1 Sv (Sievert) is 1000 mSv (millisieverts) or 1,000,000 μSv (microsieverts).
These are the scientific measurement units for radiation. Note that a microsievert is a smaller unit than a millisievert, perhaps counter intuitively, in a scale of thousands. I have recently made this mistake (math is not my strong point and I am no nuclear expert, I must say, but this investigation might still help here, given the experts recalcitrant attitude, please check these figures).
A radiation dose of 6000 millisieverts (mSv) is usually considered lethal pretty soon, and 9000 to be immediately lethal (human individual bodies react slightly differently).
The standard unit used is millisievert, or mSv, since it relates fairy well to the range in which it affects humans. I will try to stick to this unit here.
According to Encyclopaedia Britannica:-
"The average total dose received from all three [natural] sources by a person residing at sea level is approximately 0.91 mSv per year (Table 6); however, a dose twice this size [which makes 1.82 mSv per year] may be received by a person residing at a higher elevation such as Denver, Colo., where cosmic rays are more intense (Table 3), or by a person residing in a geographic region where the radium content of the soil is relatively high (Table 4). In the latter type of region, the radioactive gas radon, which is formed in the decay of radium, may enter a dwelling through its floor or basement walls and accumulate in the indoor air unless the dwelling is well ventilated periodically; occupants of such a dwelling may therefore receive a dose as high as 100 mSv per year in their lungs from inhalation of the entrapped radon and its disintegration products "
Today, as I write this at 18 March, NHK World TV reports (Japan) claim that at a reactor at Fukushima 1 plant the hourly rate is 20 mSv, and at the gate it is 3 mSv (we are left to assume per hour, given this is standard), while 30 km away it is reportedly 0.15 mSv (we assume per hour).
While a single chest scan may be up to about 18 mSv for the short duration, the reported 0.15 mSv at 30 km in Fukushima prefecture is somewhat less than that. But it is, at a cumulative rate (0.15 x 24 x 365 =) 1314 mSv per year, which is not good. The yearly 1.82 mSv possibly received in a fairly high natural radiation dwelling in Denver Colorado (as according to Britannica) is in fact a lot less. Also even the higher 100 mSv per year in a stuffy dwelling with radon gas coming out the floor is less. But it is still not highly dangerous, if the figures are correct.
I have been wondering therefore why are people being made to stay indoors between 20-30km if this is within this relatively low range? Will this radiation quickly fade away or not? Why are other nations recommending evacuating to a much greater distance from the plant? Questions and questions, and for me they are not urgent (perhaps), but I can imagine for anyone nearby they are crucial and they want, more than anything, clarity.
If this reading was instead actually 150 mSv at 30 km (not 0.15 mSv), then it would be perhaps more understandable but highly dangerous, because in only 48 hours it cumulatively becomes 7200 mSv and that is lethal. 6000 mSv, remember, is lethal.
Or, if it actually meant 1.5 microseiverts per hour as on the later graphic from the New York Times that I refer to below, it is quite low but not normal. 0.10 microseiverts would I think be a standard hourly rate (micro not milli remember).
Consider this: an earlier report on NHK last week quoted 400 'something', possibly 400 mSv (which was what I and others understood it to mean) read at the gates of the Fukushima 1 plant. But I was not absolutely sure if this meant mSv or microseiverts. Some press reported this as millisievert and were duly shocked. If millisieverts it would indeed be very worrying. But today a similar reading was reportedly only 20 mSv (definitely stated as millisievert) at a reactor and 3 mSv at the gates of the plant, which is quite a difference and a lot less alarming. With the figure of about 0.15 mSv at 30 km, this then tallies and makes some sense, because the figures are reducing steadily at a greater distance. But could the readings have been reduced so dramatically over the intervening time from 400 mSv (if correct) to 3 mSv? We will assume this must in fact be the apparent reduction from 4 mSv to 3 mSv at the gates and the 400 in fact meant microsieverts not millisieverts.
I hope the latter is so, for it would be some good news for the Japanese people given the figure is reducing. But it looks as though the calamity has worsened rather than improved, so it does run counter intuitive. Also, we can understand being advised to stay indoors except for necessary trips at this distance in this case.
But the figure still seems a bit low to cause such serious concern, while the 400 mSv might make more sense given the attitudes being demonstrated by most of the commentators and the foreign experts. But it seems too alarmingly high and makes the other readings also seem highly suspicious and inaccurate. Is this just evidence of media sensationalism?
If occupants of a stuffy dwelling up a hill in Denver Colorado can receive 100 mSv a year naturally as Britannica says, certainly (to repeat, 0.15 mSv x 24 hours x 365 days =) 1314 mSv per year at 30 km from Fukushima 1 would not be too much to be concerned about if it were to reduce anyway in the year, as it has already been reported to have done a little at the gates of the plant.
