CHICAGO (Reuters) - Does depleted uranium pose a cancer risk, or is it a benign legacy of modern warfare?

 Many scientists believe the low-level radiation emitted by depleted uranium, used in armor-penetrating NATO munitions in the Balkans and the Gulf War, is too weak to be carcinogenic.

 But while most dismiss the "Balkans Syndrome" cancer scare among peacekeeping troops patrolling old battlegrounds, they say a more immediate concern is kidney damage from ingesting the metal, which is denser and heavier than lead.

 Close monitoring of 63 US Gulf War veterans wounded by "friendly fire" show the depleted uranium (DU) shrapnel in their bodies that surgeons could not remove has yet to cause any cancers. While the veterans have high levels of uranium in their urine, none as yet has suffered kidney damage or cancer, nor have they fathered children with birth defects, said Dr. Kelley Brix, deputy chief of research and development for the US Department of Veterans Affairs.

 The most common scientific argument made against the existence of Balkans syndrome is that the radiation from DU munitions have added only 1% to background radiation normally absorbed from the ground, food and other sources.

 "Whether we like it or not we live in a sea of uranium. What the soldiers were exposed to is much lower than the naturally occurring level," said John Boice, scientific director of the International Epidemiology Institute near Washington and an expert on radiation exposure.

 But he was speaking before traces of highly radioactive plutonium were detected in DU munitions, reigniting the furor.

 Scientists say it is difficult to gauge the risks from DU in the Gulf War because of simultaneous exposures to a veritable "toxic soup" in the battlefields.

 Possible exposure to chemical and biological weapons, smoke from Kuwaiti oil well fires, an array of powerful pesticides and the controversial vaccines and anti-nerve gas pills administered to soldiers have all been blamed for a range of ailments collectively known as Gulf War syndrome.

 LAB RESULTS PROMPT WORRIES

 Even the most sanguine scientists say more research is needed to clarify laboratory findings that reveal genetic abnormalities in cultured human cells exposed to DU.

 "Depleted uranium is a transforming agent to cells in cell cultures...(causing) measurable genomic instability," said Alexandra Miller, a scientist at the Armed Forces Radiobiology Research Institute in Bethesda, Maryland.

 Genetically damaged cells can die, get repaired by the body's enzymes or replicate wildly and grow into a tumor.

 Exposed cells may exhibit abnormalities after dividing only twice, or after 40 divisions, Miller said. "It's the offspring that manifest the damage and it is measured primarily in alpha particle exposures and not as readily with gamma."

 Gamma radiation is emitted by more highly radioactive uranium-235, one of the isotopes separated from natural uranium for use in nuclear fuel or nuclear weapons. The relatively small gamma particles can pass through the body.

 In contrast, depleted uranium, which makes up more than 99% of natural uranium and is left over after the more radioactive isotopes are extracted, is primarily an alpha particle emitter. The relatively large alpha particles, which were compared to basketballs rolling on a billiard table, are blocked quickly and cannot even penetrate clothing or skin.

 But DU could gain entry to the body through the lungs. Upon impact, shards from pulverized DU ammunition catch fire and burn, leaving an extremely fine dust that can be inhaled.

 Based on animal studies, DU is highly soluble once inside the body, and can spread to the organs or to bones including the skull, Miller said. It is not clear how inhaling DU that has been oxidized by fire might react inside the body.

 Higher-than-normal lung cancer rates among uranium miners, especially smokers, is blamed on years of inhaling radon gas that permeates underground shafts, Boice said. A product of decaying uranium, radon gas decays much faster than DU, meaning it emits many more alpha particles.

 "The radioactivity is indeed less (in DU versus radon), but on the other hand an alpha is an alpha. If it traverses a cell it causes the same type of damage," Miller said.

NO HEALTH EFFECTS SEEN IN URANIUM PROCESSORS

 Boice pointed to piles of evidence accumulated from uranium processing workers that reveals little risk of cancer even after years of inhaling uranium dust, which is referred to as "yellow cake" in the industry for its sweet taste.

 "These (uranium processing) workers have ingested or inhaled uranium since the early days of the (1940s-era) Manhattan Project and have demonstrated no health effects, either in terms of cancer or kidney damage," Boice said.

 Miller, Boice and other scientists agree that the dozen leukemia cases among European peacekeepers serving in the Balkans cannot be blamed on DU exposure because the illness requires at minimum two years to develop.

 But Boice and Miller disagree about whether DU can migrate to the bone marrow, where leukemia originates.

 Iraq has blamed post-Gulf War outbreaks of cancer and birth defects on what it terms NATO "cancer bombs."

 Doug Rokke, an environmental physicist involved in Defense Department research into DU and assigned to clean up Gulf War vehicles hit by friendly fire, insists he and other members of his team have been sickened by DU exposure.

 Rokke termed government denials a "massive coverup." In his battle to win government medical care for himself and his colleagues, he said their illnesses were a product of the "toxic mess" left from the 1990-91 war.

 "We have cancer, respiratory problems, birth defects, rashes, kidney problems, eye problems and immune system problems to mention only a few of those who were exposed to depleted uranium," Rokke said.