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

Japan Hit By Severe Earthquake And Tsunami

A severe earthquake hit Japan today at 2.46pm local time (0546h GMT).

The epicentre of the quake was about 100km off the east coast of main island Honshu, at a depth of about 20km.

The Japan Meteorological Agency reported it to be 8.8 on the Richter scale, which makes it the most intense earthquake ever recorded in Japan.

In addition, the northeastern coast of Honshu was hit by tsunamis as high as 10m. Boats, cars and even houses were swept away in the flood caused by a series of tsunamis. The worst hit areas appear to be in the region around the city of Sendai in Miyagi Prefecture.

Earlier today I watched some of the live TV coverage of the earthquake on NHK World; for those of you without cable TV, here are a couple of videos:

Severe Tremors at NHK Sendai Newsroom (from NHK World)



CCTV Footage of Tsunami Wave at Sendai Airport (from Russia Today)



I think this footage of the wave front sweeping into the airport - almost at a walking pace but gradually covering the entire tarmac - is especially chilling.

It is disturbing to see how unstoppable and how much damage the visually unspectacular flood flow was causing, if you compare these scenes to the hyped-up special effects in Hollywood disaster movies.

Videos taken by the NHK helicopter also show the wave front moving gradually and relentlessly, engulfing everything in its path. As it moved inland over farmlands and residential areas, the wave front picked up so much mud and debris that it became dark in colour and had a chunky appearance.

Live video from the air also helped to convey the huge scale of the tsunami waves as they approached the coast. The photo below is a screen capture from NHK video that shows several such waves, each of them hundreds of metres long, about to reach the coast of Sendai (treeline at the lower left):



















Based on latest reports, at least 300 people are dead.

Please click here for a compilation of the latest reports on casualties and damage.


Would you like to know more?

Latest warnings:
- Japan Meteorological Agency: Tsunami Warnings/Advisories

Latest news:
- NHK World English
- LIVE: Japan earthquake (BBC News)

Latest casualty and damage reports:
- Massive earthquake brings a destructive tsunami in Japan (Earthquake-Report)