Radiation at the Fukushima Daiichi plant peaked around 400 millisieverts an hour, before falling back to 0.6. How bad was that?
The question turns out to be hard to answer. Why? Well, the total dose matters, but so does the rate. After all, DNA repairs itself, so getting exposure over a year and over a day are very different things. That said, there are some general rules about acute exposure that all radiologists know, and since I happen to be married to one, I figure I’ll put them up.
400 millisieverts an hour is enough to cause a transient sunburn over a period of 4-5 hours, although that won’t happen to everyone. (If you got 2,000 millisieverts in a matter of minutes, then you would start to burn.) After 10 hours, you might have some hair fall out 2 or 3 weeks later, but you’d have to sit out in it the radiation for almost a full day to lose the hair permanently.
The threshold for radiation-induced cataracts is about 12 hours at 400 millisieverts per hour, although symptoms might not show up for as long as 8 years. On the other hand, men would go sterile after 8 hours, and sperm counts would start to fall (definitely temporarily, but possibly permanently) after only 30 minutes exposure. (This is why men in careers that involve a lot of flying should probably consider freezing their sperm.)
Now, in theory an acute dose of 10,000 millisieverts is enough to kill you within a few weeks, by destroying the epithelial lining of the gastrointestinal tract. In practice, you’d need to get that dose very quickly: 25 hours unprotected inside the Fukushima Daiichi facility at current levels probably won’t be enough to kill you. But it is not something that you would want to try.
Cancer, well, that’s probabilistic, but your long-term risk would start rising after about five hours or so.
Whether this relieves or concerns is, I think, a matter of temperment. Given that the Yokosuka Naval Base appears to have detected a radiation level of less than a fiftieth of a millisievert per hour, redirecting naval vessels seems like a bit of an overreaction.