Complete radiation survival curves for mammalian cells as a function of partial pressure of oxygen have been obtained with considerable medium present. The results show an apparent decrease in extrapolation number (n) when irradiation is carried on during conditions of moderate hypoxia (5000 ppm oxygen) with the n value returning to the air level when the hypoxia becomes extreme. This phenomenon can be explained by radiochemical depletion of oxygen. At less than 25 ppm oxygen, the cells are not capable of sublethal repair, while at 200 ppm oxygen repair proceeds at almost the aerobic rate.

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