Well, here we go, my amateur attempt at calculating how much radiation I've been exposed to and whether I'm at risk. Today I'm going to look at external radiation.
According to Wade Allison's book (see last post) the average annual radiation exposure of the UK population from all sources is 2.7 mSv (millisievert) per year, of which 75%, just over 2 mSv/year, is from background radiation and the rest from internal and medical exposure. The average annual exposure from natural radiation in Japan was between 0.35 and 0.53 mSv/year with a recommended limit of 1.5 mSv/year.
Some background radiation is from cosmic radiation (from the Sun and activity even outside our galaxy), but most is from gamma radiation and radon from water, soil and rock.
Before we start, let's get one thing clear. There are 1,000 microsieverts (μSv) in 1 millisievert (mSv).
For every day since 11 March I noted where I was, put in the radiation per hour for that place, then how many hours I spent inside and outside. For inside I used one tenth of the outside values since when I'm inside I'm in concrete buildings. (Current reading in this apartment is 0.09 μSv/hr, outside readings just below 1.) Problem at first. I couldn't find any readings for Koriyama until 23 March. Do you remember the fuss when Koriyama City was found to be taking readings from the roof of City Hall? Well, those records have been removed from the city website. However, I found averages on an NHK online site and used those. Readings went from a perfectly normal 0.06 μSv/hour on 14 March to 8.26 μSv/hour on 15 March (Yikes! That was the day I biked one hour each way to and from Sakuragaoka!), but levels quickly fell to under 3 by end March, under 2 by mid-April and under 1 by end July. I totted up the list and reckon that in the 5 months to 20 August I've been exposed to 1.19 mSv. Borrowing some data from the source below this could be 2.1 mSv for the year to 11 March 2012. Well, the same as the UK average but well above the Japanese recommended limit.
There's another way to do this. The Ministry of Education has put out maps with calculations done for you. On the link below there are three maps. The top one shows current radiation levels in the air on 11 August, the second an estimate of what your total would be in a years time (by 11 March 2012), and the third map shows accumulation up until last week, 11 August. (There are tables after the maps.) Koriyama is on the left hand side of the map about half way down, marked by contours which show the city to be in the 5 to 10 mSv/year range. According to this estimate, accumulation up to 11 August for someone in Koriyama is 4.5 mSv and the estimate for the year to next March is 8.6 mSv. These estimates are based on being outside for 8 hours a day and being inside a house made of wood which lets in 40% of external radiation. So they're pretty generous.
Incidentally, the government adopted a figure of 20 mSv/year (lime green on the map) as the basis for evacuation.)
The calculations I did for me (if they're right) are a lot less than the averages given so I'm pleased that my efforts to stay indoors have reduced the numbers. The maps raise more questions. The centre of Koriyama, where I live, has accumulated relatively high levels so far (7-10 mSv/year) whereas on the other side (east) of the Abukuma River, it is only 1. The factory will be moving over the river next month so that's good news.
Whether these levels are dangerous is another story. Probably not. Levels in Koriyama now are the same as for Cornwall. So having lived in Japan, with its relatively low levels for 25 years, the three-month spike is not going to make any difference over a lifetime. Tomorrow I'll look at internal exposure.
The main news here is that Maehara, has decided to stand in the election for party leader. He's 49, good looking, popular and came to attention two years ago when the Japan Democratic Party (minshuto 民主党) first came to power. As Minister of Transport he put the party's credo 'people not concrete' into practice by immediately cancelling several high profile dam projects. Six months ago he was Minister of Foreign Affairs. I saw a piece on TV in which he apologised to some Australian PoWs. They were thankful and moved by the experience. Then the very next day he resigned! He'd been found to have received political donations from a Korean living in Japan and politicians are not allowed to receive donations from foreigners. The amount was small. He resigned on a matter of principle. He wasn't going to stand because of this but changed his mind and this has really set the cat amongst the pigeons. He would get my vote. But I don't get to vote. The party's MPs decide and Ozawa, that wily old-school politician and kingmaker controls 100 of the votes.
Bye for now