Ionization and Cell Damage
As previously discussed, photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials. This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material. This creates electrons, which carry a negative charge, and atoms without electrons, which carry a positive charge. Ionization in industrial materials is usually not a big concern. In most cases, once the radiation ceases the electrons rejoin the atoms and no damage is done. However, ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other. This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed. Ionization may cause unwanted changes in some materials, such as semiconductors, so that they are no longer effective for their intended use.
Ionization in Living Tissue (Cell Damage)
In living tissue, similar interactions occur and ionization can be very detrimental to cells. Ionization of living tissue causes molecules in the cells to be broken apart. This interaction can kill the cell or cause them to reproduce abnormally.
Damage to a cell can come from direct action or indirect action of the radiation. Cell damage due to direct action occurs when the radiation interacts directly with a cell's essential molecules (DNA). The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA). DNA is found in every cell and consists of molecules that determine the function that each cell performs. When radiation interacts with a cell wall or DNA, the cell either dies or becomes a different kind of cell, possibly even a cancerous one.
Cell damage due to indirect action occurs when radiation interacts with the water molecules, which are roughly 80% of a cells composition. The energy absorbed by the water molecule can result in the formation of free radicals. Free radicals are molecules that are highly reactive due to the presence of unpaired electrons, which result when water molecules are split. Free radicals may form compounds, such as hydrogen peroxide, which may initiate harmful chemical reactions within the cells. As a result of these chemical changes, cells may undergo a variety of structural changes which lead to altered function or cell death.
Various possibilities exist for the fate of cells damaged by radiation. Damaged cells can:
- completely and perfectly repair themselves with the body's inherent repair mechanisms.
- die during their attempt to reproduce. Thus, tissues and organs in which there is substantial cell loss may become functionally impaired. There is a "threshold" dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome. Exceeding the threshold dose increases the level of harm. Such outcomes are called deterministic effects and occur at high doses.
- repair themselves imperfectly and replicate this imperfect structure. These cells, with the progression of time, may be transformed by external agents (e.g., chemicals, diet, radiation exposure, lifestyle habits, etc.). After a latency period of years, they may develop into leukemia or a solid tumor (cancer). Such latent effects are called stochastic (or random).
Exposure of Living Tissue to Non-ionizing Radiation
A quick note of caution about non-ionizing radiation is probably also appropriate here. Non-ionizing radiation behaves exactly like ionizing radiation, but differs in that it has a much greater wavelength and, therefore, less energy. Although this non-ionizing radiation does not have the energy to create ion pairs, some of these waves can cause personal injury. Anyone who has received a sunburn knows that ultraviolet light can damage skin cells. Non-ionizing radiation sources include lasers, high-intensity sources of ultraviolet light, microwave transmitters and other devices that produce high intensity radio-frequency radiation.