Almost everything on earth is made of atoms, and radioactivity is a property that atoms can have. Below is a technical explanation of this property, including its role in nuclear power and the concept of half-life. Other pages get into the types, units of measurement, effects on the human body, and general hazards of radioactivity.
There is a size limit to how large atomic nuclei can get and remain stable. Every element heavier than lead (atomic number 82) is radioactive to some extent. But small atoms can be unstable also, if they have more neutrons than protons. For example, the most common form of carbon on earth is carbon-12. Carbon has an atomic number of 6, meaning it has six protons in its nucleus. When it also has six neutrons, this adds up to an atomic weight of 12, and this isotope of carbon is stable. However, if you add two more neutrons you get carbon-14, which is unstable – that is, radioactive.
Likewise, the most common isotope of hydrogen has a nucleus composed of just a single proton. It is a stable atom. If you add one neutron to the nucleus, you get an isotope called deuterium that is rarer but still stable. However, if you add two neutrons, you get a form of hydrogen called tritium that is unstable and undergoes radioactive decay.
The isotopes of hydrogen – deuterium and tritium – are the only ones to have their own names. All other isotopes are designated by giving the name of the element followed by atomic weight of the isotope, as with carbon above.
When these unstable nuclei decay, they transform themselves into a different element, releasing radiation and heat as part of the process. This new element, or radiation product, may itself be radioactive and decay further – releasing more radiation – or the product may be stable, in which case the process stops.
The heat from radioactive decay is what makes these elements a usable source of power; the radiation is what makes them dangerous. In nuclear reactors, that heat is used to heat a liquid that drives a turbine, which in turn generates electricity. Sometimes the radiation is used to transform materials into others that are useful for medicine or science; but mainly the radiation is a hazard, which continues to be a problem long after the material no longer generates enough heat to be useful in a reactor.
Half-life is the other relevant parameter for radiation. We never know when specific atoms in a substance are going to decay; but we do know the rate at which some random atoms in it will decay. This rate is expressed as the element’s half-life, which is the time it will take half the atoms of a radioactive isotope to decay. So if a substance has a half-life of four years, after four years half of it will have decayed into some other element; and after another four years, there will only be a quarter of the original isotope (half of that half) left.
Half-life can be a misleading characteristic: when you read that the half-life of plutonium-239 (the most common isotope of plutonium) is 24,100 years, that makes it sound like this horrible, horrible thing that’s going to be around forever. Which is true in some senses, but it also means that the plutonium is decaying so very slowly that it just doesn’t emit much radiation. Its chemical toxicity (see below) is a much more powerful effect on the human body.
On the other hand, a very short half-life means that a given isotope is stunningly radioactive, but this doesn’t last very long before it decays to a manageable level. So waiting for relatively short periods of time can also reduce the radioactivity of dangerous materials tremendously. Storing certain kinds of spent nuclear fuel for only three years lowers the heat generated by radioactive decay “by a factor of nearly 12,000” at which point it’s a lot safer to handle.
The sweet spot for a half-life that’s most troublesome to humans belongs to two isotopes that are common products of the radioactive decay of uranium and plutonium: cesium-137 and strontium-90. They both have a half-life of about 30 years, which is strong enough to make them dangerous emitters of beta and gamma radiation, but long enough so that once an area is contaminated with these radionuclides, it’s going to stay dangerous for the rest of your life.
Alpha radiation consists of clumps of two protons and two neutrons. These are usually called alpha particles instead of alpha radiation. These are the lowest-energy of the three types of radioactive emissions, and are the largest particles. Alpha particles can’t penetrate far into the human body by external exposure, but they are quite dangerous when ingested.
Beta radiation consists of electrons – one electron per radioactive decay. Although we are generally taught to regard electrons as a particle, when they are ejected from a radioactive decay event, they are often moving close to the speed of light and behave more like a form of energy. For this reason they are sometimes called beta particles and sometimes called beta radiation. They are much, much smaller and higher-energy than alpha particles, but have a lower energy than gamma rays. Its effects on the body fall between alpha and gamma radiation.
Gamma rays are high-energy, high-frequency electromagnetic radiation, similar to x-rays. Neutrons and larger clumps of protons and neutrons are highly damaging radiation, but they’re not normally emitted except as part of an uncontrolled chain reaction. In practical terms, gamma radiation can be the most dangerous type.
The effects of radiation on the human body are different depending on the type of radiation, whether the radiation is coming from inside or outside the body, and what part of the body is irradiated.
All three types of radiation cause damage when they smash into atoms in our body and ionize those atoms, creating free radicals inside the cell. These free radicals are the same as any other kind of free radical – there’s nothing special or extra toxic about the ones created by radioactivity: if the rate of damage is small, the cell can repair it just like it does with free radical damage from any other source. However, if the radiation causes ionization faster than the cell can repair the damage, the cell may die or become cancerous.
