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Effects on the body |
Our knowledge of the effects of radioactivity on the body is derived from (a) short- and long-term studies of the victims at Hiroshima and Nagasaki, (b) follow-up studies of medical uses of radiation, (c) research with animals. Radiation damages a cell when a single radioactive particle disrupts a cell molecule by the process of ionization -- that is, breaking the bonds that hold electrons to the molecule. The most characteristic physiological effect is damage to the bone marrow cells responsible for producing blood products. Among other things, a failure to produce white blood cells can expose the body to infection. Damage to DNA or other molecules can lead to an increased cancer risk over a period of several decades. |
Relation to dose |
Doses in the range 100 to 200 rem can produce fatigue, vomiting, hair loss, and mild blood effects, but survival is close to 100%. A dose of 500 rem produces severe blood effects and a survival rate about 50%. At 1000 rem the survival rate is 0%. These numbers, however, are somewhat controversial even at this late date. At the high exposure end, very few cases have been studied altogether. In the intermediate range of several hundred rem, studies based on the Japanese victims are uncertain because these people were subject to other bomb effects which could have caused illness or death. Some researchers claim that the dose at which the survival rate is 50% should be reduced from 500 rem to about 200. |
Low level radiation The linear hypothesis |
Controversy and uncertainty exist also about the effects of low level radiation. Many experts believe that in the range up to 10 rem or so there are no permanent harmful effects because there has always been a continuous low level background radioactivity, and living things would have evolved mechanisms to repair damage to cells from this. [The average lifetime dose from the environment is about 7 rem.] Others assume that effects of low level radiation exist and are proportional to the dose; e.g. the increased cancer risk from 15 rem is one-tenth of the increased cancer risk from 150 rem. (This is called the linear hypothesis.) The issue here is important in estimating long-term cancer risk due to nuclear accidents like the one at Chernobyl. If there is increased risk from low level radiation, it will be small for each individual, but the number of people subject to low level radiation may be very large -- and so the overall consequences could be many additional cases of cancer.
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Medical x-rays |
X-rays, such as those used for medical diagnosis, affect the body in ways similar to radioactivity. X-ray dose is also measured in rem. A chest x-ray is about 0.03 rem; a mammogram about 0.3 rem. These doses are small compared to the environmental dose, and so the risk is usually considered small enough to make the procedure worth doing. On the other hand, some laboratories have recently been advertising a "full-body scan", which might detect tumors and other abnormalities over the whole body. Here the dose is higher, about 3 rem, and so there is some concern that the procedure, especially if done a number of times, might subject the patient to too much increased cancer risk. |
KEY CONCEPTS
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