Explained: rad, rem, sieverts, becquerels

A guide to terminology about radiation exposure

Sometimes it must seem as though reports on releases of radioactive materials from Japan’s Fukushima Daiichi nuclear powerplant in the wake of the devastating earthquake and tsunami are going out of their way to confuse people. Some reports talk about millisieverts while others talk about rem or becquerels, when what most people really want to know is much simpler: Can I drink the milk? Is it safe to go home? Should people in California be worried?

There are a number of reasons for the confusion. In part, it’s the usual disparity between standard metric units and the less-standard units favored in the United States, added to the general confusion of reporters dealing with a fast-changing situation (for example, some early reports mixed up microsieverts with millisieverts — a thousandfold difference in dose). Others are more subtle: The difference between the raw physical units describing radiation emitted by a radioactive material (measured in units like curies and becquerels), versus measurements designed to reflect the different amounts of radiation energy absorbed by a mass of material (measured in rad or gray), and those that measure the relative biological damage in the human body (using rem and sieverts), which depends on the type of radiation. (Rem, rad and gray are all used as the plural as well as the singular form for those units).

“Just knowing how much energy is absorbed by your body is not enough” to make meaningful estimates of the effects, explains Jacquelyn Yanch, a senior lecturer in MIT’s Department of Nuclear Science and Engineering who specializes in the biological effects of radiation. “That’s because energy that comes in very close together,” such as from alpha particles, is more difficult for the body to deal with than forms that come in relatively far apart, such as from gamma rays or x-rays, she says.

Because x-rays and gamma rays are less damaging to tissue than neutrons or alpha particles, a conversion factor is used to translate the rad or gray into other units such as rem (from Radiation Equivalent Man) or sieverts, which are used to express the biological impact.

So, regardless of what units we use, how high does the exposure have to be before it produces significant effects? “If only we knew the answer,” Yanch says. We do know, at the high end, what levels produce immediate radiation sickness or death, but the lower the doses go, the less certain the data are on the effects. “There’s a very large variation in background levels” of radiation around the world, Yanch says, but so far no study has been done that correlates those differences with effects on health, such as cancer incidence. “It’s very hard to get a good answer to how significant low levels of radiation are,” she says. But if those effects were large, she says, it would be obvious, and “we don’t see obvious differences” in health, for example, in regions (such as parts of China) where the natural background radiation is ten times higher than in typical U.S. cities.

Some things are clear: A radiation dose of 500 millisieverts (mSv) or more can begin to cause some symptoms of radiation poisoning. Studies of those exposed to radiation from the atomic bomb blast at Hiroshima showed that for those who received a whole-body dose of 4,500 mSv, about 50 percent died from acute radiation poisoning. By way of comparison, the average natural background radiation in the United States is 2.6 mSv. The legal limit for annual exposure by nuclear workers is 50 mSv, and in Japan that limit was just raised for emergency workers to 250 mSv.

The highest specific exposures reported so far were of two workers at the Fukushima plant who received doses of 170 to 180 mSv on March 24 — lower than the new Japanese standard, but still enough to cause some symptoms (reports say the men had rashes on the areas exposed to radioactive water).

“Everything we know about radiation suggests that if you get a certain dose all at once, that’s much more serious than if you get the same dose over a long time,” Yanch says. The rule of thumb is that a dose spread out over a long period of time is about half as damaging as the same dose delivered all at once, but Yanch says that’s a conservative estimate, and the real equivalence may be closer to one-tenth that of a rapid dose.

Basic conversions:

1 gray (Gy) = 100 rad
1 rad = 10 milligray (mGy)
1 sievert (Sv) = 1,000 millisieverts (mSv) = 1,000,000 microsieverts (μSv)
1 sievert = 100 rem
1 becquerel (Bq) = 1 count per second (cps)
1 curie = 37,000,000,000 becquerel = 37 Gigabecquerels (GBq)

For x-rays and gamma rays, 1 rad = 1 rem = 10 mSv
For neutrons, 1 rad = 5 to 20 rem (depending on energy level) = 50-200 mSv
For alpha radiation (helium-4 nuclei), 1 rad = 20 rem = 200 mSv

Topics: Energy, Nuclear power and reactors, Health effects, Measuring radiation, Physics, Japan, Nuclear science and engineering


