Japan’s unfolding nuclear disaster has introduced Americans to the confusing practice of measuring radiation exposure. According to some stories, the water nearby to the No. 2 Fukushima reactor has a radioactivity level of 1,000 millisieverts per hour . But other articles describe radiation levels in terms of millirem per year . And a few sources have referred to exposure in terms of millirad or nanogray per hour . Why don’t all radiation experts just use the same unit?

Because some people are afraid to switch to the metric system. 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].) On the basis of this concern, the U.S. Nuclear Regulatory Commission still requires plants to report radiation releases in rem, while the rest of the world uses sieverts. For the record, one rem is equivalent to one-hundredth of a sievert.

Sieverts and rem are just two of the many units you might see associated with radiation levels. Scientists use different terms to describe radiation depending on where it is and what it’s doing at the time of measurement. For example, when radiation is first emanating from its source, physicists refer to the rate of emission in curies(customary system units) or becquerels(SI units). A curie is a huge denomination—one curie equals 37 billion becquerels—probably because scientific instruments weren’t as sensitive back when the curie was defined. You might also see electronvolts or joules associated with radiation emissions. These are measures of the energy, rather than rate, of emission.

Once the radiation has cleared the source and is floating ominously through the air, we need a new set of units. Ambient radiation levels are expressed in roentgens (customary) or coulombs per kilogram (SI).

If the radiation leaves the air and enters a person, animal, or object, the units switch again. The raw amount of radiation that an object absorbs is expressed in eitherrad (customary) or gray (SI). As with sieverts and rem, one gray equals 100 rad.

So where do sieverts and rems fit into this whole picture? They provide a measure of the potential harm caused by radiation in a sample of living tissue. That’s different from measuring the amount of energy or rate of emission, because different types of radiation affect the body in different ways. Alpha particles, for example, are 20 times more dangerous to human tissue than gamma rays at the same dosage level. And certain tissue types are more sensitive to radiation than others. A blast of radiation to the spleen, for example, will cause more damage than the same dose to the brain, because splenic tissue divides and multiplies much faster. Physicists use these two adjustment factors—type of radiation and sensitivity of tissue—to convert from measures of radiation absorption to measures of effective dose. Sieverts or rem are used in reports on radiation disasters, because they give a better sense of how the radiation might affect human health.

While effective dose is a great step forward from the days when we simply measured the quantity of radiation absorbed by a patient, it’s still a rather inexact science. Researchers can’t perform randomized trials on the effects of radiation, because it’s unethical to give people cancer. As a result, the tables health physicists use to convert between gray and sieverts (or between rad and rem) are constantly changing to reflect new data.

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Explainer thanks Kelly Classic of the Health Physics Society and Chris Clement of the International Commission on Radiological Protection.