There is no doubt that radioactive materials or radiation in general can be quite efficient at destroying living things. However, radiation is, in the strictest sense of the word, simply a form of energy propagation, and is absolutely necessary to all forms of life. What most people want to know is, "What amount of radiation is bad?" This leads to the more detailed question, "How can we judge meaningfully what amounts of what types of radioactive materials are dangerous?" While the ultimate answers to those questions may be highly debatable, the way in which these issues are addressed inevitably requires the use of different units of measurement. This entry is intended to shed some light on the different ways that radiation (or more accurately, the effects of ionising radiation from radioactive materials and other sources) are measured, and what the measurements really mean. This will hopefully aid the lay person in understanding what is being talked about when discussions involve the general topic of "amount of radiation." Also, it might be helpful to review and understand some of the basics about what the nature of radioactivity actually is.
What's Up With All These Units?
As humans began to work with radioactive materials, it became necessary to develop specific units that expressed what people wanted to know about the radioactive material1 involved. For example, the way tritium affects the human body is very different from the way that, say, plutonium-238 affects the human body. The differences are at the heart of why we need and use three different units for measuring radiation. What one has to realise is the different things that they measure. There are three fundamental concepts that are important when discussing radiation and its effects on physical objects: (i) there is the actual radioactivity involved, (ii) there's the amount of energy the radiation imparts on other objects, and (iii) there is the question of the biological effects of that radiation, particularly on humans. These concepts are behind the three units most commonly used to measure radiation. As it turns out, the sport of boxing provides a rather useful analogy for understanding these three concepts. From the point of view of a boxer facing a tough opponent, the three concepts of radiation mentioned above could be thought of as: (i) how many punches are being thrown, (ii) how powerful the punches are, as in a jab versus an uppercut, and (iii) how much the punches actually hurt when they land.
The Three Units
The three units that measure the three concepts mentioned above are: the becquerel (ultimately after French nuclear physicist Atoine Henri Becquerel who shared a 1903 Nobel Prize in nuclear physics), the gray (after British radiobiologist Louis Harold Gray), and the sievert (after Swedish radiologist Rolf Maximilian Sievert).2
The becquerel (Bq) is the simplest unit to understand of the three. It is also perhaps the most misleading (of the three) for answering the question, "Will this hurt me?" A becquerel is simply a measure of the number of disintegrations per second a radionuclide undergoes. Thus, a becquerel has base units identical to that of frequency, or 1/s. It is important to notice that the Bq does not measure energy, and does not differentiate between ionizing radiation and non-ionizing radiation (refer again to the radioactivity article for what this means). Frequently the "amount of radiation" is expressed simply by referring to the overall activity of "whatever-the-heck radioactive stuff" is present. It is these cases where the unit becquerel is most commonly used. How useful this is depends on what the observer wants to know. If one wishes to state how much of any radioactive decay is present, independent on what type of material or what type of decay is present, the becquerel is the correct unit to use.3 If we return to the boxing analogy, a becquerel would just be a measure of how many punches are thrown without regard to whether they are roundhouses, hooks, jabs, or even if they connect at all. They also say nothing about how much the punches hurt, once landed. For these things, we need a couple more units.
Frequently what people really want to know about radiation is how much harm it will do. Careful reading of the above will show that becquerels are unhelpful for measuring the effects of radiation and radioactive materials on various physical objects. The amount of energy absorbed by a physical object due to radiation is the ultimate source of all the damage. However, not all of these radioactive disintegrations are going to impart the same amount of energy on the object, or even be absorbed at all (this is a slight over-simplification for the purpose of illustration). Thus, measuring how much energy is actually imparted by the radiation is a good indication of how much damage can be inflicted. The unit gray (Gy) is used to express the energy absorbed from a dose of radiation. A gray has base units of J/kg and expresses the amount of absorbed energy per unit of mass of the affected system. The unit Gy is most commonly seen expressing a dose received from large amounts of neutron and gamma radiation (though it can and will be used for expressing doses from beta and alpha radiation as well), such as what someone or something might receive from a certain type of nuclear accident or due to someone's or something's proximity to a nuclear explosion.4 Using our hapless boxer again, a gray would be analogous to a unit that measures whether the punch is a strong uppercut or just a little jab. However, the gray wouldn't show the cumulative effect of something like 100 jabs to the exact same spot on the cheekbone versus one hard punch to the solar plexus. We need one more unit to express this.
