Slate has an interesting look at what's up with all the different ways of measuring radiation exposure. Sieverts, rems, grays—what's the difference? It is boils down to two issues. First, some units measure specific things. Sieverts, for instance, are different from grays because sieverts are a measure of the effect of radiation on human tissue, while grays are more about radiation absorbed by any object.
And the second issue: It's a case of the Metrics.
As with distance, weight, and temperature, doses of radiation can be expressed in either SI units (sieverts) or U.S. customary units (rem). U.S. scientists and engineers in most fields had switched to metric units by 1964, when the National Bureau of Standards (now the National Institute of Standards and Technology) officially adopted the international system. But nuclear physicists never made the full switcheroo. That's because a wholesale change in measurement could lead to mistakes, at least during the transition–and even a small mistake can be very dangerous when it comes to radiation exposure. (There is an historical argument for being cautious: In 1999, NASA lost contact with the Mars Climate Orbiter because of a mix-up between metric and customary units [PDF].)
