What Is Ionizing Radiation and What Are Its Effects?
Ionizing radiation is a type of energy released by atoms in the form of electromagnetic waves or particles. These high-frequency, high-energy waves (X-rays & Gamma rays) are classified as “ionizing” (as opposed to “non-ionizing”) radiation because they contain sufficient energy to displace an electron from its orbit around the nucleus of an atom. The most important consequence of this displaced electron on human tissue is the potential damage it can inflict on DNA, which may occur either directly or indirectly. The direct damage occurs when the displaced electron hits and breaks a DNA strand. Indirect damage occurs when the electron reacts with a water molecule, creating a powerful hydroxyl radical (associated with chemical attack which then damages the cell’s DNA).
Ionizing Radiation Effects and Their Risk to Humans 
Radiation and DNA:
Damage to a cell’s DNA in either direct or indirect ways can have several consequences. A single-strand DNA break is usually repaired appropriately by the cell with no subsequent deleterious complications. However, a break affecting both strands of DNA allows the potential for an abnormal reconnection of the strands, which probably accounts for all the adverse biological effects ionizing radiation has on human beings.
Ionizing Radiation Effects and Risks:
Risks associated with ionizing radiation have been known for almost as long as ionizing radiation itself. Within a year of the discovery of X-rays by Röntgen, skin burns had been reported and within 7 years a case of skin cancer was observed.
The deleterious effect that ionizing radiation has on human tissue can be divided into two types: “non-stochastic” (deterministic) or “stochastic” effects…
Deterministic (Non-Stochastic) Effects:
Deterministic effects only occur once a threshold of exposure has been exceeded. The severity of deterministic effects increases as the dose of exposure increases. Deterministic effects are caused by significant cell damage or death. The physical effects will occur when the cell death burden is large enough to cause obvious functional impairment of a tissue or organ.
The severity of stochastic effects is independent of the absorbed dose. Under certain exposure conditions, the effects may or may not occur. There is no threshold and the probability of having the effects is proportional to the dose absorbed.
Examples: Cancer and Hereditary Defects (e.g. Down Syndrome
*Note: Nuclear power workers receive a lower annual radiation dose than pilots, flight crews, and frequent flyers.
Summary of Adverse Health Effects of Ionizing Radiation:
 – T.R. Goodman, MD, Yale University School of Medicine, New Haven, CT – http://imagewisely.org/Imaging-Modalities/Computed-Tomography/Imaging-Physicians/Articles/Ionizing-Radiation-Effects-and-Their-Risk-to-Humans