Every second, more than 100 cosmic rays fly through your body. At the same time, several radioactive atoms in the air you have inhaled disintegrate inside your lungs and a few thousand potassium atoms and two or three uranium atoms, which arrived in your food and drink, break apart inside you and bombard your body with radiation. As if that isn't enough, more than 50 000 gamma rays enter your body from the soil and the building materials around you. All this is entirely natural. Many of us are also exposed to the artificial radiation that is used in medicine, industry, agriculture and research.

Radiation consists of various kinds of energetic particle. When such fast-moving particles plough through a living body, they alter or destroy the molecular machinery of its cells.

At high intensities, this can kill cells. At low levels, the damaged proteins and other molecules can usually be ...