Cells are especially vulnerable to damage by free radicals when they are dividing, which means that cell types that are constantly dividing are more prone to damage by radiation. Bone marrow – which is constantly dividing to replenish our supply of red blood cells – is a type of tissue that is heavily affected by radiation. This is why leukemia is so often a consequence of radiation damage, and why radionuclides that accumulate in your bones (like plutonium, uranium, and strontium) are especially dangerous. Other highly vulnerable types of cells include the gonads, and the villi that line the small intestine.
Alpha particles, being the largest, cause the most damage since they smash into (and ionize) the most atoms. That said, they smash into things so much that they slow noticeably in air (due to their collisions with air molecules) and they are stopped completely by clothing or a sheet of paper. Should they impact the body from the outside, they don’t penetrate more deeply than the skin.
This means that damage from alpha-emitters like uranium and plutonium is significantly enhanced when they are ingested, either by eating contaminated food or inhaling dust contaminated by those elements: not only are you suddenly getting irradiated from the inside by highly damaging alpha that wouldn’t even penetrate your skin from the outside, both of these metal tend to accumulate in your bones, where they irradiate the extremely vulnerable bone marrow. As if that weren’t enough, both uranium and plutonium are toxic metals, like lead or arsenic, so you get poisoned also. In fact, the chemical poisoning effect is usually more harmful than the radiation.
Beta particles come in a variety of energies. They are much, much smaller than alpha particles, so they penetrate further into your body; but they also do much less damage along the way. Higher-energy beta does more damage, but is otherwise the same.
Gamma rays generally go right through you, like x-rays. However, your bones stop more gamma rays than the rest of you, which of course is how x-ray imaging works. This means that your bones (and your bone marrow) accrue more damage, because they’re absorbing more of the gamma instead of letting it pass through. Gamma is about as damaging as beta, but all your vulnerable areas get irradiated with gamma, because it goes all the way through you. With gamma, every dose is a full-body dose.
Radiation can prove hazardous through external exposure (handling or standing too close to radioactive matter) or ingestion (consuming radioactive food or water). Furthermore, some radioactive substances have additional chemical effects.
The health risks of a high dose of radiation are straightforward: it causes cell death and damage, and above a certain point will kill a human being. These are called deterministic effects. Except for the case of Alexander Litvinenko, all instances of acute radiation poisoning have been accidental; so there isn’t really a body of research involving humans. However, these effects are routinely used in (for instance) radiation therapy for cancer, and this plus the informal data from various exposures that have occurred have given a good picture of the danger to humans. (However, even though sieverts and other units specifically measure biological damage, it’s not always straightforward to correlate risk with dosage.)
25 microsieverts (millionths of a sievert) is the world-wide average cumulative yearly dosage from environmental background radiation. It is also the maximum allowable one-time dose from a single airport screening.
One millisievert (thousandths of a sievert) per hour is the US Nuclear Regulatory Commission’s threshold definition for a high-radiation area.
A cumulative dose of 0.1 sieverts (Sv) at a rate of 0.1 Sv/hour is the threshold at which deterministic health effects start to occur. These effects include so-called radiation burns, death of white blood cells and bone marrow, and general incapacitation.
Above the 0.1 Sv dose, effects increase with more radiation and mortality starts occurring at 1.5 Sv. About half of exposed people die (LC50) at 3 Sv of exposure. Above 5 Sv, almost everyone dies.
Below doses of 0.1 mSv, deterministic effects do not exist; however, there are other effects that can only be stated as statistical possibilities: for instance, there is a general scientific consensus that each sievert of cumulative low-level dose increases a person’s chances of getting cancer by 5.5% over their lifetime. Beyond that, there is little agreement. (While it is beyond our scope to engage the scientific controversy over the effects of low-level radiation, we do note elevated cancer rates in areas that have been exposed to chronic non-acute radiation.)
Strontium-90 is of extra concern because it has similar chemical properties to calcium: it can accumulate in bones and stay there for the rest of a person’s life, irradiating their bone marrow. Furthermore, it decays to an even more virulently radioactive isotope: yttrium-90, with a half-life of 64 hours. Thus, in calculating the radioactivity of strontium-90, one usually includes the yttrium-90 radiation along with it.
Caesium-137 has a different set of special chemical properties: it very quickly forms compounds that dissolve readily in water, thus making it a major contaminant of the water supply – for animals and crops as well as for humans. In Fukushima, for example, where most of the radiation leakage is through contaminated water, caesium is the major contaminant being found in meat and vegetables produced nearby.
