The uranium minefield

It is almost impossible for science to prove a negative, whether it's to show that mobile phones are not the cause of brain tumours, to demonstrate that the measles, mumps and rubella combined-vaccine does not cause autism or to prove that depleted uranium is not the cause of illnesses suffered by veterans of the Gulf war and Balkans conflict. Disproving something is about the hardest request that can be made of a scientist.

It is almost impossible for science to prove a negative, whether it's to show that mobile phones are not the cause of brain tumours, to demonstrate that the measles, mumps and rubella combined-vaccine does not cause autism or to prove that depleted uranium is not the cause of illnesses suffered by veterans of the Gulf war and Balkans conflict. Disproving something is about the hardest request that can be made of a scientist.

In the case of depleted uranium, however, scientists at least have the benefit of several studies, some going back 50 years, to investigate the health risks. This research may not provide all the facts necessary to answer the depleted uranium question, but at least it is a start. It is by understanding what these studies can, and cannot tell us, that has led many scientists to be sceptical of the claims that the depleted uranium shells and bullets fired in the Gulf War and the Balkans have caused illnesses ranging from respiratory disease to leukaemia.

Melissa McDiarmid, professor of medicine at the University of Maryland School of Medicine in Baltimore, is more familiar with this scientific literature than most. She has studied the effects of depleted uranium first hand, having cared for some of the 60 US soldiers who became accidental victims of these deadly munitions as a result of "friendly fire" during the Gulf War. The irony of the questions being raised over depleted uranium are not lost on her: "This is a surreal situation. We are being asked 'what are the health risks of something that is supposed to blow you up?'."

There are essentially two reasons why the military have chosen to make armour-penetrating bullets out of depleted uranium: it is cheap and highly effective. As a by-product of the nuclear industry, depleted uranium is abundant and freely available. Extremely dense, it packs an enormous punch as a "kinetic energy" penetrator, using its considerable momentum to crack open the toughest armour. Depleted uranium penetrators also self-sharpen during impact and are pyrophoric, meaning they ignite when slicing through several inches of toughened material.

Britain's main battle tank, Challenger 2, and the American A1M1 tank, are both armed with 120mm armour-piercing, anti-tank rounds made with depleted uranium. During the Gulf War, about 300 tons of depleted-uranium rounds were fired -- from 20mm bullets to 120mm shells -- from tanks, aircraft and ships.

Depleted uranium is chemically identical to uranium, a natural element extracted from uranium ore mined since the beginning of the nuclear age. Uranium is radioactive but its three isotopes have relatively long half-lives, the time it takes for half of its mass to decay to something else. Generally speaking, the shorter an isotope's half-life, the more danger it poses. The isotope uranium-238 accounts for about 99.3 per cent of the natural uranium found in the ground and its half-life is 4,500 million years, whereas uranium-235 and 234 have half-lives of 710 million and 250,000 years respectively.

This can be compared with one of the by-products of uranium decay, a gas called radon, which has a half life of 3.8 days and is some 10,000 times more radiologically active than its parent element.

The nuclear industry extracts as much of the more active uranium-235 and uranium-234 as it can from uranium ore to make the enriched raw material for nuclear fuel. What is left behind is therefore "depleted" and possesses only 60 per cent of the radioactivity of natural uranium, which is itself not very radioactive.

Over the decades, many thousands of workers have been involved in the mining, processing and milling the uranium ore used in nuclear reactors and atomic bombs. Concerns over the risks of dealing with such material in a working environment are therefore not new.

There have been 11 big studies of uranium miners who have been monitored for a range of cancers and other illnesses. In addition to the radiological hazard, scientists have also assumed that workers will be exposed to toxicological hazards. Uranium is a heavy metal and as such can cause serious poisoning problems if ingested in large enough doses. Heavy metals, for instance, are known to harm the kidneys.

However, when the US Centers for Disease Control and the Agency for Toxic Substances and Disease Registry reviewed the evidence revealed by these studies, they concluded that "no significant differences in cancer [of the lungs] was found between workers who are occupationally exposed to uranium and control populations".

A committee of the US National Academy of Sciences and Institutes of Medicine have also reviewed the scientific literature and again concluded that there is no evidence to prove that uranium exposure in these workers has resulted in cancer, but neither have the studies been able to rule it out unequivocally.

The difficulty of "disproving" a health risk with depleted uranium was also highlighted in a study by Steve Fetter, from University of Maryland, and Frank von Hippel, an eminent nuclear scientist at Princeton University in New Jersey. Their review of the scientific literature was one of the most extensive undertaken, and they looked at both exposure to soldiers and civilians living in the area were depleted uranium was used.

"Due to the low radioactivity of DU (depleted uranium), radiological hazards to individuals would become significant in comparison to background radiation doses only in cases of prolonged contact, for example, when shards of a DU penetrator remain embedded in a soldiers body," they wrote in a paper published in the journal Science and Global Security. "Although the radiation doses to virtually all civilians would be very low, the cumulative 'population dose' resulting from the dispersal of hundreds of tons of DU, as occurred during the Gulf War, could result in up to 10 cancer deaths."

"Our tentative conclusion is that concerns about the public health and environmental effects of DU are overblown. The risks appear to be very low to surrounding populations and to persons who were not in direct, unprotected contact with vehicles struck with DU munitions or areas heavily contaminated by burning DU munitions. DU contamination is unlikey to have any measurable effect on public health in Iraq or Yugoslavia," they say.

The ongoing study into the 60 American servicemen who were victims of friendly fire involving depleted uranium has also failed to identify a cancer risk, or any other illness not directly associated with being blown up by a deadly munition. Yet these people, about a quarter of whom still have DU shards embedded in their bodies, are known to have been exposed to the highest doses imaginable. "Thus, the argument for uranium being the cause of leukaemia in peacekeeping forces is thin," says Professor McDiarmid.

This will hardly be of comfort to the many servicemen who became seriously ill after returning from the Gulf and the Balkans. Neither will it impress the low-level radiation campaigners who believe that particles of insoluble oxides created when DU burns become lodged in the lung, where they can emit dangerous alpha radiation to surrounding tissue or to the cells of the lymph glands. They have suggested that this could account for the half dozen cases of leukaemia in Italian soldiers, even though cancer specialists find it difficult to believe that these blood cancers could have arisen so soon after the apparent time of DU exposure in Kosovo.

Professor McDiarmid is aware of the problems she and other scientific sceptics face: "You come off sounding as if you're dismissing what has happened to these young people. I am not. I think I do have an open mind and I don't want to miss something new that might be occurring but equally we cannot ignore what we already know."

As the British Government prepares to call in its own high-level committee of radiation advisers to look once again at the depleted uranium question, the first item on the agenda will be to pull together everything we have already learnt about this much-feared element.