Ionizing radiation is any energy that causes the ionization of the substance through which it passes. As the radiation is emitted from a source, it detaches a charged particle from an atom or molecule, leaving the atom or molecule with an excess charge. This charged particle is called an ion.
To remove an electron from an atom or molecule, the ionizing particles must have a kinetic energy exceeding the binding energy of the target species, typically a few electron volts. (An electron volt is a unit of energy defined as the work it takes one electron to move across a voltage difference of one volt.) Radiation of sufficient energy for this process to occur is commonly produced in nature.
Some common ionizing charged species are electrons, positrons, protons, and β particles (Helium nuclei). Electrons, positrons, and β particles are emitted by radioactive elements. Photons, the uncharged particles of light, can also be emitted by radioactive nuclides, but can also be generated by x-ray devices. All the charged species, as well as neutrons, are currently produced at man-made particle accelerators, and lasers now generate photons of sufficient energy to exceed the binding energy of many atoms and molecules.
Most elements formed during the very early expansion of the universe were radioactive in the past, but over time became stable. Some, such as uranium, thorium, radium, and radon are still unstable, and spontaneously emit ionizing radiation. Here on Earth many rocks and minerals emit radon gas, a radioactive gas formed by the decay of radium. Other radioactive elements (3H [Tritium] and 14C) can be produced by atmospheric interactions with cosmic rays (energetic particles continuously entering the earth's atmosphere from outer space).
Ionizing radiation is more damaging to human tissue than non-ionizing thermal-type radiation, as it is more likely to be localized and have a higher intensity (energy deposited per area per second). The damage is initialized by the ionizing particle when it knocks an electron off an atom or molecule in a living system, leaving an unpaired electron behind. The target atom or molecule is then a free radical, a highly reactive type that can spawn many more free radicals in the body. The induced chemical changes have been shown to cause cancer and genetic damage.
A unit called the rem (roentgen equivalent man) is used to measure the absorbed dose of ionizing radiation in living systems. An absorption of 0.5 rem annually is considered safe for a human being. By comparison, about 0.1 to 0.2 rem per year is contributed by natural sources, about 0.002 rem comes from dental x rays, and about 0.05 from a chest x ray.