Developing mice were exposed to tritium in body water continuously from conception to 14 days after birth. Concentrations close to 1, 3, 6, and 9 μCi/ml were maintained by giving mothers <tex-math>${}^{3}{\rm HOH}$</tex-math> in their drinking water throughout pregnancy and the first 2 weeks of lactation. When the offspring were 14 days old, primary oocytes in their ovaries were counted microscopically and compared with controls. Confirming earlier findings, survival of these cells decreased exponentially with exposure; there was no threshold; the LD50 level was 2 μCi/ml, delivering 0.44 rad/day. For comparison, oocytes were similarly enumerated in mice continuously exposed for the same period to60 Co gamma rays at 1, 2.1, and 3.2 rad/day. Cell killing in this case was not as great. Nor was it exponential; lower doses showed decreasing effectiveness. The data were in general accord with the linear-quadratic dose relation of the theory of dual radiation action. The effective LD50 level for gamma rays was about 1.1 rad/day. Owing to these differences in dose-response, the RBE (relative biological effectiveness) of tritium compared to gamma radiation, as measured here in the intact animal, varied inversely with dose. At effective gamma-ray doses of some 40 rad, it was about 1.6. However, at lower exposure levels, giving effective doses of only a few rads, the RBE of tritium rises to approximately 3. This is of special interest from the standpoint of health and environmental protection.

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