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The Bad Stuff: Ingested Radionuclides

The information given above would seem to indicate that medical X-rays were the worst radiation hazard around, except for natural sources we can't do much about. Unfortunately this is a distortion based on the difficulty of measuring the most dangerous kind of radiation: $\alpha$-emitting radionuclides (radioactive isotopes). Many heavy elements have isotopes which naturally fission into lighter elements plus a helium nucleus, with the latter being emitted with a substantial kinetic energy as an alpha "ray." The range of most $\alpha$ particles is only a few cm in air and less than a mm in tissue, so the damage they cause is localized. While this may be reassuring when the isotopes are at arm's length, it can be bad news if you have breathed them into your lungs or swallowed them so that they can collect in your bones, where they can do the most damage! Since there is such a wide variety of radioactive elements with assorted chemical properties, it is wise to be aware of the specific hazards associated with each. I have neither the expertise nor the space to provide a comprehensive survey here, but I can mention a few of the most common culprits.

• Radon: All rock contains some amount of naturally occurring radium which gradually decays, releasing the chemically inert noble gas radon. Radon in turn is a radioactive element which decays by emitting a rather low energy $\alpha$ particle that is quite difficult to detect since it has such a short range it can't penetrate the window of a typical Geiger counter. Thus until recently there was little known about radon in our environment, even though it is generally believed that Madame Curie died from exposure to radon emitted by the radium upon which she performed her famous experiments. It is now felt by many that radon is the most widespread and dangerous of all radiation hazards, because it accumulates in the air of any building made of rock, brick or concrete (especially those with closed circulation air conditioning!) and thence in the lungs of the people breathing that air. Lungs in fact make a superb filter for the radioactive byproducts of radon, so that one of the most effective radon detection schemes is to measure the radioactivity of the people who live in high-radon environments. In the lung tissue, the short-ranged $\alpha$ particles expend all their energy where it does the most harm, raising the incidence of lung disease and cancer. Rocks from different regions have a tremendous range of radium content, so that a stone house may be perfectly safe in one city and hazardous in another.¹⁴

• Potassium and Carbon: Radioisotopes of potassium and carbon are continually created in the atmosphere by cosmic ray bombardment; these isotopes build up to a constant level in all living tissues, only to decay away in a few thousand years after death. This means that the most radioactive component in your household is probably you!¹⁵ It also provides a handy method of estimating the time since formerly living matter was alive (¹⁴C and potassium-argon dating).

• Man-made Radionuclides: There are too many of these to make a comprehensive list here.¹⁶ The most famous is plutonium, ²³⁹Pu, the stuff of which fission bombs are made. Plutonium is both a deadly chemical poison and a nasty radioisotope. If a miniscule grain is caught in your lungs or other tissues, it may not do much damage to your body as a whole, but it exposes the tissue immediately around it to a huge dose of radiation, drastically increasing the likelihood of cancer in that tissue. Cancer is just as deadly no matter where it begins, which makes the ingestion of radionuclides the worst possible sort of radiation hazard.

It is important to note that the food chain may serve to concentrate "harmless" levels of radionuclides in (e.g.) sea water to a level which is worthy of our concern. Were it not for this effect, and the fact that the waste products of nuclear fission include a large variety of radionuclides with various chemical properties that naturally occurring isotopes do not exhibit, it would be a sensible strategy to dispose of radioactive waste by diluting it and spreading it far and wide in the oceans - since the net radioactivity of reactor fuel actually decreases in the process of digging up the uranium, burning it in a reactor and storing the spent fuel rods for 10 years until the short-lived isotopes decay away. Because of the biological concentration effect, however, it is wiser to seek safe long-term containments for radioactive waste.