[janetjanet998]

The past few hours some in japans goverment have said the radiation is now below the level to cause harm to humans....while at the same time the defense minister said the helo operation to dump water on the reactors was called off becuase of high radiation. so which is it?

[palmer]

Here's a link regarding potential harm to humans: http://www.merckmanuals.com/professional/sec21/ch317/ch317a.html#sec21-ch317-ch317a-605 It doesn't answer your specific question, but at the peak reading from yesterday (400mSv/hour) there is 20% increased risk of cancer from 2.5 hours of exposure plus up to five percent chance of death from radiation sickness from anything longer than that.

[VanShuyten]

Apparently, the helecopters just dump lots of water at once, just like when they are used to fight forest fires. The concern was that such a sudden dump would produce a blast of radioactive steam that would affect the flyers. In addition, they were worried about additional damage done by dumping lots of water on top of the pools.

They want to pump water in using firetrucks so they can control the rate better and avoid any of that.