Ionizing (or ionising) radiation is radiation composed of particles that individually can liberate an electron from anatom or molecule, producing ions, which are atoms or molecules with a net electric charge. These ions tend to be especially chemically reactive, and the reactivity produces the high biological damage per unit of energy that is a characteristic of all ionizing radiation.
The degree and nature of such ionization depends on the energy and type of the individual particles composing the radiation, and less upon the radiation particle number. For example, if a non-ionizing type of radiation does not heat a bulk substance up to ionization temperature, even an intense flood of particles or particle-waves will not cause ionization. In such cases, each particle or particle-wave does not carry enough individual energy to be ionizing (an example is a high-powered radio or microwave beam, which will not ionize if it does not cause high temperatures). Conversely, even very low-intensity radiation will ionize materials at low temperatures and powers, if the individual particles of radiation carry enough energy (e.g., a low-power X-ray beam). These radiations are termed "ionizing." In general, particles or photons with energies above about 10 electron volts (eV) (some sources require 33 eV) are considered ionizing, no matter what their intensity. This particle-energy occurs in electromagnetic waves in for X-raysand gamma rays, and also the extreme part of the ultraviolet spectrum (not all ultraviolet is ionizing).
Free neutrons are able to cause many nuclear reactions in a variety of substances no matter their energy, because in many substances they give rise to high-energy nuclear reactions, and these (or their products) liberate enough energy to cause ionization. For this reason, free neutrons are normally considered effectively ionizing radiation, at any energy (see neutron radiation). Examples of other ionizing particles are alpha particles, beta particles, and cosmic rays. The radiations cause ionization due to the kinetic energy involved in the production of the individual particles, which inevitably exceed the threshold of 10 or 33 eV, and commonly exceed thousands or even millions of eV of energy.
Lower-energy radiation, such as visible light, infrared, microwaves, and radio waves, are not ionizing. The latter types of low-energy non-ionizing radiation may damage molecules, but the effect is generally indistinguishable from the effects of simple heating. Such heating does not produce free radicals until higher temperatures (for example, flame temperatures or "browning" temperatures, and above) are attained. In contrast, ionizing radiation produces free radicals, such as reactive oxygen species, even at room temperatures and below. Free radical production is a primary basis for the particular danger to biological systems of relatively small amounts of ionizing radiation that are far smaller than needed to produce significant heating. Free radicals easily damage DNA, and ionizing radiation may also directly damage DNA by ionizing or breaking DNA molecules.
Radiobiologists see a degree of overlap between truly ionizing radiation and the lower ultraviolet (UV) spectrum from 3.1 to 10 eV that contains molecularly-damaging radiation that is not formally ionizing, but has somewhat similar biological effects that are larger than are predictable from thermal considerations alone. Non-ionizing ultraviolet can also cause damage to skin as a result of photoreactions in collagen. DNA molecules may also be directly or indirectly damaged by UV radiation carrying enough energy to excite certain molecular bonds to form thymine dimers (pyrimidine dimer)s (this causes sunburn).
Ionizing radiation is ubiquitous in the environment, and comes from naturally occurring radioactive materials and cosmic rays. Common artificial sources are artificially produced radioisotopes, X-ray tubes and particle accelerators. Ionizing radiation is invisible and not directly detectable by human senses, so instruments such as Geiger counters are usually required to detect its presence. In some cases it may lead to secondary emission of visible light upon interaction with matter, such as in Cherenkov radiation and radioluminescence. It has many practical uses in medicine, research, construction, and other areas, but presents a health hazard if used improperly. Exposure to ionizing radiation causes damage to living tissue, and can result in mutation, radiation sickness,cancer, and death.