When alpha, beta, and gamma rays strike the complex molecules that compose the cells of our bodies, they can actually cause parts of these complex molecules to break away and ionize. Or the radiation can interact with these complex molecules, causing the formation of free radicals. Free radicals and ions can interact with other complex molecules in the body, causing their destruction. If the molecules that are destroyed are necessary for important biological processes, the whole organism can suffer and even die. For example, if the molecule attacked by the radiation is DNA, the cells involved will not be able to reproduce themselves.
Of course, the radiation could simply damage the cells, not destroy them. But if this should happen, there is a possibility that the damaged cells will not reproduce themselves in the normal fashion, which means that they could become cancerous.
Radiation can also react with one of the major compounds in our bodies-water! Cells contain about 80% water, and ionizing radiation can cause the water to go through a series of free-radical reactions, forming a toxic substance, hydrogen peroxide. This could be fatal to the cells. The reactions would look something like this:
H20 + radiation OH- + H+
The effects of radiation in cells can be divided into two stages. Stage 1 effects are the actual changes in the chemical makeup of the cells. Stage 2 effects result from the cells' inability to carry on their normal functions, which results in a body malfunction. For example, when iodine-131 is administered to an individual, it will localize in the thyroid gland. The radiation from the iodine-131 will destroy some cells in the thyroid gland. The thyroid gland will not be able to produce as much thyroxine. For an individual with a hyperactive thyroid, this represents an excellent therapeutic method.
The cells most sensitive to radiation are those undergoing rapid growth, for example, cells of the bone marrow and reproductive organs. Also, infants and children are very susceptible to radiation because a large number of cells in their bodies are undergoing rapid growth. Because cancer cells grow very rapidly compared to normal cells, radiation is very effective against these types of malignancies. Radiation directed at an area containing fast-growing cancer cells will have a greater devastating effect on those cells than on normal cells.