The way that wonderful and complex system known as the human body is affected by radiation is expressed using yet another unit. Being people, we are naturally very interested in protecting our bodies from the harmful effects of radiation. If we aren't subjected to a massive radiation flux (for which the Gy unit would apply), the long-term effects of radiation that can seriously and drastically effect the healthy balance of our bodies is what we're more interested in. Our skins, it turns out, are very good at protecting us from the most significant source of radiation that we are likely to experience in our lifetimes: our Sun. However, our innards are not so protected if something radioactive gets inside us. Problems can occur when we ingest, inhale, absorb through the skin, or otherwise take internally any sort of radionuclide, simply because our bodies haven't developed defenses against them. In order to take all of these factors into account, a third unit is used, called the sievert, or Sv.
A sievert is a derived unit that attempts to express an equivalence of absorbed dosage taking into account biological harm. The official definition of the unit Sv goes like this: "A unit of ionizing radiation absorbed dose equivalent obtained as a product of the absorbed dose measure in grays and a dimensionless factor, stipulated by the International Commission on Radiological Protection (ICRP), and indicating the biological effectiveness of the radiation." The Sv is therefore a measurement of dose equivalent, and has the same base units as the Gy (that is, J/kg). It is important to understand the difference between the Gy and the Sv. While both units express the direct effects of radionuclide decay on other masses, the Sv is designed to express the effects of specific radionuclides on some of the more critical body organs and/or the body as a whole over a relatively longer period of time than those situations where the Gy is used. The "dimensionless factor" mentioned above is also known as a "Quality Factor" (or Q), which takes into account the fact that different types of radiation have different energies associated with them. How the Q is determined is complicated, and has a lot to do with the size and speed of the "radiation" or "particles" (and thus their momentum), as well as how they interact with other atoms and molecules on the atomic and subatomic levels. To make matters ever more complex, the way various organs respond to and heal themselves after the harmful effects of these various types of radiation needs to be taken into account. Even still, we haven't mentioned the fact that all people are not alike, and other factors such as age and general good health can have a huge effect on how the body responds to harmful radiation. This is an extremely complicated and difficult subject to tackle, so the approach taken by regulators across the world has been to err on the side of conservatism and place factors that are more likely to over-estimate the effects of a given amount of inhaled/ingested radionuclide. These risk factors are determined by various government oversight orgnizations and are continually being revised and debated and revised again as we learn more about the harmful effects of radiation on the human body.
If we return once more to our hero, the boxer bravely facing down his tough opponent, the Sv would illustrate that he'd been hit by an array of punches, some of them jabs that did little damage by themselves but maybe set him up for a gasping punch to the stomach, followed by a jarring uppercut that in turn sent him reeling where a final roundhouse was landed unchecked right against the jaw. Assuming this didn't put him out for the count, so to speak, a Sv would even be useful in determining the likelihood that this poor fellow might suffer some long-term damage as a result of this pummeling. In short, a Sv is the most useful but complicated and subjective unit for measuring radiation effects on people. Because of this, the Sv is most frequently involved in expressing the effects of nuclear accidents such as the Chernobyl disaster, as well as in performing safety and risk analyses for both the nuclear power industry and in other industrial safety issues involving the use of radioactive material, such as medical, military, and energy research.
Converting and using other units
Of course, Sv, Gy, and Bq are all SI units. The US still uses older versions of the three SI units discussed above. These are, respectively, rem (for Roentgen Equivalent in Man), rad (often claimed to be an acronym for Radiation Absorbed Dose, but actually originated as just a made-up word), and the curie (Ci) or sometimes rutherford (rd) - both those last two are measures of disintigrations/s like the becquerel, but in more convenient orders of magnitude. For the sake of reference, a rem is equal to 1/100th of a Sv - or to put it the other way, 1 Sv = 100 rem. Likewise, a rad = 1/100th of a Gy, or 1 Gy = 100 rad. For converting between Gy and Sv (or between rad and rem), it depends on the type of radiation and sometimes the energy of the radiation involved. For a rough idea, in the case of gamma/x-ray radiation and beta (electrons and positrons), 1 Gy = 1 Sv (and 1 rad = 1 rem). For neutrons and alpha particles, a "quality factor" (Q) is used (see above discussion about the sievert), which varies from 20 (the generally accepted number for alphas) down to a range of 3 to 20 (for neutrons), depending on which standard one abides by (as discussed in the "Who makes up all this stuff" heading below).
