There is a lot of information available already. Different isotopes of uranium have exactly the same chemical and biological behaviour, which is why chemical methods cannot be used to separate them to produce enriched uranium. Therefore the chemical toxicity of DU is the same as that of natural uranium. The radiological toxicity of DU is lower than that of natural uranium, because the specific activity is lower. When uranium went into large-scale production to produce reactor fuel, the possible chemical and radiological hazards were recognised. Animal experiments were carried out to investigate them. These experiments (mostly carried out many years ago) showed that if the exposure was high enough, the most likely effect was damage to the kidneys.
Estimates of the risks associated with exposure to ionising radiation are based mainly on studies of people who were exposed to high levels of radiation. The most important study is that of the survivors of the atom bomb attacks on Japan, because this is a large group, including all ages, a wide range of doses, and the whole body was irradiated. Furthermore, the health of these survivors has been studied over several decades. However, studies on various other groups of patients and workers, and results of animal experiments, are also used in assessing radiation risks. These include internal as well as external exposures. In particular, bone cancers were seen in workers who ingested large amounts of radium while applying luminous paint to dials in the early part of the 20th century. Radium deposits in bone in a similar way to uranium, but has a far higher specific activity, and so ingestion of relatively small amounts can give high doses to bone. Using all this information, the risk of cancer from any radiation exposure (external or internal) is estimated from the amount and type of radiation each organ receives (per unit mass). Excess radiation-induced cancers cannot be seen at very low doses either in human studies or animal experiments, because the excess at low doses is small, and the same types of cancers occur naturally. For radiation protection purposes it is generally assumed that the risk of cancer is proportional to the radiation dose: if the dose is halved, the risk is halved. Some scientists believe that there is a threshold for radiation effects, partly because life evolved in a radioactive environment, and so it is reasonable to expect that at low doses the body would repair any radiation damage. NRPB, however, supports use of the assumption that all radiation doses, however small, carry some additional risk, which is proportional to dose.
An exception to the standard dosimetric approach to assessing radiation risks is made in the case of radon, a radioactive gas, which for most of the population gives rise to about half the dose from natural background radiation. A clear excess of lung cancers, which increases with increasing exposure to radon, is seen in groups of miners who were exposed to high levels of radon. Risks from radon are based on the excess lung cancers in these miners, because the comparison is more direct than the standard approach, which predicts rather more cancers than are seen in the miners, i.e. it seems to somewhat overestimate the risk in this case. Risks from radon at lower levels are again based on the assumption that the risk is proportional to the exposure.
Many thousands of workers have also been exposed to uranium compounds over many years, through the processing of uranium from the ore to the production of fuel elements. Studies have been carried out on the health of such workers. While some studies have reported excesses of cancers, unlike the miners, no clear excess of any cancer related to increased exposure has been demonstrated. The only clear finding is a 'healthy worker effect'; mortality is lower than in the general population. This is expected in such workforces, because of selection for employment, and the benefits of a regular income.
