Explanation Of Millisievert

With the international media spotlight on the Fukushima 1 nuclear power plant emergency, radiological terms such as "millisievert" have entered the public consciousness.

Here is a video that clearly explains what the sievert is, plus a demonstration of two instruments that are used for measuring radioactivity:



It is understandable that the international media is more focused on Fukushima than the devastation caused by the tsunami, since they reflect the concerns of foreign nationals living in Japan.

However, some of their headlines seem sensationalistic and have the potential to cause unnecessary worry among Japanese residents.

Take for example this headline - "Tokyo radiation levels 23 times normal: officials". This brief article states that radiation levels in Tokyo "surged to 23 times normal on Tuesday" and reported a measurement of 0.809 microsieverts, presumably over the hour (10am local time) mentioned.

Without explaining the significance of this reading, a 23-fold increase in radiation levels sounds very serious indeed.

However, if we take into account the average annual background radiation dose per person, which is around 2000 microsieverts, we can see that the reported radiation level spike is actually about 1/2470 of the average annual dose.

Further, by doing this form of comparison, we can find out that 0.809 microsieverts is about:

- the dose received during 6 hours of flying time in a jet airliner.
- 1/120 of the dose of a chest X-ray.
- 1/12400 of the dose of a CT scan.
- 1/24700 of the annual limit of a radiation worker (20mSv).
- 1/124000 of the lowest level that can cause a measurable increase in cancer rates.
- 1/309000 of the lower limit for acute radiation sickness.

Just for comparison, a news report about more recent readings in Tokyo has this headline - "Tokyo Area Radiation Around Typical Background Levels - City Government", which I think is a more responsible headline.

The article states a measurement of 0.05 microsieverts per hour, slightly higher than the background level of 0.035 microsieverts per hour - which could have led to a headline like "Tokyo radiation levels 143% of normal!", but their editors didn't do that.


Would you like to know more?

About units used for measuring radiation dose:
- Sievert (SI unit)
- Roentgen
- Rem
- Gray
- Rad