The physical absorption of radiation in a cell that results in ionization causes a chemical reaction, referred to as free-radical formation. A free radical is an atom or molecule that has a single unpaired electron in one orbit. Free radicals are highly reactive and create damage through chemical reactions in the cell. Changes in the cell, whether from a direct or indirect reaction, can leave the cell in one of three states, changed, dead, or the cell can repair itself and the cell be unaffected. For more information on this subject, please review Chapter 3 of the Radiation Safety Training Guide, "Radiobiological Concepts".
Stochastic effects are those in which the probability of an effect occurring within a population increases with dose, without threshold. In other words, there does not have to be a minimum dose to cause an effect. Carcinogenic (cancer) and genetic (inherited) effects are examples of stochastic effects. Non-Stochastic (deterministic) effects are those in which the severity of the effect varies with the dose. For these types of effects, a threshold dose may exist. Examples of such effects are cataracts, skin ulcerations or burns, depletion of blood forming cells in bone marrow, and impairment of fertility.
