Ionizing radiation (or ionising radiation) consists of particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, thus ionizing them. Direct ionization from the effects of single particles or single photons produces free radicals, which are atoms or molecules containing unpaired electrons, that tend to be especially chemically reactive due to their electronic structure.
The degree and nature of such ionization depends on the energy of the individual particles (including photons), not on their number (intensity). In the absence of heating or multiple absorption of photons (a rare process), an intense flood of particles or particle-waves will not cause ionization if each particle or particle-wave does not carry enough individual energy to be ionizing (e.g., a high-powered radio beam). Conversely, even very low-intensity radiation will ionize, if the individual particles carry enough energy (e.g., a low-powered X-ray beam). Roughly speaking, particles or photons with energies above a few electron volts (eV) are ionizing, no matter what their intensity.
Examples of ionizing particles are alpha particles, beta particles, neutrons, and cosmic rays. The ability of an electromagnetic wave (photons) to ionize an atom or molecule depends on its frequency, which determines the energy of its associated particle, the photon. Radiation on the short-wavelength end of the electromagnetic spectrum—high-frequency ultraviolet, X-rays, and gamma rays—is ionizing, due to their composition of high-energy photons. 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, damage done by ionizing radiation produces free radicals, even at room temperatures and below, and production of such free radicals is the reason these and other ionizing radiations produce quite different types of chemical effects from (low-temperature) heating. Free radical production is also 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. Read more from Wikipedia, at http://en.wikipedia.org/wiki/Ionizing_radiation
Comment: Some people don’t fear that which they cannot sence, feel, or otherwise become aware of, and others do: It is what makes the world a bit more level. Diversity does that. The caution that some have, over the daring other don’t. The faith in something, larger than the frailty innate in nature, and so much more existent in humans. When I was a little boy, my mom took me to the Black Sea sunny beaches, where she bought for me a toy bucket and shovel, and then she assisted me in making, what I thought it was the most magnificent sand castle ever built in entire world: it was just before the high tide, and before I had time to enjoy it a more daring wave leveled it, before i could do a thing about it. Of course I cried my heart out: Wouldn’t anyone. Aren’t we made to cry over spilled milk? O, yes, more than anybody else in nature, we do people. Back to that day, though, the day of my sand castle: I learned something fundamental that day: Think of consequences, before any action, be ready for a unforseable outcome, revere that which you cannot sence, pay attention to details, as much as you pay to the large picture. Don’t build a sand castle unless you’re ready to face its demise. I also understood that day that Nature is bigger than me, and could care less about my sand castle. Later in life I have encountered many occasion to verify the value of that lesson: Words can be so deceiving, where a picture says a thousand words.