All biological damage effects begin with the consequence of radiation interactions with the atoms forming the cells. All living things are composed of one or more cells. Every part of your body consists of cells or was built by them. Although we tend to think of biological effects in terms of the effect of radiation on living cells, in actuality, ionizing radiation, by definition, interacts only with atoms by a process called ionization. For ionizing radiation, the kinetic energy of particles (photons, electrons, etc.) of ionizing radiation is sufficient and the particle can ionize (to form ion by losing electrons) target atoms to form ions. Simply ionizing radiation can knock electrons from an atom.
There are two mechanisms by which radiation ultimately affects cells. These two mechanisms are commonly called:
- Direct effects. Direct effects are caused by radiation, when radiation interacts directly with the atoms of the DNA molecule, or some other cellular component critical to the survival of the cell. The probability of the radiation interacting with the DNA molecule is very small since these critical components make up such a small part of the cell.
- Indirect effects. Indirect effects are caused by interaction of radiation usually with water molecules. Each cell, just as is the case for the human body, is mostly water. Ionizing radiation may break the bonds that hold the water molecule together, producing radicals such as hydroxyl OH, superoxide anion O2– and others. These radicals can contribute to the destruction of the cell.
A large number of cells of any particular type is called a tissue. If this tissue forms a specialised functional unit, it is called an organ. The type and number of cells affected is also an important factor. Some cells and organs in the body are more sensitive to ionizing radiation than others.
Sensitivity of various types of cells to ionizing radiation is very high for tissues consisting of cells that divide rapidly like those found in bone marrow, stomach, intestines, male and female reproductive organs, and developing fetuses. This is because dividing cells require correct DNA information in order for the cell’s offspring to survive. A direct interaction of radiation with an active cell could result in the death or mutation of the cell, whereas a direct interaction with the DNA of a dormant cell would have less of an effect.
As a result, living cells can be classified according to their rate of reproduction, which also indicates their relative sensitivity to radiation. As a result, actively reproducing cells are more sensitive to ionizing radiation than cells that make up skin, kidney or liver tissue. The nerve and muscle cells are the slowest to regenerate and are the least sensitive cells.
The sensitivity of the various organs of the human body correlate with the relative sensitivity of the cells from which they are composed. In practice, this sensitivity is represented by the tissue weighting factor, wT, which is the factor by which the equivalent dose in a tissue or organ T is weighted to represent the relative contribution of that tissue or organ to the total health detriment resulting from uniform irradiation of the body (ICRP 1991b).
If a person is irradiated only partially, the dose will depend strongly on the tissue, which was irradiated. For example, a 10 mSv gamma dose to the whole body and a 50 mSv dose to the thyroid is the same, in terms of risk, as a whole-body dose of 10 + 0.04 x 50 = 12 mSv.