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How much radiation is too much ?

In Japan many citizens are concerned about the radiation levels which they are being exposed to. Before anyone can decide if radiation levels are too high, a choice needs to be made as to what is too high. I have recently seen that in one area of Japan the radiation levels are 0.25 to 4.62 microsieverts. Now before I get going I want to stress that journalists must learn not to truncate the units. The radiation levels are normally expressed in a dose per unit time.

If I was working in a place where the dose rate reached 1 microsievert per second I would step away very fast and ask what was going on (in one hour I would be getting over 20 % of my yearly limit), but if the dose rate was 1 microsievert per minute then I would ask “what is going on ?” while if the dose rate was 1 microsievert per hour then I would think “about normal for the west coast of Sweden”.

If you are a journalist who finds that your coworker has been truncating units in this way then please tell them off (please no whacking them with a carpet beater) So once we have got the real dose rate then we start to think about it. I will assume that the units were microSv per hour.

My advice is to translate these doses into yearly doses. Which gives us an level between 2.2 and 40.5 mSv per year. The 2.2 mSv per year is about the same as the background in Sweden but the 40.5 mSv is a very high level. I would like to point out that some places exist with natural high backgrounds such as the coast of south west india which have had yearly dose rates of about 40 mSv per year for years. But I think that if the dose rate in a location is increased by an act of man by 40 mSv per year then something needs to be done !

We have the problem of how do we set the limits for human exposure. The highest current limits in Europe for a classified radiological worker is 50 mSv in one year but no more than 100 mSv over five years (Sweden / Czech Republic) which works out about the same as the UK (+ most of Europe) limit of 20 mSv per year. In the UK and Sweden a second type of radiation worker exists which has a lower limit, the UK law is that if a person is unlikely to get a dose below one third of the classified worker limit then they can be placed on a regime where they are no longer subject to quite the same level of medical supervision.

The limit for occupational exposure for non radiation workers and the general public is 1 mSv per year.

You might ask how can a person who is not a radiation worker be subject to occupational exposure, it is quite simple. Imagine a person who tends the lawns at a nuclear site, maintains the site fence and paints the outside of the buildings, this person might never enter a radiation area or need to be trained as a radiation worker. But the person might in principle be exposed to a tiny amount of radiation which passes through concrete walls. This exposure of the handyman needs to be kept to a very low level, unless a limit is set society can not be sure if the person is being exposed to what is an undue dose. Classified radiation workers is they are kept under medical surveillance to increase the chance that a radiation induced disease is spotted at an early stage while it is still treatable.

One non government organisation (The ECRR) wants to lower the annual general public limit for exposure to artificial radioisotopes to 0.1 mSv and the limit for nuclear workers to 2 mSv per year (http://www.euradcom.org/2011/ecrr2010.pdfpage 190). While other bodies wish to set the limits higher. My own view is that the 20 mSv limit in Europe is a reasonable limit for occupational exposure of radiation workers. The problem with a dose limit of 2 mSv is that while it might reduce by a factor of ten the radiation dose to humans, it is likely to cause a series of unwanted effects in society.

One problem would be the treatment of cancer by gamma emitting radioisotopes, we need to consider the service engineers who tend and look after the equipment. In the UK these workers often get more than 2 mSv in a year, if we were to limit these workers to only 2 mSv per year then it is likely that the cohort of people tending the machines would have to increase greatly and each person would be able to work for fewer hours per year doing work such as source changing.

This could well result in a great reduction in the skill level of the workers, this is because it would be harder for a worker to get the hundreds of hours needed to become competent. My worry is that a larger and far less experienced cohort of workers would work more slowly and be more likely to make mistakes as a result of being more poorly trained. This could lead to an increase in the radiation exposure of humans if we consider the number of manSv resulting from the use of radiotherapy equipment.

If radiological vigilantes start to mark out “dangerous areas” using very different criteria to those of the national radiological protection authority then it is likely to lead to lots of trouble. If we take a dose rate of 0.48 microsievert per hour as the limit for a public area (this is based on a 40 hour a week and 52 weeks a year exposure time) then we might get a 100 square meter exclusion zone around a point source. But if we choose a 0.11 microsievert per hour limit then we would get a much larger exclusion zone (436 m2).

Alternatively we could take the Louisiana rules for industrial radiography and view 50 microsievert per hour to be the dose at the edge of a radiation area. This would give a much smaller exclusion zone of about one square meter.

We can imagine what would happen if a series of different groups of radiation measurement people are going around where one lot rope off a “high radiation area” which another group regard as “close to nothing”. What will result is total confusion !

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