It has been stated that a daily radiation exposure of 3-12 rads per day will cause no short term effects. This is true to a limited extent. However, in radiation therapy where the typical daily dose is 15 Rads whole body, is has been noted that serious blood changes occur when day after day exposures hit around 150-200 R cumlative. Even if exposure is halted at that point, blood components will continue to deteriorate for the next 25-35 days. Ionizing radiation causes the formation of free radicals in the body.
Radiation sickness is a free radical disease just as a hang-over is.
The body can handle a certian amount of free radicals before the free radicals overwhelm the body's free radical scavenging system. When overwhelmed, part of the free radical damage consists of damage to the body's free radical scavenging system creating a vicious cycle. This is based on genetics, age, state of physical conditioning, overall health, and quality of nutrition.