The Open Thread 21 has passed 500 comments and is getting a little bloated, so time for a new one.
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There was quite a bit of discussion in the previous OT on radiation levels and the Fukushima evacuation zone. Relevant to this is the recent announcement that Japan will lift the entry ban on some cities within the prefecture. To quote:
In areas where annual radiation measurements are below 20 millisieverts per year, a government safety guideline, residents will have free access to their homes during the day and will be allowed to return permanently at the earliest opportunity post-decontamination. Where readings are between 20 to 50 millisieverts annually, evacuees will also have unrestricted access during the day although their permanent return will come later. In areas where measurements top 50 millisieverts, residents will not have free access and they will not be allowed to return for a minimum of five years.
A past BNC guest poster, engineer Chris Uhlik, analysed the situation a private email distribution list, and I thought his summary with respect to LNT (linear no-threshold hypothesis of radiation damage to living organisms) was very useful. With Chris’ permission, I reproduce it below:
The official position of every regulatory agency & scientific body, and even the people who will tell you “we don’t know what’s going on under 50 mSv”, the weight of the evidence favors LNT.
Here’s what I think is going on:
Under 50mSv/year we can’t find any epidemiological data to support LNT. There is simply too much noise and other effects to see sub-0.5% changes in cancer rates in populations where the variations from other effects (smoking, stress, chemical exposures, etc) are in the range of 20–45%.
The rates of different kinds of cancers are affected differently by radiation. Some kinds appear to increase while others decrease. Some kinds of cancer are more treatable than others and thus result in different mortality rates, even if the occurrence rate increases. Simple statements like “cancer death rates show a LNT response to radiation exposure” are way too simplistic to be true, but such statements are easy to base regulations around. When regulators feel a need to support a regulation with some math, they’d rather choose simple math than more-correct, but difficult to understand and explain math.
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