I'm no radiologist but from what I can tell I don't think Tokyoites have much to worry about.
The first chart comes from a blogger in Tokyo who has rigged his computer to a Giger counter. Recorded radiation levels seem to be normal (the spike at 9am occurred when he accidentally unplugged the counter). As you can see, readings from yesterday and today are more or less stable at 20 CPM.
Second chart comes from the Japan Times. It says the body can take up to 500 millisieverts of radiation before it has a toxic effect on the body.
Math works like this: 20 CPM is equivalent to about 0.0002 millisierverts per hour. The radiation in Tokyo is lethal, it'll just take 285 years to (partially) kill you.
That's not to say that we shouldn't exercise caution. The situation in Fukushima could take a drastic turn for the worse. Luckily radiation measurements are reliable and precise (anthropologists use it to date artifacts). If Tokyo were in trouble we'd definitively know.
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