During our recent trip to Fukushima, we carried two geiger counters with us, one inside the bus, and one taped to the outside of the window, facing inwards.
These geiger counters measure gamma radiation, and give a readout in micro Sieverts per hour, or μSv/h. Most people in the world are not familiar with these units, but, of course, here in Japan there is a high level of familiarity and interest in radiation levels. For example, on the right is a map stuck on the lockers in an office I went to.
In a previous blog about the trip to Fukushima, I mentioned some geiger counter readings we took, including one of 0.924μSv/h. This was on the ground outside one of the temporary houses built for evacuees from Iidate village. This blog explains a bit about radiation measurement – it is unfortunately a highly technical issue.
What is safe?
This is a highly debated issue, since it seems feasible to some that a single gamma ray could damage the DNA in one cell, and this could result in cancer. Of course, there is always some radiation hitting us, from cosmic rays, and naturally occurring isotopes. This is about 2.4mSv per year on average. Some scientists say that doubling this to 5mSv doubles the chance of getting cancer, and others disagree. There is no definitive research that has been done proving this one way or the other. There is no disagreement however, that high doses of radiation result in cancer – you might have noticed your dentist wearing a lead apron when x-raying your teeth.
Firstly, ‘safe’ doses are usually describe in milli-Sieverts per year, or annual mSv. In the USA, the acceptable level after a radiation leak is 20mSv for the first year after the leak, and 5mSv thereafter. To convert from μSv/h to mSv/year, you multiply by 8760, so the reading of 0.924μSv/h translates into 8.1mSv/year. This means that this area would be considered unsafe for human habitation in the USA. Bear in mind this is where people, including children, have been evacuated to.
There is a nasty element of practicality here – if all areas which were above 5mSv were evacuated, where would everyone go? The expense of doing this means that safety standards are changed, so that the number of people the government is obliged to evacuate is lower.
A measurement in Sieverts does not describe the actual radiation, rather it is a measure of the dose received by a person. So by walking over a contaminated area, a certain Sieverts will be received, lying down for a while in the same area would result in another figure, and putting some soil in your mouth would result in far more.
Where to measure?
The above highlights a serious issue – when measuring Sieverts, where do you measure? The government in Japan has decided that one metre off the ground is the standard, and all figures released are according to this. But children are not one metre off the ground, so an area declared safe for an adult to walk through could be extremely dangerous for a child to play in. Also, gutters tend to be radiation hot spots, because the top layer of soil washes into the gutter when it rains. One of the ‘solutions’ to radiation on the roads is to wash them down with high pressure hoses. This, of course, just concentrates the radiation wherever the water ends up.
This is why children over a wide area are not allowed to play outside, even though they will not be evacuated. This is likely to stay the same for decades, or perhaps longer. What will the long term effect of this be on the society? Only time will tell, but it is unlikely to be positive. It is after seeing this, and talking to concerned parents about it, it is not possible to read pontificating statements about how its not so bad because ‘only three people died due the Fukushima nuclear disaster’ without becoming very angry. There are also many other effects, such as families torn apart by being evacuated to different areas, and men continuing with their jobs while the wives and children are evacuated. A fireman we spoke to see his young child and his wife about once a month now. This is likely to continue for some time.
It has fallen on civil society organisations in Japan to do measurements according to their own standards, such as measuring on the ground, and also 5cm off the ground.
While the devastation from the tsunami of March 11 is easily seen, the radiation contamination is not visible. It remains dangerous over a wide area, which even now is not limited to the arbitrary 20km zone, and it continues to be spread by wind, and water. This has a multitude of effects over and above the increased risks of cancer, and results of this are sure to be felt for generations to come.