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of the effective dose equivalent. The safety standards assume that the probability of harm to any tissue or organ is proportional to the dose to that tissue. However, because of the differences in radiosensitivity among the various tissues, the value for the proportionality factors differs among the tissues. Generally, the higher the mitotic rate of a cell line, and the less differentiated a cell line is, the more radiosensitive is that cell line. (This observation was first enunciated by two French physiologists, Bergonie and Tribondeau, in 1906.) The differences in radiosensitivity are accounted for by tissue weighting factors, wT, Table 2. The effective dose equivalent is given by
(8)
TABLE 1 Values for the quality factor, Q, and the radiation weighting factor, wR.
| Radiation | Q or wR |
|---|---|
| X‐rays | 1 |
| Gamma rays | 1 |
| Beta particles | 1 |
| Alpha particles | 20 |
Note that these specific recommendations are unchanged for ICRP 26, 60, and 103.
TABLE 2 Values for tissue weighting factors, wT.
| Tissue (or organ), wT | ICRP 26 | ICRP 60 | ICRP 103 |
|---|---|---|---|
| Gonads | 0.25 | 0.20 | 0.08 |
| Breast | 0.15 | 0.05 | 0.12 |
| Red bone marrow | 0.12 | 0.12 | 0.12 |
| Lung | 0.12 | 0.12 | 0.12 |
| Thyroid | 0.03 | 0.05 | 0.04 |
| Bone surface | 0.03 | 0.01 | 0.01 |
| Colon | Not given | 0.12 | 0.12 |
| Stomach | Not given | 0.12 | 0.12 |
| Bladder | Not given | 0.05 | 0.04 |
| Liver | Not given | 0.05 | 0.04 |
| Esophagus | Not given | 0.05 | 0.04 |
| Skin | Not given | 0.01 | 0.01 |
| Salivary glands | Not given | Not given | 0.01 |
| Brain | Not given | Not given | 0.01 |
| Remainder | 0.30 | 0.05 | 0.12 |
| Total | 1.00 | 1.00 | 1.00 |
The values are based on a reference population of equal numbers of both sexes and a wide range of ages. In the definition of effective dose, they apply to workers, to the entire population, and to both sexes.
Example. A laboratory technician had accidentally inhaled 131I. Using data from whole body counts and bioassay measurements, the health physicist calculated a thyroid dose equivalent of 6000 mrem (60 mSv) and a whole body dose equivalent of 13 mrem (0.13 mSv). What was the technician's effective dose according to the USNRC criterion?
Substituting into the equation for effective dose equivalent,
6 RADIATION BIOEFFECTS
Quantitatively, the harmful effects of radiation overexposure are the same whether the radiation was delivered from a source outside the body (external exposure) or from radionuclides within the body (internal exposure). The effects of a given dose are the same whether the dose was from an external radiation exposure or from an internally deposited radionuclide.
There are two qualitatively different categories of radiation bioeffects. Deterministic (or nonstochastic) effects are those for which there is a threshold dose, for which the magnitude of the effect depends on the size of the dose, and for which there is an unequivocal relationship between the radiation exposure and the effect (such as skin burns and hair loss). For deterministic effects, the threshold dose for any harmful effect must be exceeded before that harmful effect is observed. These threshold doses are very large relative to radiation safety standards. For example, the threshold dose for radiation illness exceeds 100 rads (1 Gy) of acute whole body irradiation. Threshold doses for some other effects are shown in Table 3.
The second category of radiation bioeffects is called stochastic effects, and it includes cancer and genetic effects. (It should be emphasized that, although genetic effects have been observed in experimental animals, we have observed no increase in genetic effects in any human population exposed anywhere at any time at any dose level.1) Stochastic effects occur by chance, and they occur to unexposed persons as well as to exposed individuals. Stochastic effects are therefore not definitively related to the exposure. Exposure to a carcinogen or to a mutagen increases the likelihood of expression of the effect in proportion to the size of the dose. At no time, however, regardless of the size of the dose, can we be certain that a cancer will develop. If cancer does develop, it cannot be certain that the cancer was due to the exposure. No pathologist can say with certainty that the cancer would not have occurred if the person had not been exposed to the carcinogen. The best that one can do is to calculate the probability, based on the person's exposure history and dose, that the cancer was due to the exposure. It has been observed that there are no increases in leukemia rates at acute whole body doses less than 10 rads (0.1 Gy), and no increase in solid tumor