Back to the Basics: Radiation Units

One of the radiation survey maps at the Fukushima dai-ichi facility (

Imagine trying to describe the world without using units. As I write today I’m watching football on TV – I’d never really paid much attention before, but now that the topic is on my mind I am amazed by the welter of units being bandied about by the commentators. Describing a player as “big and tall” doesn’t convey as much information (or as much awe) as saying that they are 6’7” and 335 pounds, and a 99-yard punt return is certainly more impressive than a “really long” run of indeterminate length. Not only that, but without units of some sort we have no objective basis of comparison – my “medium” drink would be far too much for a toddler and would be a trivial amount for the bonus-sized football player mentioned earlier. This is why we use units – to help us quantify the world around us, if only to help with these comparisons. Radiation safety is no different – it’s just that the units are inscrutable to most non-practitioners. My goal here is to take a step back from some of the more involved postings to discuss what the radiation safety units mean and to give a feel for levels that are considered normal, interesting, or alarming (by “interesting” I mean levels that are obviously elevated but that are not dangerous). I’ll also define both the SI units and those that are still most commonly used in the United States (in spite of the best efforts of the Health Physics Society to switch entirely to SI units). With each of the units discussed remember that the typical multipliers (micro, milli, kilo, mega, etc.) apply, so there are 1000 millirads in 1 rad and a million microGy in one Gy.

Radiation is nothing more (or less) than the transfer of energy from one place to another. The photons bombarding me right now have brought energy from incandescent gases on the Sun (93 million miles or 150 million km distant) into my apartment, and a fastball is a form of particulate “radiation” that transfers energy from a pitcher’s arm to a catcher’s mitt (or to the batter’s bat). What we are concerned about in radiation safety is ionizing radiation, which is radiation that has enough energy to strip an electron from an atom, creating an ion pair that can go on to cause health problems.  With radiation we are primarily concerned about radiation dose and the dose rate – discussed below.

Radiation dose is a measure of the amount of energy deposited in a substance by the radiation that is striking it and it is measured in units called the Gray (SI) or rad (US). The formal definition of the Gray is the deposition of 1 Joule of energy per kilogram of absorber and depositing 100 ergs of energy per gram of absorber exposes it to 1 rad – do the unit conversions and you find that 1 Gy = 100 rads. Note that this has nothing at all to do with the amount of biological damage caused by the radiation, these units refer only to the amount of energy deposited. Some notable radiation doses (from a standpoint of health effects) are noted here:

Dose (rad / Gy) Health effect from uniform exposure to the whole body
25 / 0.25 Minor changes to blood cell abundance
100 / 1 Onset of radiation sickness in some of those exposed
450 / 4.5 Lethal dose to ½ of those exposed who do not receive medical care
800 / 8 Lethal dose to ½ of those exposed who do receive medical care
1000 / 10 Lethal dose to all exposed

Skin burns and cataracts can also be caused by radiation exposure at dose (to the skin or to the eye) of about 300 rads (3 Gy).

Equivalent dose takes into account the fact that some types of radiation cause more harm – and are more likely to kill cells or to cause cancer – than others and is measured in units of rem (US) or Sievert (SI). Alpha particles, for example, are big and are much more likely to cause irreparable DNA damage than are the lighter beta and gamma radiations. So a person exposed to 1 rad (0.01 Gy) of alpha radiation will accumulate about 20 times as much DNA damage – will be 20 times as likely to develop cancer later in life – as a person exposed to the same level of beta or gamma radiation. To calculate equivalent dose we just multiply the absorbed dose (rad or Gy) by the relative biological effectiveness of the radiation in question. Thus, a person exposed to 1 rad of alpha radiation will accumulate 20 rem of equivalent dose. Radiation regulations are based on both short-term and long-term risk so regulatory dose limits are typically given in terms of dose equivalent (rem or Sv) rather than absorbed dose (rad or Gy).

Dose rate is just as it sounds – the rate at which energy is being deposited in an absorber. In a sense we can consider the dose rate as the speedometer and the total dose as the odometer – a high dose rate gets you to a dose limit (or a given health effect) more quickly than a lower dose rate. A person exposed to a dose rate of, say, 5 mr/hr (50 microGy/hr) will take 1000 hours to reach their regulatory dose limit. This table might help to give a feel for the significance of various dose rates. For the sake of compactness I’m listing only the US units – for SI units simply divide by 100.

