5. Effective Equivalent Dose
In some cases, only a part of the body is irradiated. For example, mainly the lungs are involved in the case of radon and radon decay products. Different organs and types of tissue have different sensitivities with regard to what is termed the late effects of radiation. Late effects are biological responses that are only observed after a substantial amount of time has passed, often years. Induction of cancer is a late effect. In order to compare the risk for late effects of different types of radiation, the so-called effective dose is used.
If one part of the body (e.g., the lungs) receives a radiation dose, it represents a risk for a particularly damaging effect (e.g., lung cancer). If the same dose is given to another organ it represents a different risk factor.
It is possible to calculate a dose given to the whole body that yields the same risk as that from the much larger dose given to one particular organ.
This calculated dose is called the effective dose (often shortened to simply the dose) and is designated E. It is defined in the following way:
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E = w1H1+ w2H2 + ……………….
where w1 represents a weighting factor for organ 1 and H1 is the equivalent dose (given in Sv) for organ number 1. The weighting factor represents the sensitivity of a particular organ and Table 4.1 gives the different weighting factors suggested for radiation protection work by the ICRP. Equivalent dose and effective equivalent dose have a meaning only when considering late effects such as cancer and leukemia.
TABLE 4-1.
Tissue weighting factors used in radiation
Organ |
Weighting factor |
Gonads |
0.20 |
Bone marrow (red) |
0.12 |
Lungs |
0.12 |
Stomach |
0.12 |
Colon |
0.12 |
Bladder |
0.05 |
Breast |
0.05 |
Liver |
0.05 |
Thyroid gland |
0.05 |
Esophagus |
0.05 |
Bone surfaces |
0.01 |
Skin |
0.01 |
The rest |
0.05 |
TOTAL |
1.00 |
protection
work
6. Other Dose Units
In addition to the units already defined, there are a number of other concepts used in radiation protection and these, unfortunately, are frequently a source of confusion. Radiation biologists are interested in the mechanisms that result in the observable macroscopic effects on biological systems, whereas radiation protection authorities are interested in guidelines for large population groups. Here is a brief overview of the concepts used when working in radiation protection.
Collective dose
The collective dose is the sum of all individual doses in a group of people. It can be obtained by the product of the average individual dose with the number of people in the group. For the collective dose the unit used is person-sievert (person-Sv). For some reasons the use of collective dose is questionable in risk analyses. Nevertheless, it is used by authorities for risk analysis in many countries.
Committed equivalent dose
When a radioactive compound enters the body, the activity will decrease with time, due both to physical decay and to biological clearance, as noted earlier. The decrease varies from one radioactive compound to another. Accumulated dose over a certain period of time, usually 50 years, is called the committed equivalent dose.