Effect of ionizing radiation leading to a functional loss of the irradiated organ or tissue, if sufficient cells are killed or prevented from reproducing or functioning due to radiation. The seriousness of this loss of organ function is directly proportional to the number of cells affected. Since the function of many organs and tissue is not impaired by a limited reduction in the number of functional cells, a threshold dose for deterministic radiation effects exists, which must be exceeded for an effect to occur. In the case of radiation doses above this threshold the degree of pathological severity increases rapidly. The deterministic effects of ionizing radiation include skin reddening (dose threshold 3 to 5 gray), opacity of the eye lens (dose threshold 2 to 10 gray) and permanent sterility (dose threshold 2.5 to 6 gray).