News blog

Confusing radiation numbers swirl around Fukushima

The television and various news sources are clogged with radiation numbers and experts trying to explain them. Unfortunately the result seems to be a confusing tangle of data that is difficult to understand.

The most fundamental problem seems to be the rampant switching from micro to millisieverts. As a science reporter I love the metric system, but I do wonder if the public can easily understand that a change in radiation from 5 millisieverts per hour to 1,000 microsieverts per hour is actually a five-fold decrease in exposure rates?

Similarly, do people appreciate that the .144 microsieverts per hour seen in Tokyo yesterday afternoon is actually 70,000 times less than the 10 millisieverts per hour today at the Fukushima nuclear plant gate? (I actually got confused just trying to write that sentence, so from here on out I’m sticking to millisieverts per hour, or mSv/hr).

Let me try to put this into perspective. As I’ve already mentioned in an earlier post, the effects of radiation depend upon the total amount of radiation received over a period of time. So at a level of 10 mSv/hr, an unprotected individual at Fukushima might be able to work for 10 hours before feeling nausea and other effects of acute radiation exposure.

For long-term health effects radiation is often measured in years. Receiving 100 mSv in a year is the line at which you see a small increase in cancer risk. This risk rises with any additional dose.

NHK television was reporting .08 mSv/hr at 25 kilometres west-northwest of the site today. A back-of-the-envelope calculations makes that 700 mSv per year (simply: .08 mSv x 24 hours x 365 days).That is a serious dose, but not as bad as it initially sounds. For one thing, the radiation coming from Fukushima seems to be sporadic, so it won’t stay at .08 mSv/hr for a long time. Additionally, you would only see the effects of that radiation if you were standing outside for a whole year.

More realistically, people will take cover to the greatest degree possible, reducing their exposure considerably. On top of that, iodine tablets and simple precautions for those outside (such as covering up, and removing clothing when moving indoors) will help a great deal. In this context .08 mSv/hr probably isn’t much to worry about, though it may become an issue for rescue workers outside for long periods.

As I mentioned, in Tokyo, Japan’s science ministry reported average rates of .000144 mSv/hr yesterday afternoon. That’s double the background rate, but should residents of the capital be worried? No.

For full coverage of the Fukushima disaster, go to Nature’s news special.

1. Sara Osman said:

Accessible and nicely explained…thanks.

2. Matthias Friedrich said:

Thanks. I wish one could do more of that. Trying to stick to facts and real measurable data instead of hysterical worst-case scenarios. Politics all over the globe fall victim to unsubstantiated angst of citizens who feel threatened by the air they breathe.

As a consequence, nuclear energy gets labeled as dirty, ignoring the relative safety of new generation plants and paving the way for a continued use of truly dirty fossil energy with its CO2 emissions.

Yet another, equally serious longterm effect of the earthquake. Too bad.

3. arvind mioshra said:

Thanks for regular upadtes enabling us closely watch the developments of this neclwear disaster…Relaible info help people to take reasoned desicions!

Such concerns of a science popularizer site is a praiseworthy step!

4. berlin said:

Thanks for regular upadtes enabling us closely watch the developments of this neclwear disaster…Relaible info help people to take reasoned desicions!

Such concerns of a science popularizer site is a praiseworthy step!