Besides lead, depleted uranium is another heavy metal used by the military that has many environmentalists worried.

The highly dense substance is used in armor-piercing cannon shells and as armor that protects U.S. service members from projectiles. The M-1 Abrams tank, the Bradley Fighting Vehicle and the Air Force's A-10 Warthog are among the weapons that fire munitions made from depleted uranium. DU armor also is found in some models of the M-1 Abrams.

Pentagon officials say DU is about 40 percent less radioactive than natural uranium and does not does not pose a risk if it is handled properly. Dust particles formed when a DU round explodes can cause health problems if inhaled, according to military officials, but they say the particles are typically washed away when it rains.

But many critics, including veterans of the Persian Gulf War, suggest that DU poses more of a threat than the Pentagon acknowledges and have called for more research. Some European allies also have raised concerns.

A report by the United Nations Environment Program made public this month concluded that there was no widespread ground contamination in areas where depleted uranium ammunition rounds were fired during the 1999 Kosovo conflict, but the report also recommended several precautionary measures to protect local residents. U.N. officials said DU rounds that remain buried several meters into the soil could contaminate groundwater and drinking water.

Army officials said research is under way into possible alternatives to larger munitions, such as 120 mm armor-piercing rounds used by the Abrams tank that propel a depleted uranium "penetrator" at hyper-velocity.

Air Force officials, however, say they are not exploring alternatives to the DU round fired by the A-10 Warthog's 30 mm – another bane of Iraqi armor during the Persian Gulf War.

Navy ships still carry some depleted uranium ammunition for the "PHALANX Close-In Weapons-System," a Gatling gun that spews 20 mm cannon shells and is designed to knock out missiles and low-flying aircraft. The Navy is slowly phasing out its DU ammunition in favor of tungsten rounds.

Although tungsten is lighter, tests demonstrated that it was as effective as the DU munitions against the likely targets of the PHALANX system. And unlike DU munitions, tungsten rounds do not require special handling aboard ships.

Some military experts suggest that there is some reluctance to discard DU munitions, which have proved to be highly effective on the battlefield - even with potential health risks and environmental concerns.

"As was demonstrated so clearly in the Persian Gulf War, the combination of the extra range and the penetrating power just blew the lids off of every type of tank the Iraqis had," said retired Army Col. Dan Smith, chief of research at the Center for Defense Information.