Radiation Doses

Update: Boing Boing has a much more comprehensive article on this here: Radiation: Dose and Risk
Update: Excellent infographic for comparing levels.
Update: Mutant Frog’s Radiation safety update is also well worth checking out.

Radiation Dose is measured in milliRems or Siervets. 1 sievert = 100 000 millirem so it is not hard to convert them. I’ll use millirem here, but Japan more commonly uses siervets.

Here is a great wee note about levels…


In nearly all of our discussions about radiation, we will be considering doses below about 10,000 mrem, which is commonly referred to as low-level radiation.

We frequently hear stories about incidents in which the public is exposed to radiation; radioactive material falling off a truck; contaminated water leaking out of a tank or seeping out of a waste burial ground; a radioactive source used for materials inspection being temporarily misplaced; malfunctions in nuclear plants leading to releases of radioactivity; and so on. Perhaps a hundred of these stories over the past 45 years have received national television coverage. The thing I always look for in these stories is the radiation exposure in millirems, but it is hardly ever given. Eventually it appears in a technical journal, or I trace it down by calls to health officials. On a very few occasions it has been as high as 5-10 mrem, but in the great majority of cases it has been less than 1 mrem. In the Three Mile Island accident, average exposures in the surrounding area were 1.2 mrem — this drew the one-word banner headline “RADIATION” in a Boston newspaper. In the supposed leaks of radioactivity from a low-level waste burial ground near Moorhead, Kentucky, there were no exposures as high as 0.1 mrem; yet this was the subject of a three-part series in a Philadelphia newspaper6 bearing headlines “It’s Spilling All Over the U.S.,” “Nuclear Grave is Haunting KY,” and “There’s No Place to Hide.” In the highly publicized leak from a nuclear power plant near Rochester, New York, in 1982, no member of the public was exposed to as much as 0.3 mrem. Yet this was the top news story on TV network evening news for two days.

And unsurprisingly we are seeing the same thing now.

The dose level measured at the Fukushima plant itself yesterday was 120.4 millirem/hr. Now that sounds high, and it is – about the same as an abdomen xray (which is way more damaging than chest or dental). Sure you don’t want to have that kind of exposure (an abdomen xray an hour) for an extended time, but this level is only at the plant itself. Even at that level, you need 40 hours exposure to reach the maximum yearly recommended dose.

There are reports that the mighty US navy has turned the USS Ronald Reagan because they detected a “dose of radiation was about the same as one month’s normal exposure to natural background radiation”. No actual figure of course. 85 mrem per year is natural background exposure so we guess they saw 7.08 milliRems over a day? So about seven dental Xrays equivalent and 705 straight days is safe at that level.

Meanwhile there is still no sign of any rise in the local radiation in Tokyo, nor anywhere else that I could find reliable data for. If you are more than 3km away from Fukushima you are in no danger whatsoever.

The reports of radiation detections so far in Tokyo are in microSiervets, and are tiny. Again this infographic is very good.