What would extreme radiation do to the body?

What impact does the rate of radiation exposure have on the human body?

• People usually talk about the dose of radiation received. I read somewhere that a CT scan of the head was equivalent to 4 round trip coast to coast flights in the US for example. Nominally the amount of radiation received in these two cases is the same however how it's received is quite different. What impact do these differences have, if any? My first thought is about the rate of radiation exposure. I recall reading somewhere that a single large dose of radiation is better tolerated by the body than a continuous small dose, as it passes a threshold that turns on mechanisms to repair damaged DNA that would otherwise go unnoticed. Is there some truth to this hypoethsis? Another difference I note is that the CT scan is directed at the head while in flight presumably it is directed all over the body. Does the impact of radiation really spread throughout the body from a fixed point of exposure or is the risk of cancer much greater in the affected area as a result?

• Answer:

  The biological effect of radiation is dependent on the exposure rate, but you have it backwards. The body tolerates a slower radiation rate much better than a fast one. Basically, the body has time to repair the damage during a prolonged exposure, but a shorter exposure can overwhelm the repair mechanisms. A dose that would cause illness if delivered in a few hours (300 rad), probably wouldn't be noticed if delivered over a month. The area of the body exposed is significant also. An exposure to only one part of the body is much less dangerous than exposing the whole body to the same dose. Basically, if every part of the body gets a certain amount of radiation, it is clearly worse than if only one part got that dose. Health physicists use "tissue weighting factors" to determine the whole body equivalent danger from an exposure to a particular part of the body. For example, if your lungs get a certain dose, it is only 12% as bad as if your whole body had received it. You didn't ask, but the type of radiation also has an effect. A CT is all relatively low energy x-rays. In flight you also receive high energy cosmic rays. That must be taken into effect. The health effects of the CT are well understood and the benefits are generally worth the minor increased risk.