A Geiger counter in Tokyo has been set up to publish the readings online. Over the past hour, the counts per minute have been rising from the baseline of 20 to nearly 60 cpm. These readings do not represent a dangerous level of radiation in Tokyo—but a worrisome development, for certain.


Now up to 80 cpm, as of 1213:


For reference, 20 counts per minute (cpm) is the normal background radiation level at this site. 80 cpm (in this detector) is roughly equivalent to 0.80 microseiverts per hour. (100 cpm is about equal to 1 microseivert per hour.)

This ustream links to another Geiger counter in Tokyo, this one reading closer to background levels.

Update 3:


Levels have come back down, closer to 40 cpm now.

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For a bit of context:
As I stated above, one hundred counts per minute in a Geiger counter like this is equivalent to 1 microseivert per hour. It takes about a full seivert (equal one million microseiverts) to have a risk of death from acute radiation poisoning. At lower levels of exposure, around 100 milliseiverts, one can suffer some permanent injury (like infertility). A dose of 1 milliseivert per year is considered a 'safe' annual dose of radiation.

These levels are nowhere near anything that would cause short-term injury. The rise in CPM is more a reflection that some radiation from the plant (hundreds of miles away) is now reaching Tokyo—a curiosity more than an actual concern. Radiation at this level would—for example—never prompt an evacuation of a city.

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