a tumour. Damage to the ovum or sperm cells can in theory lead to offspring of the irradiated person being affected, however no such hereditory effects at levels that are statistically significant have, as yet, been observed in humans (refs 2, 3 and 4).
Exposure to high levels of radiation can lead to early effects for which there is a threshold below which the effect will not occur. For example exposure of the skin, in a short time period, to a dose in excess of 2 000 mSv will result in a sunburn-like reddening of the skin (erythema). Below about 2 000 mSv the effect will not occur.
There are a number of important early effects at high acute doses, that is doses received within, say, a few hours. These high dose early effects are known as 'non-stochastic' effects and are characterised by having a dose threshold below which the effect will not occur. Doses to the whole body of between about 3 000 to 5 000 mSv will cause damage to the infection fighting white blood cells. As a result of this the body is less able to protect itself from disease and death may occur several weeks after the exposure as a result of, say, pneumonia. As the dose increases above about 5 000 mSv damage is also caused to the gut wall and death may occur within a few days to weeks. At doses above about 10 000 mSv damage will be caused to the lungs and at higher doses to the central nervous system. Following an acute dose of 10 000 mSv survival is only possible with extreme medical care usually including a bone marrow transplant. At acute doses much greater than 10 000 mSv death is inevitable within, at most, a few days.
may not
At lower radiation doses of around a few hundreds of millisieverts the person receiving the radiation dose notice any physiological changes at all, although an examination of his or her blood may show a temporary drop in the white blood cell count. There will be no obvious detrimental effect on the health of that exposed person, but studies have shown that people who have received doses in the order of a few hundreds of millisieverts have on average a slightly higher risk of dying of cancer, about 10-30 years later, than a similar unexposed group of people. These long term effects, known as 'stochastic' effects, are assumed to have no threshold and a risk proportional to dose, even though long term effects are not observable in groups of people exposed to low doses of radiation, say, a few tens of millisieverts.
3.6 Radiation Risks
Exposure to an acute dose of ionising radiation in excess of about 1 000 mSv will cause observable physiological changes which will generally be detrimental to health. The severity of these early non-stochastic effects is fairly predictable if the dose is known.
The long term stochastic effects of radiation are more difficult to predict. A person exposed to say 500 mSv of radiation is unlikely to suffer any long term effects, however studies of large groups of people exposed to levels of radiation of this order indicate that a small number may develop cancer or leukaemia as a result of the exposure. These data on radiation risks are obtained from studies on, for example, the survivors of the atomic bombs exploded over Hiroshima and Nagasaki in 1945 and groups of people exposed to radiation for medical reasons. It would be easy to gain the impression from anecdotal accounts of the radiation effect following the Hiroshima and Nagasaki bombs that the majority of survivors suffered from long-term radiation effects. A careful study of about 80 000 of the original 280 000 survivors, who received large doses of radiation has been carried out (ref 5). Up to the end of the reported study period (1978) about 24 000 of the 80 000 study group had died, of which about 5 000 were due to some form of cancer. This was about 250 more than would have been expected from studies of
a control group of Japanese people who had not been exposed to radiation. It appears therefore that cancer was responsible for about one in five of all deaths, which is a typical figure in most countries, and that cancer induced by radiation from the bombs may have been responsible for about one in a hundred of all deaths.
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