Fukushima radiation detection and measurement

Here is a back-of-the-envelope calculation of the radiation level required to cause immediate skin burns.

The specific heat of tissue is about 4 joules per gram-degree C. So it would require about 80 joules/gram to raise the skin temperature 20 deg. C (like spilling boiling water on skin).

Because the definition of a Sievert is 1 joule of energy deposition per kilogram, we have

20 deg C temp rise = 80 joules per gram = 80,000 joules per kilogram = 80,000 Sieverts.

This sounds like a lot. For comparison, I know (from personal experience) that I could not feel 42 doses of 1.8 Sieverts (per session) of focused gamma radiation for prostate cancer treatment last year.

Just a (very late) comment on this: the skin burns by radiation don't come from THERMAL heating of the tissues, which only play a role, as you calculate, at crazily high doses where thermal heating is actually the last of your worries. The "burning" actually means tissue destruction by the ionising effect of the radiation, which results in major chemical damage to the cells to a point where they are actually destroyed (their proteines are disrupted, and the membrane is broken). You can even have an effect at lower doses, where the cell's biological function has been destroyed, and will soon die off as it has no correct selfsustaining metabolism anymore.