As Japanese government and TEPCO's PR presentation was very poor and not comprehensive, it is natural response that many foreigner left Japan for the time being.
I also had impression from their announcement that it is difficult to get reliable, truthful, concrete information about the situation, true risks, short term and longer term assessments, etc.
Although I'm not scientist, not familiar with nuclear technology, what I learned so far is that radiation level in Tokyo is pretty much safe level, we don't have to be panic, not necessary to rush out of Tokyo to evade from radiation.
First, what we need to understand is the radiation measure unit, what Sievert ("Sv") means, and the difference of micro SV, mille Sv, and Sv.
- Micro Sv x 1,000 = Mille Sv.
- Mille Sv x 1,000 = Sv
So, from what level is harmful to human? It is said as above 200 mille Sv. This means 200K micro Sv.
So far, radiation in east Japan is reported as micro Sv level, e.g. Tokyo Shinjuku area's radiation level as of March 20 is 0.0452 micro Sv per hour. If a guy stands out of building most of a day (10 hours), it means this guy receive 0.452 micro Sv. (But as you imagine, this is unlikely behavior.) As long as TV or news report "micro Sv" level, there is no concern at all in human health. Just for comparison, CT scan health check is 6,900 micro Sv.
Tokyo is 200-300km far from Fukushima Daiichi plant. As IAEA reported recently on their web site, "Tokyo and other areas outside the 30-kilometre zone remain below levels which would require any protective action. In other words, they are not dangerous to human health." IAEA verified Japanese government announcement was not lie.
Also, compared with Hiroshima atomic bomb in 1945, my grandparents and parents lived in a city just 100km away from Hiroshima. Atomic bomb radiation level was estimated as about 30 Sv (please be noted this means 30 millions micro Sv!) in 500meter from the ground zero point. This radiation level leads to immediate death or die within 1 month. But even in this extremely high radiation level, the people living 100km away from Hiroshima, they didn't have any health concern and my parents still live healthy, never suffered from cancer in 70 years time. I know it is not scientific analysis to refer the atomic bomb more than 60 years ago, but this idea personally relieve myself.