Japan's Public Radiation Dose

Japanese infants are receiving far more radiation dose more than I have been expecting. I apologize for my earlier optimism.


According to a press release, http://mdn.mainichi.jp/mdnnews/news/20110324p2a00m0na037000c.html, "The Nuclear Safety Commission of Japan (NSC) has made its first announcement on the estimated amount of radiation exposure in areas located around 20 and 30 kilometers from a quake- and tsunami-crippled nuclear plant in Fukushima Prefecture. The commission concluded the accumulated amount of radiation exposure per person was 500 millisieverts in areas within the "indoor standby zone" between 20 and 30 kilometers from the Fukushima No. 1 Nuclear Power Plant -- where people have been instructed by the government to remain indoors to avoid radiation risks -- over the course of 12 days following the March 11 disaster."

That is 50 Rem, for U.S. readers. The U.S. limit for dose to the skin is also 50 Rem. But, most of this dose is to the thyroid gland in the neck. And the calculation is for infants.

So, what does this mean?

Fortunately, the data indicates that there is little risk of effects at this exposure level. Unfortunately, there is not enough data to be definite about that. Thyroid doses are less dangerous than whole body doses, by about a factor of 20. http://www.oecd-nea.org/rp/chernobyl/c04.html But, infant doses are more dangerous, partly because they are growing quickly, and partly because they will live long enough to develop cancer or other diseases.

I wish that I could be more optimistic.

Chernobyl area residents got about 1.4 gray of dose, or 140 Rem. So that is about three times as much as Japanese infants near the plants have gotten - and the total they are likely to get is about what Chernobyl residents got.

That is far more than I have been expecting. I apologize for my earlier optimism.

But, these doses are calculated for infants, and the dose is to the thyroid. As best I can tell, no one else will get doses that high.

Therefore, cancers will be fewer than were caused by Chernobyl.

And it is still true that U.S. residents will receive far less dose. Washington State has a web page for airborne radiation, showing that, even with the releases from Japan, dose rates here are below the average annual rates from last year. http://www.doh.wa.gov/Topics/japan/monitor.htm