Coverage of the recent problems with Japanese nuclear reactors has increased public awareness of radioactive isotopes of cesium, iodine, and uranium, but it hasn't helped people understand what makes a given isotope dangerous. It's no surprise, really; the threat posed by a particular isotope depends on a combination of factors, including its half-life, mode of decay, and what happens to the isotope once it gets inside the body. We'll look at each of these issues separately to help clear up some of the confusion.
Half-life isn't just a game
Radioactive decay is largely a random process. There is no way to predict when a specific atom will decay, but it is possible to get a sense of how often an average atom will survive before decaying. The most common measurement for this average is a half-life: the amount of time it takes for half the atoms in a sample to undergo decay. For some isotopes, the half-life is a fraction of a second; within a few seconds, nearly all of it will be gone. For other isotopes, a half-life can be hundreds of thousands of years or more, so you need a substantial amount of the material for the radiation to really register. If you only had 100,000 atoms of a long-lived isotope, chances are low that there would be any decays during a short exposure.
Of course, a radioactive isotope is a bit of a moving target. An element with a long half-life can decay into one with a substantially shorter one. For example, thorium-232 has a half-life of over a billion years, but all the isotopes between it and a stable form of lead have half-lives of less than two years, and few of those are less than a second. So, once a single atom decays, it can set off a rapid series of further decays that produce a lot of additional radiation.
When it comes to nuclear risks, the half-life dictates the time scale of the risk. For the isotopes with the shortest half-lives, the dangers are immediate. Someone can be exposed to dangerous matter only at its source, since the material won't get very far before it decays.
Isotopes with half-lives of days to months pose immediate critical risks. They live long enough to spread far from the source, but decay fast enough that they can deliver high doses of radiation in a short span of time. This is why the news in Japan has focused on isotopes of iodine and cesium (iodine-131 and cesium-137), which fit this profile.
The actual nuclear fuel, and some of the nastier isotopes associated with it, create more of a long-term risk. Unless present in high quantities, the fuel won't deliver a critical dose of radiation in a short time period. Because of their long lives, however, isotopes deliver extended, chronic doses of radiation in any contaminated areas; since they'll outlive anybody alive today, they're typically dealt with either by extensive cleanup or prohibiting any humans from occupying the area of contamination (e.g., the exclusion zone around Chernobyl).
Know your radiation
Radiation doesn't come in a single form; nuclear decays can emit a number of different types of radiation, each of which presents distinct risks. From the human health perspective, dangers come from ionizing radiation, which can break chemical bonds, damaging DNA and proteins. We'll present the types of radiation in order of increasing unpleasantness.
Alpha particles: The alpha particle is basically a helium nucleus, composed of two protons and two neutrons. They're heavy and slow-moving and, as a result, don't require much in the way of shielding to be stopped; a small bit of plastic or even the dead cells in the body's surface skin is sufficient. That doesn't mean that alpha particles aren't dangerous, though. If an alpha-emitter gets inside the body, the charged particles have the potential to cause severe ionizing damage to tissues.
Beta particles: Beta particles are either a regular electron or its antimatter equivalent, the positron. They can be emitted at much higher energies than alpha particles, so they require much greater shielding to be handled safely; they can also penetrate deeper into the body if not blocked. Since they're charged, they also cause ionizing damage. A positron will also undergo antimatter-matter annihilations with regular matter, which can result in high-energy radiation.
Gamma rays: Gamma rays are not particles; instead, they're photons, and comprise the most energetic photon category. Due to their high energies, gamma rays are extremely dangerous; they are able to penetrate extensive shielding and cause lots of damage to human tissue. Gamma rays are generally emitted as part of a decay process that also produces an alpha or beta particle, making an otherwise dangerous event much more problematic.
Neutrons: As their name implies, neutrons are uncharged particles that normally reside only in the nucleus of atoms. These are typically produced by fission reactions, although they can be emitted on their own by some unusual isotopes that simply pack too many of them in the nucleus. The neutrons emitted are very high energy, and thus can be dangerous on their own, and they can trigger the emission of high-energy photons as they bump into atoms and lose energy. At a certain rate, they'll be captured by the nuclei of the atoms they bump into, often converting those into radioactive isotopes in the process.
Because of the risk of secondary radioactive elements produced by neutron bombardment, neutron shielding has to be fairly specialized. Typically, it involves lightweight elements, like boron, that can slow down neutrons during collisions, and shielding for any radioisotopes created by their absorption.
External exposure to radioactivity can be bad, but it's far worse to have ingested a radioactive isotope, since the energy from its decay is pretty much guaranteed to damage a cell. Unfortunately, our own bodies work against us when it comes to specific isotopes, since they are either identical or closely related to the elements our bodies rely on to perform basic functions.
One example is potassium, which our body uses to maintain salt balances in cells and transmit electrical signals. In general, your body tries to hang on to all the potassium it gets, but the same mechanisms that hold onto potassium also work to keep cesium in the body, and a radioactive form of cesium has been released at Fukushima. Radon can be dangerous because it is gaseous, and may end up decaying deep in the lungs.
In some cases, radioactive elements are concentrated in specific tissues. For example, strontium-90 is a major hazard of spent nuclear fuel. Its health risks are enhanced by its close chemical similarity to calcium, which leads the body to concentrate it in the bones. The thyroid gland incorporates iodine into the hormones it produces, so the element is actively concentrated in that tissue. Unfortunately, Fukushima has released significant amounts of a radioactive form of iodine (131I), some of which eventually ended up in the Tokyo water supply.
(On the plus side, radioactive iodine makes a great treatment once thyroid cancer develops.)
So, the hazard related to a given radioactive isotope depends on a complicated mix of its half-life, the particles and energy it emits, how you get exposed to it, and what happens if and when it gets inside your body.
To give a concrete example: iodine-131 is one of the isotopes that has been released at Fukushima. It emits beta particles and gamma rays, but with energies that can be stopped by reasonable shielding. Its half-life is just a bit more than a week, so, should you happen to step in a contaminated puddle, you could probably stick your shoes in an unused corner of the basement for a few months, and use them without risk afterwards. However, if you ingest any, it will end up concentrated in your thyroid, where there is no possible shielding.
It's important to understand these sorts of details not only to prevent an unnecessary panic when the word "radiation" is uttered—knowing the real risks involved is the only way to truly protect yourself from any radiation you will encounter.