Editing Radiation (section)

=== Ultraviolet radiation ===
{{Main|Ultraviolet}}
Ultraviolet, of wavelengths from 10&nbsp;nm to 200&nbsp;nm, ionizes air molecules, causing it to be strongly absorbed by air and by ozone (O<sub>3</sub>) in particular. Ionizing UV therefore does not penetrate Earth's atmosphere to a significant degree, and is sometimes referred to as [[vacuum ultraviolet]]. Although present in space, this part of the UV spectrum is not of biological importance, because it does not reach living organisms on Earth.

There is a zone of the atmosphere in which ozone absorbs some 98% of non-ionizing but dangerous UV-C and UV-B. This [[ozone layer]] starts at about {{convert|20|miles}} and extends upward. Some of the ultraviolet spectrum that does reach the ground is non-ionizing, but is still biologically hazardous due to the ability of single photons of this energy to cause electronic excitation in biological molecules, and thus damage them by means of unwanted reactions. An example is the formation of [[pyrimidine dimer]]s in DNA, which begins at wavelengths below 365&nbsp;nm (3.4&nbsp;eV), which is well below ionization energy. This property gives the ultraviolet spectrum some of the dangers of ionizing radiation in biological systems without actual ionization occurring. In contrast, visible light and longer-wavelength electromagnetic radiation, such as infrared, microwaves, and radio waves, consists of photons with too little energy to cause damaging molecular excitation, and thus this radiation is far less hazardous per unit of energy.