Ionizing radiation is generally considered capable of causing physiological effects and of harming human or animal bodies. By definition ionizing radiation produces ions, directly or indirectly, in its passage through matter.  It includes X-rays, gamma rays, neutrons, and other forms of penetrating radiation. Under specialized circumstances, sources of non-penetrating energy, such as α-particles, may be considered as a form of of ionizing radiation if the source enters the body such that normal bodily measures do not protect against its effects. For instance, when an α-particle source is swallowed or inhaled, the human skin does not protect against the harmful effects of the α-radiation.
Ionizing radiation can have effects measurable by both chemical and physical processes. The World Health Organization defines it as "...radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionized. For electromagnetic radiation (i.e., photons, but not particles such as electrons and neutrons, or sound energy), the shorter the wavelength in the electromagnetic spectrum, the more energetic the radiation and the greater are its effects.
If delivered in sufficient intensity, over a short period of time, to the entire body, ionizing radiation may cause acute radiation syndrome. In the U.S. and other countries, sources of ionizing radiation (e.g., a X-ray machine used in diagnostic imaging) that are claimed to have health benefits are considered radiation-emitting products. Some diagnostic apparatus may emit non-ionizing radiation such as ultrasound or low-energy electromagnetic radiation (MRI).
Sources of ionizing radiation, above a certain level of intensity and ability to penetrate, may also fall under the regulation of other organizations, such as the U.S. Department of Energy. Radioactive materials that emit significant ionizing radiation at all times can be considered hazardous materials. An improvised device that cause uncontrolled release of sources of ionizing radiation may be considered a radiological weapon.
There are an immense number of beneficial applications of ionizing radiation, but the sources need to be handled properly so that they do not become a hazard.