The Planck-Einstein relation explains and quantify the amount of energy carried by electromagnetic (EM”) radiations (see our article: What is light?). The energy difference between both ends of EM spectrum can be over 1020 times. Physicists found that part of the EM radiation carries sufficient energy to liberate or remove electron(s) from atoms and turn atoms into ions, i.e., ionizing an atom. This can happen to molecules as well. Gamma rays, X-rays, and the higher ultraviolet part of the EM spectrum are generally considered as ionizing EM radiation; the rest of EM radiations in the EM spectrum are considered as non-ionizing EM radiation.
Figure 1: Ionizing and Non-ionizing electromagnetic spectrum
The boundary between ionizing and non-ionizing EM radiation is not clear cut, since different molecules or atoms ionize at different energy levels. In Physics, conventional definition places at the photon energy higher than 12.4eV or at the wavelength lower than 100nm. Ionizing radiation can be categorized into energetic subatomic particles, ions or atoms moving at high speeds (usually above 1% of the speed of light) and the ionizing EM radiation.
Most people should feel a lot of anxiety about the ionizing radiation, as it is commonly-known that it causes cancer or tumor. Although the lasting effects of nuclear weapons used in the World War II took away tens of thousands of lives due to radiation injury, today nearly all radiation cases result from medical or nuclear accidents, for example Fukushima nuclear accident in 2011. Damages done to tissue caused by ionizing radiation are usually far more destructive and longer lasting, although similar injuries to tissues may be caused by strong electrical currents and by some drugs and toxins.
Such kind of radiation-induced ionizations may act directly on the cellular component molecules or indirectly on water molecules, causing water-derived free radicals, i.e., removing electrons from neutral water molecule to produce H2O+. Free radicals are extremely reactive and are strong oxidizing agents, at the molecular level, which destroy biologically active molecules by either removing electrons or removing hydrogen atoms. At the cellular level, it represents the damage of membrane, nucleus, chromosomes, or mitochondria of the cell. The affected cells may turn into or produce malignant cells (i.e., cancer cell) or are inhibited to divide or and at worst, result in cell death.
However, ionizing radiation may not be necessary evil. Exposure to ionizing radiation can also result from irradiation from artificial source, such as medical X-rays/CT Scan instruments or radiation therapy equipment. Such external irradiation can stop when the radiation source is properly shielded.
Figure 2: CT Scan Instrument
Does it mean non-ionizing radiation not being harmful to us?
Most non-ionizing radiation, such as radio waves, microwaves and infrared, is considered harmful only to the extent of the amount of heat energy it transfers to whatever it hits. This is why that microwaves boil water and cook food. Ultraviolet rays are different. Despite of the fact that only part of higher-frequency UV rays are ionizing, the non-ionizing UV radiation have the capacity to cause harmful effects similar to what ionizing radiation can cause, such as an increased risk of cancer due to damage to DNA molecules.
UV radiation denoting EM radiation of wavelength shorter than that of the violet end of the visible light spectrum, having wavelengths between 10 and 400 nm. The shorter the wavelength of the UV ray, the more energy the UV ray carries, the more biological damage it causes. The most common three UV radiation classification is below:
- UVA or UV-A, its wavelengths between 320 and 400 nm, lesser energetic UV rays
- UVB or UV-B, its wavelengths between 280 and 320 nm, medium energetic UV rays
- UVC or UV-C, its wavelengths between 100 and 280 nm, most energetic UV rays
Figure 3: infrared, visible light and ultraviolet spectrum
However, there are other UV radiations, more powerful than UVC, for example, extreme UV, its wavelengths between 10 and 120nm, which is considered as ionizing radiation. There are many subcategory of UV radiation under the ISO Standard ISO-21348.
UVC is commonly used to kill micro-organisms, like bacteria and virus, in sanitation system used in hospital or sterilization system in water facilities.
Figure 4: UV Sanitation used in hospital
 Lars Persson, On the Boundary between Ionizing and Non-ionizing Radiation, Sweden Radiation Protection Institute, Box 60204, S-10401 Stockhom, Sweden
 ISO 21348 Definitions of Solar Irradiance Spectral Categories (Courtesy of Space Environment Technologies firstname.lastname@example.org, http:SpaceWx.com)