The British government announced it will test three people for possible radiation exposure related to the apparent poisoning of Alexander Litvinenko. The former Russian spy was found to have radioactive polonium-210 in his body. Could he have irradiated people around him before he died?
No, but he might have secreted radioactive material. The polonium isotope discovered in Litvinenko's body emits radiation only in the form of alpha particles, which are the charged nuclei of helium atoms. Unlike other forms of radiation, like gamma rays or X-rays, alpha emissions can be blocked by something as insubstantial as a piece of paper or the layer of dead cells on the surface of the skin. Even without these barriers, alpha particles usually can't travel more than a few centimeters.
After Litvinenko swallowed or inhaled the polonium, the alpha particles it emitted became trapped inside his body. The radioactive material entered his bloodstream, where its emissions wouldn't be able to travel more than a few dozen microns from their source. They couldn't have breached the surface of the skin, but they were well within range of his immune system. The polonium would also have been able to seep through Litvinenko's capillaries into very close proximity with every one of his tissues.
Litvinenko couldn't have irradiated his friends and family directly, but it's possible that he exposed them to the radioactive polonium. Once he'd been poisoned, the toxic element would have entered all of his bodily fluids. (Doctors confirmed the presence of the material by testing his urine for alpha emissions.) That means that anyone who came into contact with his urine, feces, or sweat might be at risk. In a certain sense, his radiation sickness could have been a sexually transmitted disease as well, since radioactive elements do show up in semen. (Researchers have found traces of depleted uranium in the semen of Gulf War veterans years after their initial exposure.)
In order for someone to catch the radiation sickness, they'd have to be contaminated by Litvinenko's bodily fluids. That means they'd have to ingest, inhale, or otherwise take up enough excreted polonium to become sick. (Everyone has a tiny and harmless amount of naturally occurring polonium-210 in their bodies.)
Bonus Explainer: If alpha particles only travel a few dozen microns, why are they so dangerous? Though they have a very short range, they cause a lot of damage. A single alpha particle, which is made up of two neutrons and two protons, will have a nastier effect as it traverses bodily tissues than, say, a beta particle, which consists of a single electron. Doctors measure these effects in terms of "rems" or "sieverts," which are related to the type of radiation and the amount of energy that gets absorbed by a given mass of tissue. A dose of alpha particle radiation will deliver more rems than the same radiation dose in another form.
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Explainer thanks Keith Eckerman of the Oak Ridge National Laboratory.