Radiation has a potential effect on a wide variety of life. Beginning with the ionization of atoms and resulting in eventual cell damage, radiation may impact higher-level biological functions. The most critical damage is that which occurs in the DNA of cells.

At the molecular level, there are four possible effects that radiation may have on humans.

The first group of effects has no negative consequences for higher-level biological functions. Either cells remain undamaged by the radiation (in this case, the ionization of materials in the cell may produce chemical reactions which occur normally in the cell) or cells may be damaged, but not irreparably so. Often, even damage to chromosomes may occur with few long-term effects because the cell is able to detect and repair a limited amount of damage. Even without radiation dosage, changes and repairs in cells, including chromosomes, occur constantly in our bodies.

The second group of effects is more critical and will most likely have a negative impact on higher-level biological functions. Cells may be damaged and either begin operating abnormally or die. If enough damage is done and a cell is unable to completely repair itself, it may perform further functions abnormally, including reproduction. This usually occurs when cells are exposed to a lower dose of radiation over an extended period of time (or chronic radiation). It is this kind of exposure that may lead to cancer and genetic effects (problems in offspring), depending on the strength of the dose. With exposure to high-dose, short-term radiation (or acute radiation), damage may occur to the point where a cell is unable to perform any further function, including reproduction, and may even die. On a large enough scale (for example, at the organ level) this kind of damage is likely to cause radiation sickness. Symptoms of radiation sickness include skin that seems slightly sunburnt, hair loss, fatigue, internal bleeding, fever, nausea, dehydration and diarrhea, bleeding ulcers, loss of coordination, confusion, coma, convulsions, shock, and more.