A lot of the rationale of supporters of non-evacuating Tokyo seems to revolve around the notion of hotspots. Hotspots are limited areas in which ground radiation spikes up compared to the surroundings. Although radiation is not negligible in Tokyo, some argue that it is even higher in similar cities where there hasn’t been any known nuclear incident, such as Hong Kong with more than 0.3 uSv/h at 1m above ground. Tokyo hotspots detected in Setagaya, around the Imperial Palace, etc. were not very satisfactorily explained in the news by the supposed presence of radium bottles left over (same explanation used several times).
Tokyo cityscape changes continually and buildings may disappear suddenly as one visits a street after a few years. Besides construction work and demolitions, low to medium intensity earthquakes regularly shake the city and cause shelves, TV sets, etc. to fall and smash on floors. It is difficult to imagine radium bottles lying around for 50 years untouched in these ever-moving conditions. Such bold statements from the government were largely accepted by the population who is eager to cling to any reassuring explanation for their hotspots.
Hotspots are perceived to be like rotten apples in an otherwise healthy basket, singularities which statisticians can dismiss in order to focus on the average radiation environment. Scarcity of hotspots seem to support this view, however monitoring is imperfect and reporting even worse.
Shortly after March eleven, a green tea grower from Shizuoka prefecture (south-west of Tokyo and near the Mount Fuji) reported that his tea was radiation-hot after he got it analyzed on a voluntary basis (Cf. link to New York Times story in Analysis Of Japanese Government Radiation Spread Report on SurvivalJapan). Panic followed among green tea growers who made sure that none of them would ever carry their tea leaves to a laboratory again. This is anecdotal but it illustrates how hotspots are discovered and buried in Japan. Therefore hotspots tend to seem isolated whereas, if the population wanted to seriously investigate, there might be more rotten apples.
Here is another anecdote: in the farming village from where I buy local vegetables and rice, I proposed a foreign friend of mine who grows organic food there to make some radioactivity measures – and in order to make it significant, to get organized with the local Japanese community and offer to check out their fields and rice paddies too. My friend replied that such discussions had already taken place and that the consensus was that, although it would nice to know that the soil is safe, it would have a devastating effect with commercial consequences, should we find anything unusual. I discreetly made a measurement there which showed it was alright (0.125 uSv/h with Inspector Alert right next to the wet, black soil) and told my friend about it. From reading the news and hearing people talks, I am convinced this is a relevant example of farmers’ attitude with respect to radiation monitoring nationwide – and hence the explanation of the scarcity of reported hotspots.
Hotspots monitoring in Japan is like searching for a sick tree at the edge of a forest, cutting the tree down and never look in this place again.
Singularities, i.e. unexpected and isolated events, are sometimes useful in science and can lead to breakthroughs. They need to be thoroughly investigated and a plausible explanation proposed, in order to make a model and, later on, forecasts. Radium bottles buried by Oni nationwide hardly qualify.
Instead of dismissing potential meaningful pieces of information such as hotspots for mere artefacts, given the under-sampling and information concealment, I would rather instead consider hotspots as indications of a wider problem, which size would be at least that of a typical rain cloud for instance.
Hence I tend to err on the side of precaution and view hotspots as trees hiding the forest. It may seem to be a detail, but it underlies everyone’s belief about risk in Japan. Indeed, Tokyo residents may consider it safe to eat produce from the Kanto Plain based upon the notion that hotspots are unlucky draws in a reasonably safe zone – while I recommend to avoid the whole area and define north-east of Japan off-limits, i.e. a no man’s land.