in radiobiology, the protective effect of lowered oxygen content (hypoxia) when exposing living organisms to ionizing radiation.

The oxygen effect is observed in all living things (microorganisms, plants, animals) and at all levels of organization (subcellular, cellular, tissular, organic, and organismic), greatly attenuating all radiobiological reactions (biochemical disturbances, mutations, stunting of growth and development) and lengthening the survival time of the irradiated organisms. The mechanism of the protective effect of hypoxia is explained by the fact that irradiation in the presence of oxygen forms peroxide radicals, which intensify the radiation’s effect on vital macro-molecules and cellular structures and/or weaken the effectiveness of intracellular protective agents.

The extent of the oxygen effect depends chiefly on the type of radiation and conditions of exposure. The effect is greatest with X rays and gamma rays. It diminishes with increasing density of ionization, and it is virtually nonexistent with dense ionizing radiation (for example, alpha irradiation). In active living things with normal water content, radiation injury is lessened only when there is hypoxia during the irradiation, but for dry objects (dormant seeds and bacterial spores) the effect is observed with post-irradiation hypoxia, when the irradiated object resumes its normal vital activities (for example, in the germination of seeds). The oxygen effect is used in radiation therapy: by increasing the amount of oxygen in a tumor and creating hypoxic conditions in the surrounding tissues, one can intensify radiation injury to the tumor cells while lessening the injury to the healthy tissues.