3. What are the health effects of radiation?
Radioactive materials emit energy as alpha particles, beta particles, gamma rays, X-rays and neutrons. All can damage cells by acting directly on molecules or by ionising molecules which themselves cause damage. The energy which may be transferred by the different forms or radiation, and their penetrating power, is different. The sensitivity of different tissues also varies. These factors are taken into account in assessing the risks of known exposures.
Radiation can cause radiation sickness due to immediate tissue damage after high doses are delivered in a short time – a few minutes. Such damage may be seen after radiotherapy, industrial accidents, and use of nuclear weapons. It is produced by the effect of the incoming energy on rapidly reproducing cells, such as those in the lining of the gut, the bone marrow, and the skin.
Radiation sickness is only seen above a threshold radiation dose. Such doses are not expected to be seen from any imaginable pathways of exposure to depleted uranium.
The other result of radiation damage is random mutation, which increases the risk of cancer and birth defects. The risk is usually assumed to increase smoothly as the dose increases from zero – that is the dose-response is linear and there is no threshold where damage begins to show. That said, most of the evidence about increased cancer risks comes from people exposed to high radiation doses, such as the populations of Hiroshima and Nagasaki.
Studies have not shown any definite link between numbers of tumours and radiation doses within the normal range due to background radiation from natural exposures. This may be due to the difficulty of performing epidemiological studies involving such doses. At low levels of exposure, studies of cancer incidence do not provide direct evidence about the relationship between dose and risk. The number of extra cancers that might be expected to result from the radiation exposure is too small to detect. However, there is also some recent biological evidence for a threshold in carcinogenic effects for both radiation and chemical damage. This might be because cells can repair the damage, possibly because low doses of radiation stimulate cells’ natural repair mechanisms. However, permissible exposure limits for workers and the public are still set on the assumption of a linear response. For the public, they are well below the average annual dose from naturally occurring background radiation, from cosmic rays and minerals in rock and soil.