Keep this chart close at hand while reading reports on the levels of radiation spreading from the ruined Fukushima reactor in Japan.
[Updated with new chart below]: If you're at all like me, while watching the nuclear crisis unfold at Fukushima, you've been really confused by the reports of levels of radiation released at the plants, and at various locations nearby. One report, for instance, says that "radiation levels near the stricken plant on the northeast coast reached as high as 400 millisieverts (mSv) an hour." But what does that mean for people?
The International Atomic Energy Agency explains the measurement like so:
A person's radiation exposure due to all natural sources amounts on average to about 2.4 millisievert (mSv) per year. A sievert (Sv) is a unit of effective dose of radiation. Depending on geographical location, this figure can vary by several hundred percent.
Since one sievert is a large quantity, radiation doses are typically expressed in millisievert (mSv) or microsievert (µSv), which is one-thousandth or one millionth of a sievert. For example, one chest X-ray will give about 0.2 mSv of radiation dose.
For further information on radiation, see Radiation in Everyday Life.\n
Canada's National Post has a really useful graphic that helps put all these numbers in context, and illustrates just how much radiation is emanating from the nuclear reactors in Japan, and what concentrations are really dangerous to human health. Click here or on the image to see the whole graphic.
It's not the best designed graphic—it's really, really hard to follow those thin black vertical lines—but it does a great job of putting Japan's current radiation concerns in context.
In short: Yes, the radiation levels immediately around the nuclear reactors are potentially quite dangerous. As we get more reports from Japan, having this chart close at hand will be really useful in making sense of the news.
Update: The folks at Next Big Future made a much more legible (if a tiny bit less comprehensive) chart that should help you even better understand radiation levels. Here it is, reprinted with permission.