Because caesium-137 and strontium-90 are so common and have these properties that make them especially dangerous to humans, the danger to people from major nuclear accidents is often expressed in terms of the quantity of caesium-137 and strontium-90 that was released.
Radiation is measured in two kinds of units. One measures the number of radioactive decays; the other kind measures some aspect of its effect on the body. Both kinds of measures have definite limitations, and it can be very frustrating to try and derive one kind of measurement from another. One of the main problems is that there are several types of radioactive energy, and the effects of each kind on the human body are different. Also, there is a problem with different units having been used at different times in history.
Below are technical definitions; another page discusses levels of hazard associated with different levels of radiation.
The raw amount of radiation that’s present in a given situation is measured in becquerels or curies. The becquerel is the currently official unit, but we’re giving both here for two reasons. First of all, when we quote older documents, they often use curies. Second, the becquerel is a much smaller unit that is great for measuring things like medical x-rays, but often cumbersome for measuring the amount of radiation in an unintentional release. It is a nice, tidy measurement of one radioactive decay per second. The curie is 37,000,000,000 decays per second. So any measurement in curies can be converted to becquerels by multiplying by that number. It’s about the same ratio as the relationship between two inches and 100,000 miles. But there are times when you don’t really want to do the conversion, even though becquerels are the more officially approved unit nowadays. For example, the material released in the Chernobyl meltdown was radioactive to the tune of 51.4 million curies. That’s a really big number in becquerels.
Digging a little deeper, you can see a problem here already: both curies and becquerels measure only the number of decay events; they don’t hold any information about the type of radiation that was emitted. To access that information, you have to know what the radioactive material was. This is something of an artifact of instrumentation: the Geiger counter (the world’s most popular device for measuring radioactivity) only counts decays without conveying any information about what sort of decay it was. So to know how dangerous a radiation source is, you need more information.
Grays and rads are units that measure how much of the radioactive energy that strikes your body is absorbed instead of just passing through. It is tissue-dependent: your bones absorb more gamma rays than the rest of you;, so when the radiation source includes gamma, they accrue more grays than your soft tissues. On the other hand, if the radiation is all alpha, none of the energy gets past your skin; so your bones don’t pick up any grays at all. Some sophisticated analyses of radiation exposure list different levels of grays for different tissues in the body. But most sources simply use a whole body measurement in grays, as if the radiation only had one level of effect on your whole body.
Grays are the modern, approved unit in this category and this time they are the bigger unit, with 100 rads equaling one gray. However, while these units measure the energy absorbed, they don’t allow for the extra-damaging effect of alpha particles (or neutrons, or big chunks of atomic nuclei, but we won’t go into that here).
The units that do attempt to capture the amount of actual ionizing damage are sieverts and rems. Sieverts correlate with grays and rems with rads. For beta and gamma radiation, 1 sievert = 1 gray (and 1 rem = 1 rad). But for alpha radiation, 1 gray = 20 sieverts and 1 rad = 20 rems.
Note that these biological-damage units only apply to mammalian flesh. For a measure of how much ionization takes place in an inanimate material (usually air), we use roentgens, which reflect an equivalent amount of ionization to rems.
The above discussion is couched in units that are already defined in terms of biological tissue damage. But how does one arrive at tissue damage units from the cruder units of radiation that only measure the number of nuclear decays over time?
Different radionuclides emit electrons or gamma photons with different levels of energy, which means that they cause different amounts of tissue damage. These energy levels are known for each radionuclide, so if you are given a gross number of decays per second (in becquerels or curies) and the identity of the radioactive material – and your distance from the material – you can calculate the amount and rate of damage in sieverts, which carries reliable correlation to human health.
Here is a preliminary table of common radionuclides, showing some physical parameters and how much damage they cause. The effects have been keyed to megabecquerels (MBq) – one million radioactive decays per second – and grams of material. For each substance we have also given the volume of 1 gram relative to the volume of a penny. The health effects are given in milliSieverts per hour; as a reminder, 100 mSv per hour is the threshold at which deterministic health effects start to occur. Radium is included as a sort of benchmark – many people have a vague sense of how dangerous it is, as it has been responsible for the most human exposure under everyday conditions.
So for cesium-137 and Sr/Y-90, one gram is enough to kill you outright in an hour at a distance of five feet, and an average of these two is about a thousand times as potent as radium.
Note also the tremendous fall-off in toxicity with distance. This means, conversely, that there’s a tremendous increase in virulence when radionuclides are taken internally, whether inhaled or swallowed. Note also that the above figures are only for beta and gamma radiation. But when taken internally, the heavily-damaging alpha radiation also needs to be taken into account.
Note: The dosage calculations were done with the beta and gamma calculators here. For Sr-90/Y-90, the assumption is that half of the radiation is coming from the strontium and half from the yttrium.