The main structures affected by radiation are cells. Radiation energy is not spread diffusely throughout the tissue; rather, the energy rays penetrate into localized areas of tissue, affecting only the cells contacted by the rays. Whether a cell dies immediately or develops molecular changes depends upon the dose of radiation and the length of exposure. Molecular changes in a cell are reflected in its ability to grow and divide to form a normal generation of daughter cells. When the radiation dose is high, cell death is rapid and extensive; there is usually no reserve tissue left to replace that destroyed. If the cell changes are more subtle, the cell may be unable to reproduce altogether or the new cells produced may be abnormal and not viable. The tissues most affected by radiation are those that undergo rapid replacement, such as bone marrow, the lining of the gastrointestinal tract, and skin. Slower-growing tissues, such as those of the brain and liver, require either high doses of radiation or prolonged exposure before they show symptoms of degeneration. The overall direct complications of radiation are cell depletion, inability to reproduce new tissue, lessened body resistance to infections, decreased numbers of blood cells, hemorrhages from disrupted blood vessels (blood vessel), body poisons from tissue breakdown, and a slower blood-clotting time. Indirect effects can be tumour growths, leukemia, a shortened life span, recurrent bacterial infections, anemia, and body ulcers (ulcer).

Local tissue injuries from radiation may manifest a number of months after the initial exposure or several years after a sequence of exposures. The skin may ulcerate, scale, swell, and slowly deteriorate. Systemic symptoms appear only after the whole body or numerous parts of it have been exposed. Radiation sickness with systemic symptoms can exhibit four stages in milder cases or cause immediate convulsions (convulsion), high blood pressure (hypertension), shock, fever, skin reddening, and death. The first phase in the slower form develops within a few minutes or hours after exposure; symptoms are nausea, vomiting, weakness, and diarrhea. A day or two after exposure, the symptoms depart, and there is a second phase of apparent recovery that may last a week or longer. Third-stage symptoms are fever, infection, vomiting, bloody diarrhea, hemorrhages, dehydration, weight loss, hair loss, and ulcers. Death usually occurs in this phase if the damage has been sufficiently severe. If the patient survives the third phase, the fourth phase (slow recovery) begins about six weeks after the exposure. The recovery may take several months, and there may be permanent disability, such as sterilization, extensive scar tissue, cataracts (cataract), bone disintegration, cancer, and blindness.