What dose of radiation will result in permanent sterility to both men and women?

The dose of radiation that leads to permanent sterility in both men and women is indeed a significant threshold due to the effects of ionizing radiation on reproductive tissues. A dose of 5 Gy (500 rad) is recognized as the critical level necessary to induce irreversible damage to the germ cells in the gonads, affecting fertility.

In terms of biological response, exposure at this level exceeds the capacity for the body's reproductive systems to recover or regenerate. For men, the spermatogonia—cells that develop into sperm—are particularly sensitive to radiation effects, and a dose around 5 Gy can destroy these critical cells. Women are similarly affected, as radiation exposure can impair or destroy the primordial follicles in the ovaries, leading to permanent sterility as well.

Lower doses, such as 2 Gy and 1 Gy, might cause temporary infertility or sub-fertility by affecting the function of reproductive cells without necessarily inducing permanent sterility. In contrast, a dose of 10 Gy is quite high and surpasses the threshold for significant biological damage, but the distinction is that 5 Gy is specifically characterized as the dose at which permanent sterility is expected for both sexes, making it the correct answer.