Fukushima Radiation Levels Explained
Geoff Brumfiel of Nature‘s The Great Beyond blog provides a series of useful primer posts on radiation numbers (see here and here, for example). The convention is to gauge radiation levels in sieverts—or really, millisieverts (mSv) or microsieverts (microSv).
Most in the health professions are familiar with quantifying radiation exposure (typically from X-rays) in units of gray (Gy), which can be equivalent in a 1:1 fashion with sievert; although the latter unit importantly considers the type of radiation and the type of tissue (eg, brain, abdomen, etc) exposed. The most important caveat, however, when considering radiation exposure from a potential environmental disaster is, not only the radiation level, but the time exposed to that level.
According to TGB blog, radiation levels in Tokyo on the afternoon of March 15 were 0.144 microSv per hour—an elevated, but still tiny, level. By comparison, yesterday’s level at the Fukushima nuclear plant gate was 10 mSv per hour (or 10,000 microSv per hour); although there was a transient spike to 400 mSv per hour on Tuesday (March 15).
Brumfiel writes that the cancer risk begins to increase at a threshold of 100 mSv per year. According to various other sources, symptoms of acute radiation sickness (eg, nausea, vomiting) may begin with levels as low as 500 mSv and are almost certain when levels exceed 2000 mSv (or 2 Sv).
According to one Danish study, a chest CT provides a one-time exposure of 6-18 mSv.* A mammogram poses a 3-mSv risk. Other routine background exposures (like those from transcontinental flights or dental radiographs) are provided by Reuters and, of course, Wikipedia.**
* For a rough comparison, the estimated exposure from an abdominal CT is 14 milligray (mGy).
** Unfortunately Wikipedia articles are not consistent in their use of sievert (vs Gy) when describing radiation risks.
03/18/11 update: Today’s TGB blog posts an animated map of the recent-past, current, and near-future radioactive fallout from Fukushima, courtesy of Austria’s weather service. The wind-directed exposure ranges from 100 mSv per hour (in a transient magenta focus at the plant) to the negligible 100 nanoSv per hour (in a diffuse purple trail, over the Pacific). The model suggests that Tokyo will suffer the greatest exposure (but still at microSv levels), due to changes in wind direction, on March 20.
03/21/11 update: A revision of the animated fallout map from Austria’s weather service is at yesterday’s TGB blog. The bulk of the fallout, in purple, represents 0.3 microSv per hour—“which corresponds to the amount of the natural background radiation dose.” In addition, Forbes’s Matthew Herper put the risks of Fukushima’s radiation in much-needed perspective.