For the same equivalent dose, the degree of biological damage depends on the organ exposed. In the case of partial exposure affecting several organs with different radiosensitivities, it is useful to define a quantity able to measure the overall risk corresponding to that of a uniform exposure of the whole body. This quantity is called the effective dose.

The effective dose E (expressed in sieverts) introduces, along with the absorbed dose D (expressed in grays) and the radiation weighting factor WR (WR = 1 to 20), a tissue weighting factor WT which indicates the radiosensitivity of the exposed organ. This gives:

E = D1 x WR1 x WT1 + D2 x WR2 x WT2 + ...
where WT = 0.12 for the lungs, WT = 0.05 for the thyroid gland, and so on. (see the table below)

Examples: An equivalent dose of 100 mSv delivered to the lungs results in the same risk of biological damage as an equivalent dose of 12 mSv delivered to the whole body. An equivalent dose of 100 mSv delivered to the thyroid results in the same risk of biological damage as an equivalent dose of 5 mSv delivered to the whole body.
In other words, for the same equivalent dose, the lungs are 2.2 times more vulnerable than the thyroid.