The biological damage of ionizing radiation occurs at the level of the cell. Radiation can tear apart the DNA in the nucleus of the cell. Most breaks in DNA are repaired within twenty four hours but twenty four percent of those repairs are incorrect. If the damage is severe enough, it will kill the cell in a process called apoptosis where the cell go through a process of programmed cell death. If the damage to DNA does not kill the cell, it can cause changes in the DNA sequence known as mutations. These can be passed to daughter cells when the damaged cell divides and cause cancerous tumors. Since the DNA contains the recipe for all the proteins in the human body as well as instructions for the manufacture of proteins, abnormal proteins or abnormal protein production can be the result leading to premature aging and cancer.
Natural radiation produced by naturally occurring radioactivity in the human body produce a low rate of DNA damage and mutation. X-rays, gamma rays, beta particles, alpha particles and neutrons can all produce DNA damage and mutations at a much greater rate. Experiments have shown that a given amount of radiation spread out over time in repeated exposures can have a more damaging effect than the same amount of radiation given in a single high dose.
Radiation damage to cells causes the release of free radicals of oxygen which are highly reactive and also cause biological damage. The oxygen can damage the bases which make up DNA as well as the enzymes that repair damaged DNA. Radiation damage can be passed to neighboring cells in human tissue. Different types of radiation cause different amounts and types of damage.
Alpha particles or helium nuclei are very energetic and produce a great deal of damage but it is highly localized in tissue. It can cause 20 times the damage that x-rays and gamma rays can cause.
Beta rays or energetic electrons are more penetrating than alpha particles but cause less damage.
Neutrons can cause damage in a number of ways. They can cause other elements to become radioactive. They can release free radicals that damage molecules. They can cause protons to be ejected from nuclei. These protons directly damage tissue.
X-rays and gamma rays can pass through tissue more easily than alpha and beta particles and the damage they cause is more evenly distributed in tissue. X-rays can also cause electron or beta particles to be ejects from stable atoms.
Cancers associated with large doses of ionizing radiation include leukemia, thyroid, breast, bladder, colon, liver, lung, esophagus, ovarian, melanoma and stomach cancer. The latent period refers to the time between exposure to radiation and the detection of cancer. The cancers caused by ionizing radiation are indistinguishable from cancers caused by other carcinogens such as cigarettes, alcohol, harmful foods, cleaning products, and industrial pollutants in air, soil and water.
Radiation hormesis is an unproven theory that exposure to low levels of ionizing radiation may help to immunize the body against damage from higher levels of ionizing radiation. The mechanism is thought to be based on activation of the DNA repair machinery by the low level radiation.