Is it true that people are slightly radioactive?
People are indeed a little bit radioactive, but this isn't something you can do anything about, so it's also something you shouldn't worry about. Radioactivity is the name for processes whereby an atomic nucleus, which is made of protons and neutrons, breaks apart, emitting energetic particles. These particles can damage cells and cause their death, or change their genes causing mutations or cancer.
In your body, most of the radioactivity comes from the decay of atoms of potassium-40. Most potassium is potassium-39 and has 19 protons in its nucleus accompanied by 20 neutrons. A very small fraction, about 1.2 percent, has an extra neutron and is called potassium-40. On average, if you start off with some potassium-40, it takes about 1.3 billion years for half of it to decay, so it's not really very radioactive and in any case you haven't got much in your body.
Potassium-40 contributes about the same dose that you get from cosmic rays from space that make it through the Earth's atmosphere to sea level. Most of the radiation doesn't make it out of your body, so there's no reason to avoid crowds or worry about how much you might be affecting the people around you. Your dose from the environment can be much, much higher depending on where you live.
Where does this potassium-40 come from? Well, according to current theory, it comes from supernovas, where it's made in a phenomenal explosion when a star dies. It, as well as all sorts of other elements, got to Earth from many such explosions. A lot of your body is literally stardust.
You may find it interesting that there is a controversial idea called hormesis that holds that some stresses to organisms actually help them to be healthier, and that some radioactivity may be good for you.
I know a nuclear physicist who is close to 80 and looks and behaves like a 40-year-old and he says (perhaps with tongue in cheek) that his constant exposure to radioactivity is the reason.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 