I tweeted a back-of-the-envelope calculation concerning radiation exposure here in Tsukuba, Japan, and in a few minutes had a slew of demands for documentation. I asserted that, at current exposure rates you'd have to stand outside 24 hours a day for two years to get as much exposure as the flight from Seattle to Tokyo. So, here is my evidence.
Tsukuba has a High Energy Physics Institute (KEK) which includes a Radiation Science Center. SInce the Fukushima accident, this group has been measuring background radiation in Tsukuba, and it is available online, in one minute intervals here. They report that typical background radiation is from 0.07 to 0.09 microSieverts per hour. At this moment the reading is 0.17 microSieverts/hr. Thus, the radiation exposure is between 2 and 3 times normal. An extrapolated daily radiation dose from background would currently be about 4.08 microSieverts. Of course, variations occur during the day, but as I said, this is back of the envelope stuff.
The Health Physics Society has a public information page that lists various references for radiation exposure due to commercial flying. Somewhere (I've lost the link) I saw a number of 3 milleSieverts for a transoceanic flight. The HPS ranges are broader, but that number is probably a pretty reasonable mark for an 11 hour flight in the northern latitudes.
So, the math:
~ 3000 microSieverts for an 11 hour transoceanic flight
~ 4 microSieverts for a day's background radiation exposure in Tsukuba at the current time
3000 / 4 = 750 days of exposure
And who stands around outside all day and all night, anyway?
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last season's Lotus pods decaying in the lake at Ueno park in Tokyo