Although in the opening paragraph you allude to the key questions people want to know, you haven't really come close to answering those questions! Like many others still in Eastern Japan, I would like the elusive straight answer to my number 1 concern at the moment: is it safe for us to be here? I am living approximately 300km away from the Daiichi plant and would be in trouble if the evacuation zone were to be expanded. Please let me know if Tokyo and the surrounding areas are in danger!
What is the amount of daily dose in mRems for nuclear professionals working at, ‘In Service Inspection’ department of a nuclear power plant.
You talk in the article about confusion, the reasons for confusion, and even quote "if only we knew the answer" for dosage.. then finish by, on one line, explaining the differences between milli and micro. Most schoolchildren know this by the time they're ten. I had come to the article looking for clarification, and leave hoping that the workers' rashes get better. Please focus next time.
Your short article on terminology is a perfect "101" for laymen (as the author of a 574 page book on technical nomenclature, I should know). As a short article coming out of MIT, I would have expected a more expansive piece, but that is just my personal expectation. Again, the title and its content match. For those complaining, please read the title; it is appropriate to the article. For those wishing more information than the title states, know that radiological events are diverse and unique. To ask if a particular person or particular place is 'safe' is way beyond what is stated here without an up to the minute data stream defining the event for analysis. What you are looking for is more like a "paper" or "work" on radiation and its effects to make an evaluation for your information needs. Better yet, read a qualified book or take a course on the subject if you genuinely have an adamant thirst.
Shizuoka1: You might be interested in the map at the end of this page from the IAEA: http://www.iaea.org/newscenter/news/tsunamiupdate01.html which implies that 300km is likely very far from the danger zone. Its not easy to 'convert' becquerels to sieverts, which can easily be converted to rem but this page apparently is a starting point: http://hps.org/publicinformation/ate/faqs/gammaandexposure.html
In today's paper (9-10-11) there is a tiny mention of about 2-3 sentances, that 15,000 TRILLION becquerels were "released into the sea" - during the Fukushima Nuclear Plant crisis in Japan. Can ANYONE put that in lay men's terms? That is more than triple the estimate given (of course) by Tokyo Electric Power Co. Will ANYONE tell us the truth? We are destroying the sea, the air and the earth. Is anything sacred anymore - other than profits? Please - someone tell me what 15,000 Trillion Becquerels means? Rucy1@aol.com - Thanks a lot!
I am attempring to translate a reading of 'berquerels per kilogram', into the 'biohazard for life' factor. For example, a soil sample of '300,000 berquerels per kilogram' translates into WHAT DANGER for eating food grown in the soil, what effects to the human body by exposure over time, etc.? The particular monitoring was apparently done to measure Cesium-137 (not the more dangerous spectrums, with longer half-lives). Sorry, the link is in Japanese, but my POST QUESTION is in English: http://www.asahi.com/national/jiji/JJT201110050035.html I agree with most of the posted comments here; Your explanations in this article are elementary, and pedantic, and condescending, to the average layman; in terms of translating 'potential' issues into REAL ones. IF YOU HAD AN ACRE OF LAND that had 300,000 bequerels per kilo as an active monitored reading (Cs137), would you eat the crops grown there (or live in the house built on this land)? Can you translate the 'potential' for radiation into a number that is an 'actuality' for exposure, all other interferences being null? Thank you for your time and consideration. The people of the Fukushima Province, Honshu, Japan, are waiting for the 'best minds on the planet' (like the boys and girls at M.I.T.) to answer this question. NOW.
One becquerel is oner radionuclide emission per second. Typically, this one event is from one radioactive atom, so it is a very tiny emission. Typically, meaningful radiation is in the gigabecquerel or even the tetrabecquerel range. To put things in perspective, there are 37,000,000,000 becquerel in one curie. Further, the 15,000 trillion becquerel released into the sea doesn't have much meaning. A becquerel is a rate of release, not a quantity. So, I am missing something when there's a report of a rate being released. My suspicion is that we are dealing with newspaper sensationalism. That's not to say the situation is not very serious, but a lot of information is questionable.
Hello all. I mean that when some sievert datum is given it'2 supposed that refers to a specific kind of radiation (X ray, alpha particle...) and over an specific kind of tissue (gonade, arms...), but this is an information that is never provided with the data. So that, would not be preferable to always give data in grays better than in sieverts? In this case you can converts grays to sievert if you know the radiation and tissue you are interested in, but not in the reverse way, from sievert to grays in you are not given with more information. Thank you for your attention.
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