Ionizing (or ionising) radiation is radiation composed of particles that individually can liberate an electron from an atom 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-rays and gamma rays, and also the extreme part of the ultraviolet spectrum (not all ultraviolet is ionizing).

Christopher Busby (born 1 September 1945) is a British scientist known for his controversial theories about the negative health effects of very low-dose ionising radiation. Busby is a director of Green Audit Limited, a private company, and scientific advisor to the Low Level Radiation Campaign (LLRC) another private company that Busby set up in 1999. Busby is a visiting professor at the University of Ulster. Busby was the National Speaker on Science and Technology for the Green Party of England and Wales.

Following the 2011 Fukushima Daiichi nuclear disaster, Busby established a television and internet presence where he discussed the risks of ionizing radiation and the Japanese Government's handling of the disaster. Busby furthermore marketed, on his Japanese language website, tests and a mineral supplement (dubbed by critics an "anti-radiation" pill) that he claimed could mitigate the effects of ingested radioisotopes.

Busby obtained a BSc in Chemistry with First Class Honours from the University of London, and then worked as a researcher at the Wellcome Foundation as an analytical chemist^{[citation needed]}. He was formerly a member of the Royal Society of Chemistry although he this lapsed in 1984^{[citation needed]}. He later gained a PhD in chemical physics at the University of Kent, researching Raman spectro-electrochemistry. In 1999 Busby stood as an Election Candidate for the European Parliamentary elections. Busby was a member of the British government sponsored Committee Examining Radiation Risks from Internal Emitters (CERRIE), which operated from 2001 to 2004. In 2001, he was appointed to the UK Ministry of Defence Oversight Committee on Depleted Uranium (DUOB). Between 2003 and 2007 he was a Fellow of the Faculty of Medicine, University of Liverpool, in the Department of Human Anatomy and Cell Biology , and he is currently a visiting professor at the School of Biomedical Sciences, University of Ulster.