Another unit that one may encounter in some applications is the roentgen (R), which measures the energy of radiation in terms of ionization, and has base units of electric charge per unit mass, or coulombs/kg. The roentgen can be converted to the very similar gray or rad for certain types of radiation, namely gamma and x-ray radiation (strictly speaking, gamma and x-ray are the same type, comprising high energy photons). Finally, one may also come across the electronvolt (eV), which measures the energy associated with a speeding particle of radiation. It should be noted that while the eV is a very common unit of energy used by scientists and engineers for measuring elemental and subatomic particles involved in radiation, prefixes indicating higher orders of magnitude are much more frequently used, like the keV and MeV, particularly amongst those involved in such matters professionally. The activity that a becquerel measures is closely related to another commonly used "measurement" of radioactivity, known as the half-life. A half-life is simply the time it takes for half of a radionuclide to decay into something else. Since the half-life has the base units of time and the becquerel has base units of 1/time, the two (activity and half-life) are in fact inversely proportional. This means that something with a shorter half life would have a higher specific activity than something with a longer half life, indicating that something that sticks around for millions of years isn't very "radioactive" at all, and vice versa. This is yet another reason for having all of these other units to express the effects of radiation outside of just the becquerel.
Who makes up all this stuff?
Quite a lot of people are involved in this, as it turns out. The International Commission on Radiation Units and Measurements (ICRU) and the International Commission on Radiation Protection (ICRP) have the overall responsibility for recommending Q values, weighting factors, and other technical guidance on how radiation should be measured. Of course, the USA and others may have their own government agencies that act alongside the ICRP for determining the various risk factors and quality factors mentioned above.5 While the differences between the various quality factors and risk factors used from nation to nation are usually subtle, the way certain radionuclides are treated differently can make expressing the effects of a nuclear accident across borders somewhat problematic, particularly from a political standpoint.
How the units are used
As was hinted at above, the one thing that makes radiation measurement so difficult to understand in simple terms is that the units most suitable for expressing what we are after by "how much radiation is bad for me" are dependent on the pathway of contamination by the substance. For example, eating contaminated food could affect the body much differently than breathing contaminated dust, even if the contamination is from the same radionuclide! One can hold certain radionuclides that are primarily alpha-emitters (such as plutonium-239) in one's bare hand with little to worry about - until one scratches an eye or touches one's nose or face and thus inhales minute amounts of the alpha-emitting plutonium. Then, without the skin to stop it, those alpha particles will do some very bad things to one's internal organs. Therefore, when assessing the possible effects on members of the public as a result of a large-scale nuclear materials release where they're likely to inhale or ingest the radionuclides, it becomes necessary to express the long-term health effects, and one must use the unit sievert. For the special cases where someone has received a massive dose from, say, standing unshielded right in front of a large, sudden neutron flux6 , the effects are so spectacularly bad that discussing dose in terms of Sv becomes moot, and actual energy absorbed (using Gy) becomes a more accurate expression of what physical harm one will suffer. For a quick expression of how much "radiation" is present due to some lump of all sorts of radionuclides, one might use the simple and convenient unit of becquerels. Each unit has its specific uses, and like any unit of measurement, care should be taken in confusing the "equivalence" of each unit. These three units do not measure the same things, but measure distinctly different aspects of closely related things. Now it becomes apparent why this stuff is so difficult to understand in real world, simple terms! Comparing doses like 2 mSv versus 200 mSv is complex enough, let along making the confusing comparison of what 100 Gy versus 100 Sv means, or what 40 billion Bq of depleted uranium might mean in terms of Gy or Sv.
To sum it all up, people are a problem ... quite literally! Due to the complexities of the human body and how widely varied individual biological systems and people are, there's no simple way to convert between Bq and Sv without making some fundamental assumptions about the exact specifics of the biological systems and/or people involved. In addition, the amounts and types of radionuclides, their energies, their chemical form, the pathways or means by which the radionuclide enters the biological system, how long it is likely to remain, the age of the exposed individuals, the organs affected, all of these factors must be taken into account when trying to answer the fundamental question of "how much radiation is bad?" That's a lot to ask of just one unit of measure, so we have three.
The topic of "what level of radiation is bad for the human body" continues to be hotly debated all over the World, with obvious socio-politicical and health/environmental ramifications. The general consensus amongst the most brutally honest scientists, medical professionals, and engineers is that no one really knows for sure - but we are continually learning and improving.