[potatolicious]

1 mSv is estimated to increase your odds of serious medical complications by 1/1,000,000. Not a significant amount, but one's regularly allowable dose should not be expected to cause health problems in the first place. Also, these limits are very conservatively set for various reasons - most of which I agree with (used to work in a nuclear facility).

So no, if you got blasted with 10 mSv all at once you shouldn't expect to vomit blood or fall over dead, but it's still a sizable amount of radiation.

Also, 400 mSv/h is 3,504,000 mSv/Y (aka 3504 Sv/Y, aka 17,000 times the radiation output in Ramsar, Iran). Of course, one doesn't expect this radiation leak to last for a whole year, but the dose rate does matter. Sucking in 400 mSv over an hour is many, many times worse than getting it over a matter of years or decades.

[beefman]

A typical CT scan will expose you to 10-15mSv. A dose of 100mSv infers a 1% lifetime cancer risk. The effects of doses as small as 1mSv are not known and are inferred by models.