Dose rate (mr/hr) Comment
0.01 – 0.1 Dose rate from natural sources in most of the world
> 0.1 Indication that there might be artificial radiation present
0.5 The highest dose rate I measured in Japan in April 2011 – in the city of Iidate, which was evacuated because of the contamination
2.0 – 2.5 Highest natural dose rate I’ve measured – in the spa city of Ramsar, Iran
5 Must be posted as a radiation area under US regulations
100 Must be posted as a high radiation area under US regulations
10,000 Will reach the allowable dose limit in about ½ hourMight start to develop radiation sickness after about 10 hours of exposure
100,000 Can receive a life-threatening radiation dose in 4-5 hours – potentially a dangerously high dose rate (although I have several colleagues who have worked safely in much higher dose rate areas because they knew exactly what they were doing and kept track of their exposure and stay time limits)


Radioactivity is a fundamental physical property of atoms that have excess energy and that give up this energy by emitting radiation. Objects may (or may not) be heavy, dark-colored, flammable, radioactive, liquid, hot, and so forth. However, unlike some of the properties noted above, an atom’s status of being radioactive can only be changed by its decay to a non-radioactive (stable) condition by emitting radiation. Radioactivity cannot be destroyed by fire, extreme pressure, or any other known physical process. Radioactivity is quantified by measuring the rate at which atoms decay (emitting radiation) in an object – it’s measured in units of becquerels (SI) and curies (US). It is important to understand that the amount of radioactivity present does not necessarily have anything to do with the total amount of material you might be looking at – one gram of radium-226 for example has the same amount of radioactivity as about 3 tons of depleted uranium. So when we refer to a radioactive source as being “high-activity” we are referring only to the rate at which atoms are decaying in the source.

The SI unit for radioactivity is the becquerel (abbreviated Bq) and a 1 Bq source will undergo 1 decay per second. The US unit of radioactivity is the curie (Ci) and a 1 Ci source will experience 37 billion decays per second. Thus, 1 Ci = 37 billion Bq (37 GBq). All things being equal, a high-activity source poses a greater potential risk, but the amount of risk posed by a source of any given activity level depends very strongly on the radionuclide that makes up the source – the amount of alpha-emitting polonium used to kill Alexander Litvenenko in London in 2006 was far lower than the amount of cesium needed to cause harm.

Radioactivity is found naturally in soil, water, air, and food around the world. In fact, I have measured radioactivity from natural potassium in a bunch of bananas and in salt substitute, and I collect naturally radioactive rocks and minerals. The amount of radioactivity found in natural objects such as these is normally measured in picoCuries (pCi) where a million pCi go into a single µCi and a million µCi comprise a curie. One Bq is equal to 27 pCi. Natural objects (soil, bananas, etc.) generally contain up to several tens of pCi of natural radioactivity per gram, although some “hot” soils and rocks might contain millions of pCi (up to a few µCi per gram).

Radioactive materials are used extensively in the research laboratory – most labs use vials with small amounts of radioactive liquids and they will contain a milliCurie (mCi) – 37 MBq – of radioactivity or less. Higher levels of radioactivity – as much as a few hundred mCi – are used in nuclear medicine.

Radioactivity is also found in industry – low- to moderate-activity sources (on the order of a few to several mCi or a few hundred MBq) are used to help control various processes and sources of up to several curies (a few hundred GBq) are used by drilling companies to help them find hydrocarbons and water. Higher activities are used for industrial radiography – these sources, with up to a few hundred curies (a few to several TBq) can be deadly and, in fact, have caused death and injury in a number of nations. Any sources with tens of curies (hundreds of GBq) of activity or more must be considered dangerously radioactive.

Other sources can be even more radioactive – blood banks irradiate blood with sources that are several thousand curies (tens to a few hundred TBq), some research facilities use sources that can be tens of times as “hot,” while even higher levels of radioactivity are used in some specialty facilities. These sources can give a fatal exposure of radiation in a matter of minutes.

So – to try to give a sense of scale on these units….

Measurement Found in nature Of interest Potentially dangerous
Radiation dose ~300 mrem/yr~3 mSv/yr 100s – 1000s of mremA few – 10 mGy 100s of rem and higherA few Sv and higher
Radiation dose rate 10s of µr/hr100s of nGy/hr A few mr/hr10s of µGy/hr 100s of r/hra few Gy/hr
Radioactivity levels A few pCi – 10s of pCi10s – 100s of Bq nCi – µCi levelskBq – 10s of kBq 10s of Ci and higherHundreds of GBq and higher


Note: there is little controversy over the levels of each of these that are found in nature or what can be dangerous. The middle column (“Of interest”) is less well-defined – these are levels where I would start to think about taking protective actions, not to limit risk so much as to ensure  compliance with the appropriate regulations. Other health physicists likely have their own trigger points for each of these.


This posting is more of a tutorial than an explanation or opinion piece. I’d appreciate your feedback as to whether or not this is useful – if so I’ll post similar pieces from time to time as a change of pace.


Dr Y is a certified health physicist, trained in nuclear power plant design and operations, with experience in nuclear power, environmental science, and planning for radiological and nuclear emergencies. He has 30 years of experience in the areas of nuclear and radiation safety.

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One Response to “Back to the Basics: Radiation Units”

  1. Cheryl Rofer January 5, 2012 at 5:50 PM #

    Radiation units are horribly confusing, particularly with the switch from rems etc. to sieverts etc. I say this as someone who has worked with them for some long time.

    Becquerels are perhaps the worst of all, providing enormous numbers with little readily apparent meaning.

    One of the things I’ve written on the subject.

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