The US says it has no plans to remove the debris left over from depleted uranium (DU) weapons it is using in Iraq.
It says no clean-up is needed, because research shows DU has no long-term effects.
It says a 1990 study suggesting health risks to local people and veterans is out of date.
A United Nations study found DU contaminating air and water seven years after it was used.
DU, left over after natural uranium has been enriched, is 1.7 times denser than lead, and very effective for punching through armoured vehicles.
When a weapon with a DU tip or core strikes a solid object, like the side of a tank, it goes straight through before erupting in a burning cloud of vapour. This settles as chemically poisonous and radioactive dust.
Both the US and the UK acknowledge the dust can be dangerous if inhaled, though they say the danger is short-lived, localised, and much more likely to lead to chemical poisoning than to irradiation.
But a study prepared for the US Army in July 1990, a month before Iraq invaded Kuwait, says: "The health risks associated with internal and external DU exposure during combat conditions are certainly far less than other combat-related risks.
"Following combat, however, the condition of the battlefield and the long-term health risks to natives and combat veterans may become issues in the acceptability of the continued use of DU."