I was heading to bed, but decided to check on Japan news. Found this article. The picture above comes from that article. The meter is reading 0.35 microsieverts in an industrial area in Sendai, which is near Fukushima.
I am going to break down that reading so its comprehensible since the article does not explain what it means. How big is 0.35 microsieverts? If expressed as a number it would be 0.00000035 Sieverts. Since exposure is more commonly expressed in rem, I will convert the Sievert measurement into rem. 1 Sievert = 100 rem or 1 rem = 0.01 Sieverts. So multiply 0.01 times 0.00000035. The above reading is thus equal to 0.000035 rem. How dangerous is this reading? Not terribly dangerous. According to the Nuclear Regulatory Commission, in an average year a human receives 0.36 rems from natural sources like radon gas in the atmosphere. So do not panic if you receive 0.000035 rem in an hour, you are not veering into dangerous territory unless you exceed 25 to 50 rem cumulative.
25 rem is 714,286 times greater than 0.000035 rem. When expressed like that, it puts the reading into perspective doesn't it?
Another area the article is fuzzy on is fallout. Fallout is bits of dirt to which particles of radiation has attached itself. Radioactive iodine has a half life of 8 days, so in 8 days half the radioactive iodide will have spontaneously converted to a more stable isotope. Cesium will take 30 years to see half its mass converted. And these isotopes are what is showing up as fallout.
Now what do they mean when a reading is described as 1/26th of a chest x-ray in a liter of water? Again looking at the Nuclear Regulatory Commission site, a chest x-ray equals 0.01 rem of exposure. So lets multiply 1/26 x 0.01 to get the exposure they are talking about. The result is 0.000385 rem. Since this is radioactive iodine 131, people in the affected area should be directed to take the potassium iodide tablets to flood the thyroid with good iodine as a precaution.
Hopefully I have made things a bit more understandable. With over 18,500 dead and over $230billion in damage, Japan has enough problems without adding misleading claims.