As I'm sure everyone in the developed world knows by now, Japan was hit with an earthquake and a tsunami that damaged one of their nuclear energy facilities. Anyone who is following the story closely may wonder, how much radiation has 'leaked' from the facility- and how much of this affects the people around the facility. Depending on the news source, you may get a value in different units, such as gray, sievert, rem, rad, becquerel, or curie (most likely the milli-sievert, but news stations have been picking and choosing at their leisure).
What the heck do all of these units mean? How much radiation is that??
The Gray is the SI unit of absorbed radiation dose, and is defined at the absorption of one joule of ionizing radiation by one kilogram of matter. The key here is that it is used to measure the strength (amount of energy as defined by the Joule) of radiation that is absorbed by matter.
The Sievert is the SI unit which measures the dose equivalent radiation. It is used to measure the biological effects of radiation. Simply, the sievert measures the amount of absorbed radiation that is absorbed by a person. Often, the unit sievert/hour (or other unit of time) is used to estimate the cummulative effect of radiation.
A curie (named, of course, after the very famous Marie Curie- the discoverer of polonium and radium, the creator of several techniques for isolating radioactive isotopes, the first woman to earn a Nobel prize, and the only person in the world to hold Nobel prizes in multiple sciences- physics and chemistry) is a measure of radioactivity based on the activity of 1 gram of the radium isotope Ra-226.
A REM is the customary unit (American, or "english" unit) for the Sievert. 1 rem= 0.01 sievert.
A RAD is the customary unit (American, or "english" unit) for the Gray. 1 rad= 0.01 gray
and, a Becquerel is the SI unit that replaces the curie. 1 Bq = 2.70×10−11 Ci
So, when we're talking nuclear meltdown, there are only two you need: the gray and the sievert. The gray tells you how much radiation is released into the surrounding environment. The Sievert tells you how much radiation is absorbed by people. And now you know :)
Just to put things into perspective...the highest level of radiation detected throughout the recent Fukishima nuclear problem is 0.4 Sieverts/hour. The highest level during the Chernobyl nuclear disaster...300 Sieverts/hour. The threshold for acute radiation sickness is somewhere around 1 Sievert/hour.