Audeen W. Fentiman
Mitchell Smith
Joyce E. Meredith
Radiation may affect living things by affecting the cells that make up the living organism. Radiation effects on a cell are random. That is, the same type and amount of radiation could strike the same cell many times and have a different effect, including no effect, each time. However, in general, the more radiation that strikes the cell, the greater the chances of an effect occurring. If a significant number of cells are affected, the organism may be damaged or even die.
All living things are constantly exposed to background radiation. Most cells have the ability to repair some damage done by this level of radiation. As a result, the effects of doses similar to background levels are impossible to measure in a single individual. Effects of these low levels of radiation are often predicted for populations rather than for individuals.
This fact sheet describes how low levels of radiation affect cells, how cell damage affects the health of individuals, and how the health effects on populations are estimated. Effects of high levels of radiation will be discussed briefly.
When a cell absorbs radiation, there are four possible effects on the cell. First, the cell may suffer enough damage to cause loss of proper function, and the cell will die. Second, the cell may lose its ability to reproduce. Third, the cell's genetic code (i.e., the DNA) may be damaged such that future copies of the cell are altered, which may result in cancerous growth. Finally, the absorption of radiation by a cell may have no adverse effect.
Cells are made up of molecules. Cell damage may be caused by interaction of radiation with these molecules. If radiation strikes a molecule crucial to the cell's function, such as DNA, damage to the cell is likely to be greater than if the radiation strikes a less crucial molecule such as water.
Some cells are more likely to be affected by radiation than others. Cells that multiply rapidly are the most susceptible. Cells can often repair radiation damage, but if the cell multiplies (splits into two identical cells) before it has had time to repair the most recent radiation damage, the new cells might not be accurate copies of the old one. Some examples of rapidly multiplying cells are those in a fetus and cancer cells.
Health effects of radiation are divided into two categories: threshold effects and non-threshold effects. Threshold effects appear after a certain level of radiation exposure is reached and enough cells have been damaged to make the effect apparent. Non-threshold effects can occur at lower levels of radiation exposure.
Threshold effects occur when levels of radiation exposure are tens, hundreds, or thousands of times higher than background, and usually when the exposure is over a very short time, such as a few minutes. Some examples of observed threshold effects and the doses which cause them are presented in Table 1. Dose is measured in rem or millirem. (1,000 millirem = 1 rem)
| Table 1. Threshold Effects | |
|---|---|
| Dose (in rem) | Effects |
| 5 to 20 | Possible latent effects (cancer), possible chromosomal abberations |
| 25 to 100 | Blood changes |
| More than 50 | Temporary sterility in males |
| 100 | Double the normal incidents of genetic defects |
| 100 to 200 | Vomiting, diarrhea, reduction in infection resistance, possible bone growth retardation in children |
| 200 to 300 | Serious radiation sickness, nausea |
| More than 300 | Permanent sterility in females |
| 300 to 400 | Bone marrow and intestine destruction |
| 400 to 1000 | Acute illness and early death (usually within days) |
Non-threshold effects can occur at any level of radiation exposure, but the risk of harmful health effects generally increases with the amount of radiation absorbed. The most studied non-threshold effect is cancer. These studies are somewhat complicated by the facts that (1) not all cancers are caused by radiation, (2) exposure to a particular dose may cause cancer in one person but not another, and (3) the cancer often doesn't appear until many years after the exposure to radiation. It is currently impossible to determine which cancers are caused by radiation and which are caused by other carcinogens in our environment.
Susceptibility to radiation-induced cancer depends on a number of factors such as the site of exposure in the body, sex, and age. Sites in the body where cells rapidly grow and multiply, and those where radioactive materials tend to concentrate, are more susceptible to cancer than others. For example, the breast and thyroid gland have relatively high susceptibilities to radiation-induced cancer, while the kidney and nerve cells have lower susceptibilities.
Many studies have been done on other possible effects of radiation on human health. The detail necessary to present accurate information on these studies is beyond the scope of this fact sheet. Health effects are thoroughly discussed in a book entitled Medical Effects of Ionizing Radiation by Fred A. Mettler, Jr., M.D. and Robert D. Moseley, Jr., M.D. This book is fairly technical, but it has an extensive glossary of terms and contains hundreds of references to studies done on the health effects of radiation.
Because it is impossible to predict the effect of low levels of radiation on any one person, studies of the human health effects of radiation are usually done by trying to predict how many people in a large population might be affected. The result of such a study is usually a prediction of how many people in a population of 100,000 or a million may get cancer due to a specific radiation exposure. The predicted cancers due to this specific radiation exposure are in addition to cancers that would normally be expected in the selected population.
The number of additional cancers expected in a population is calculated in two steps. First, the dose (in rems) to an average person in a population is multiplied by the number of persons in that population. The answer is given in person-rems. Then that answer is divided by the number of person-rems that produce one cancer in the population. The final result is the number of additional cancers expected.
If you want to read more about the health effects of radiation, some of the references and other fact sheets listed below may be helpful.
Fred A. Mettler, Jr., M.D. and Robert D. Moseley, Jr., M.D. Medical Effects of Ionizing Radiation, Grune & Stratton, Inc., Orlando, Florida, 1985.
National Research Council, Committee on the Biological Effects of Ionizing Radiation, Health Effects of Exposure to Low Levels of Ionizing Radiation, BEIR V, National Academy Press, Washington, D.C., 1990.
RER-21 What is ionizing radiation?
RER-22 What are sources of ionizing radiation?
RER-23 How is ionizing radiation measured?
Dr. Audeen W. Fentiman is an Assistant Professor in Nuclear Engineering at The Ohio State University. Mitchell Smith a Graduate Research Associate in Biomedical Engineering. Joyce E. Meredith is a Graduate Research Associate, Ohio State University Extension.
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