19 March 2011: -
To quote the BBC live blog at 0933: "Outside the exclusion zone around Fukushima nuclear plant, radiation levels are absolutely miniscule and nothing to worry about, points out the BBC's Tim Willcox in Tokyo. That's interesting, he says, given how many foreign nationals have left Tokyo and other parts of the country."
from http: news="" world-middle-east-12307698="" www.bbc.co.uk=""
By 19 March 2011, BBC reports say readings of 0.05 microseiverts in Tokyo, which if per hour (the reporter does not give this crucial information), is extremely low but a very tiny bit higher than normal (apparently this is about 0.03 microseiverts according to a blogger! - I can get next to no trustable information from official sources). In fact it seems so low as to be a bit unlikely, since a sum of 0.10 microseiverts would be standard safe reading per hour at sea level. A figure of 0.10 microseiverts per hour (or 0.91 mSv per year - Britannica) is considered about normal.
The NY Times reports with a graphic expressed in microseiverts per hour. It implies it comes from the Japanese government and the data appears to be for Thursday 17 March 2011 (two days ago). It says on Friday levels of 150 microseiverts per hour were reported 19 miles (about 35 km) northwest of the plant, down slightly from 170 per hour on Thursday.
150 microseiverts (0.15 mSv) per hour is quite worrying given it is significantly higher than a standard rate. In the graphic one area reports 18.3 microseiverts per hour (0.018 mSv) not far from Fukushima city at about 55 km from the plant, but only 2.1 microseiverts in the stay-indoors zone only 30 km from the plant. This could be the wind factor given that along that line are some other higher readings. The graphic gives an example: 23 microseiverts per hour cumulating for a year gives 200,000 microseiverts. By my calculation that is 200.0 mSv per year. This is not very high but would be of concern if it remained at that level because it is associated with slightly higher cancer risks, it is about twice the naturally produced higher level in our fictional 'Denver' hovel.
However the same graphic says the maximum level so far recorded at the gates of the Fukushima 1 plant was 647 microseiverts per hour (the graph was published today 19 March, and it looks as though the English text is written over the top of a Japanese (?) graphic by the NYT, but it may be reporting the figures for Thursday 16 and Friday 17 March), or 0.64 mSv per hour, but this last figure contradicts the 3 mSv (3000 microseiverts) per hour reported at the gates of the plant from the verbal TV report yesterday 18 March, unless this is now an out of date maximum and this new maximum has risen quite significantly, which would make intuitive sense given we see the greater destruction but contradicts claims about falls in readings.
From all this I suggest the government seems to be giving out some correct and some incorrect data, and in ways that are confusing, and these figures are reported in confusing ways after that. Radiation levels have generally increased, but the Tokyo figures are suspiciously too low.
See NY Times:
My figures are, apart from newspapers websites, from sites such as the IAEA (which in fact is useless since it has little hard data) and the BBC, but are mostly drawn from Japanese NHK TV presented by a commentator as well as presented through an interpreter of Japanese to English, so there may be mistakes, but these reports are repeated in video so can be written down, also they are direct translations of the figures that Mr. Edano of the Japanese government reports from TEPCO (the managers of the plants), and some are also written on the boards that the presenters use in the diagram of the exclusion zones of the radiation figures for Japan.
One thing I must conclude: the constant unnecessary changing from micro to millisieverts, coupled with the figures of the authorities fluctuating (apart from when they do), as well as not stating the time periods that apply for dosage and exposure, cumulative or not, leads to great confusion, which the Japanese people, suffering in this truly awful situation could really do without. The government has an undeniably difficult task and is trying very hard, but remember what this industry has promised the world in safety and accountabiity terms.
I must also say the measurement of radiation seems also to be deliberately shrouded in techno babble at the outset, and has changed so many times that it is already hard to understand. This adds to the sense that the Nuclear industry has a lot to hide even before such a calamity as this possible meltdown of no less than six reactors existed.
In this context Professor David Spiegelhalter, of the Public Understanding of Risk, Cambridge University, says:
"Radiation does, however, feel acceptable when used in benign circumstances such as medical imaging. You can pay £100 ($160) and get a whole-body CT scan as part of a medical check-up, but it can deliver you a dose equivalent to being 1.5 miles from the centre of the Hiroshima explosion."
from http: news="" world-asia-pacific-12785274="" www.bbc.co.uk=""
Which is decidedly disingenuous (if not actually ridiculous) considering it once more blithely leaves out the factor of the length of time of the exposure and any actual dosage figures, and if you do that you can almost claim anything. And, you see, he is a scientist!
But to be fair he also notes:
"The electricity company appears to be as secretive as its reputation suggested and although the Japanese media are mostly giving the government an easy ride, individuals able to follow western sources are faced with a barrage of conflicting opinions."
from BBC: - http://www.bbc.co.uk/news/world-asia-pacific-12785274
With the gaps in communication between the big corporations, the Japanese government, and the media being exposed, as it seems to be here, as well as being compromised already by the vested interests involved, confusion and then anger results.
I cannot find an expert who will lay out the bare facts and figures in a convincing and straightforward manner. This alone tells us something.