I suspect many of you, at this point, are pretty damn confused about Bq, Sieverts, Curies and the like. I know most reporters, struggling with the Fukushima story, have no clue at all what these scientific terms mean. Let's make you smart:
Measures of How much radioactivity: The Becquerel.
Radioactivity is when an unstable atom falls apart, releasing energy as it becomes a happier, more stable, atom. This process is called a 'decay'. We can detect a decay using a tool like a Geiger counter by looking for the energy that was released.
A Becquerel is a decay per second. So, when a news report says a liter of water in Tokyo has 210 Becquerels of radioactive iodine, we can translate that to a liter of water (about a quart) as having 210 radioactive Iodine atoms decaying each second.
How do they know it's radioactive Iodine that's creating the decays? By measuring the exact amount and kinds of energy being released in each decay. Every radioactive atom has a fairly unique pattern of energy release when it undergoes decay. This signature can be used to identify the radioactive atoms present.
The other implication of this is, not all decays are equal. Some atoms release far more energy during a decay. Both the kind and amounts of energy released per decay—as well as how many decays in a certain amount of time—all determine the effect of the radiation.
Now, we flip perspectives, from the radioactive atoms decaying and releasing energy to that of the recipients of all this energy.
What we often care about is the effect this energy might have on the recipient—such as the delicate thyroid gland of a young child...
...The first step is to figure how much energy an object is absorbing from a radioactive source. We measure energy in Joules. Next, we have to consider how much stuff this energy is being absorbed into. Putting the two together, we get Gray, or Joules (energy) per kilogram (amount of stuff).
Sieverts are a slight modification of Gray, reflecting an even deeper level of complexity about how living tissues deal with different kinds of radioactive decays. For the kinds of radiation people are being exposed to from Fukushima (gamma waves and beta decays), Gray and Sieverts are the same.
So, when someone says there is 1 sievert per hour of radioactivity around a reactor at Fukushima, what they're trying to tell you is per kilogram of person, there will be a joule of energy delivered by all the radioactive elements decaying nearby.
The energy delivered by radioactive elements to living tissue can cause problems in two ways: It can kill cells or it can damage the DNA in the cells more subtly, increasing the risk of cancer in the future, causing premature aging and perhaps other problems.
We understand far better how energy absorbed from radioactive decay can cause problems, and kill off parts of the body in the short-term. We have a much fuzzier sense of how lower doses of energy absorbed from radioactive decay increases the risks for subtler problems down the line.
I'm highly suspect—and you should be too—when people attempt make sense of the risks posed by small doses (measured in Gray or Sieverts) of radiation by comparing to things like chest x-rays, CT scans or airplane flights—or bananas eaten. We're just now beginning to appreciate that medical imaging isn't quite as safe as doctors or patients have long assumed. We're also beginning to realize that the duration and frequency of radiation exposures matter a lot for cancer risk—in ways scientists are just now beginning to tease out—important details completely ignored when you're measuring radiation exposures in Sieverts, and scaling linearly. In strange ways, quickly delivered, localized, higher doses of radiation are sometimes 'safer', killing off damaged cells rather than allowing them to live on, damaged, and cause cancers later. In short, living things are not all that much like an ananimate block of matter—and therefore radiation exposure shouldn't be measured for living things in the same way either.
Frankly, and honestly, it's next to impossible to accurately predict right now the ultimate health effects from the Fukushima disaster—despite all we've learned from other radioactive disasters.