistan President Hamid Karzai has accused U.S. and NATO-led coalition troops of littering his country with weapons that use "nuclear components."

Karzai made this comment last week during an address to the Afghanistan Youth International Conference, throughout which he broadly criticized coalition forces and pointed out that the U.S. has been in negotiations with the Taliban in an attempt to end the fighting set off by the September 11, 2001, terrorist attacks. U.S. Secretary of Defense Robert Gates, during an appearance June 19 on CNN's State of the Union news program, confirmed such negotiations had taken place. Less clear, however, are exactly which weapons Karzai was referencing and their long-term impact on the Afghani people and their country.

Karzai's comments likely refer to ammunition that uses depleted uranium (DU) to pierce armor or, conversely, to strengthen armored vehicles, according to scientists as well as intelligence and policy analysts. They also note that DU is not "nuclear" in the sense that brief exposure to it would not cause radiation sickness or cancer in the way that fallout from a nuclear warhead or meltdown would. DU, the main by-product of uranium enrichment, is a chemically and radiologically toxic heavy metal that is "mildly radioactive," with about 60 percent of the activity of natural uranium, according to the United Nations Environment Programme (UNEP).

"In short, DU munitions are not even remotely on the same scale of danger as having a war in the first place," says Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies and publisher of the ArmsControlWonk blog, which addresses disarmament, arms control and nonproliferation.

U.S. Central Command (CENTCOM), a Unified Combatant Command unit of the U.S. armed forces whose territory includes the Middle East, claims that no DU weapons are currently being used in Afghanistan, although a spokesman acknowledges that "DU-type munitions were used in Iraq in anti-tank and anti-armor weapons." The U.S. military itself  has reported on its use of Fairchild Republic A-10 Thunderbolt II jet fighter aircraft in Afghanistan. Whereas the A-10's standard 30-millimeter rounds normally contain DU, CENTCOM says that the A-10s in use in Afghanistan are not using DU munitions.

Why use DU?
"Wherever we send our A-10s, soon enough we hear reports of uranium contamination thanks to depleted uranium," says Chris Bronk, an information technology policy research fellow at Rice University's James A. Baker III Institute for Public Policy and a former U.S. State Department diplomat. Still, it is unclear how much DU ammunition has actually been used in Karzai's country (either by the U.S. or its NATO allies) and the long-term impact of DU on the environment, he adds.

DU kinetic-energy rounds are an effective way of penetrating armored vehicles. "You want something dense, and DU is denser than lead, something on the order of 1.6 times the density of lead," says Kristian Gustafson, deputy director of the Brunel Center for Intelligence and Security Studies (BCISS) at West London's Brunel University. "You've now upped your energy transfer by significant quantity." Still, U.S. and NATO air-strike targets in Afghanistan are more likely to be mud–brick buildings than armored vehicles, and DU rounds "are useless for anything other than smashing armor," he adds.

DU is used in anti-tank shells because it is a heavy metal that can slam through shielding plates on armored vehicles, agrees Hans Kristensen, director of the Federation of American Scientists' Nuclear Information Project.

How dangerous is DU?

The DU used in munitions is neither the same as natural uranium ore nor the radioactive uranium used in a nuclear reactor. DU is mostly composed of the isotope uranium 238 (U238); its more radioactive content, U235, is at least three times less than that of natural uranium, according to the World Health Organization (WHO). "Natural uranium ore contains almost entirely U238 but also a small amount of U235," Kristensen adds.

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."