At what radiation exposure level is permanent sterility typically observed in men?

Permanent sterility in men is typically observed after exposure to radiation levels of around 500 rad. This level of exposure can cause significant damage to the testicular tissue, which affects spermatogenesis—the process of sperm production. At approximately this radiation dose, the male reproductive system experiences irreversible changes that lead to the inability to produce viable sperm, leading to permanent sterility.

Exposure levels below this threshold may cause temporary infertility or reduced sperm count, but not permanent damage. Conversely, much higher levels, such as 1000 rad or 1500 rad, though they can certainly cause severe biological effects, are beyond what is typically required to achieve the specific outcome of permanent sterility in men. Thus, 500 rad is recognized as the threshold for permanent sterility.