Reply #16: It's not what you hope to see

[Yo_Mama]

But it's puzzling given the strontium numbers I have seen from Fukushima, where surely there should have been more. The highest I remember from the Fukushima area was 250 becquerels/kg. The highest number I remember from the soil at the Fukushima Daiichi plant itself was 480 becquerels/kg:
http://www3.nhk.or.jp/daily/english/01_h01.html

I think the highest contamination for strontium 90 during the nuclear testing era for Japan was around 50 becquerels per kg.

I have to wonder how a number that high could be on a roof in Tokyo! There was only one brief period early on in the disaster when the wind blew south toward Tokyo; unless almost all of the Strontium 90 emissions occurred early on during that period there should be no way the roof could be that contaminated.

In terms of danger to humans:
Strontium 90 isn't that bioavailable. 20 - 30% of it is retained. Strontium 90 contamination is pretty widespread globally. On the rooftop it is not much of a danger. You wouldn't want to see it in water. In agricultural soils it can be a problem due to uptake and subsequent ingestion by humans.

Yokohama is going back and testing their ditch samples for strontium 90 now - they'll know more in a bit:
http://www.nippon-sekai.com/main/articles/fukushima-daiichi-nuclear-power-plant-crisis/yokohama-city-tests-soil-for-strontium/

Note the extremely high contamination levels from cesium already detected! (40K becquerels/kg.) That's the real threat, although it doesn't translate well into human exposures, because you expect cesium to concentrate in drainage areas.

If I had to guess, I would say that the rooftop reading is either an error or stems from contaminated building components, probably steel. If that were true, it would be no threat to humans at all.

So far, the strontium-90 concentrations reported in Japan seem unlikely to affect human health. They are similar to a lot of US exposures during the nuke-ourselves era (I am a proud graduate), and because of the filtering effect, lifetime exposures are probably going to be less than say, mine. Anyway, if you want background info, try these links:
http://www.epa.gov/superfund/health/contaminants/radiation/pdfs/strontium.pdf
(General)

http://www.epa.gov/radiation/rert/nuclearblast.html
(specific, with some graphs)

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1915172/pdf/pubhealthreporig00096-0053.pdf
(analysis of milk exposure in human diet - US)

http://www.epa.gov/rpdweb00/docs/federal/frc_rpt4.pdf
(very detailed analysis of dietary exposures extrapolated to lifetime risk for human population during blast era)\

Short summary if you don't want to read all that:
http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/tooth-fairy.html

By far, the largest source of Sr-90 in the environment (~99%) is from weapons testing fallout. Approximately 16.8 million curies of strontium-90 were produced and globally dispersed in atmospheric nuclear weapons testing until 1980 (UNSCEAR 2001)2. With a 28 year half-life, Sr-90 still remains in the environment at nominal levels. Numerous measurements were made during weapons testing which found that the worldwide average radiation dose from ingesting Sr-90 (1945 to present) is 9.7 millirem (about equal to radiation doses from a transpolar flight), and the dose from inhaling strontium-90 (1945 to 1985) is 0.92 millirem (about equal to the dose from an arm or leg x-ray). These doses are well below those doses known to cause any effects on health (NCRP 1991)3. The doses from Sr-90 in the environment are about 0.3% of the average annual dose a person in the United States receives from natural background radiation (~300 millirem).

As a result of the Chernobyl accident, approximately 216,000 curies of Sr-90 were released into the atmosphere. An increase in the incidence of childhood thyroid cancer in the area directly affected by the accident has been attributable to radioiodine ingestion. No other increase in overall cancer incidence or mortality has been observed that can be attributed to radiation from the accident (UNSCEAR 2000)4.

There's a lot of nonsense written about low-dose radiation, including ridiculous death estimates. Strontium 90 exposures outside of the non-local area of the Fukushima Daiichi plant, based on everything we know and the unfortunate experiments conducted upon human subjects such as yours truly, are unlikely to cause detectable human health problems. The LNT model is obviously false - here's a pro/con argument pdf:
http://www.columbia.edu/~djb3/papers/rpd1.pdf

The con guy - Raabe - is an authority on studies in animals. See the graph on page 5, and note the dosage in Grays. Those are massive doses compared to anything we can reasonably expect the Japanese population to intake. The fact that no correlation seems to show for myeloma/bone cancers in the US for my age group implies that beagles and humans have a pretty high threshold for exposure without injury.