What is UV Light?

Ultraviolet light is that portion of the electromagnetic spectrum that lies beyond the “purple” edge of the visible spectrum and has wavelengths between 100 and 400 nm. The UV spectrum is further divided into ranges as follows:

Range Name	Wavelength Range (nm)
UVA	315-400
UVB	280-315
UVC	200-280
Vacuum UV (VUV)	100-200

The UVA range causes “sun tanning” in the human skin. The UVB range causes “sun burning”. The UVC range is absorbed by DNA and thus can cause cancer and mutations. This is also the range that is most effective in inactivating bacteria and viruses. The Vacuum UV range is absorbed strongly by water and air and thus can only be transmitted in a vacuum.

Ultraviolet photons are particularly energetic and when absorbed in molecules can cause bonds to be broken (photochemistry).

What is a “photon”?

Ultraviolet light (along with all others forms of electromagnetic radiation) comes in discrete energy packets called “photons”. The energy of a photon is given by:

where h is the Planck constant (6.626755 ´ 10-34 J s), c is the speed of light (2.997925 ´ 108 m s-1), n is the frequency (Hz) of the light, is the wave number (cm-1 or m-1) of the light and l is the wavelength (nm or m). Usually photochemical “events” involve absorption of only one photon per molecule.

Why is UV important for the disinfection of water and air?

The portion of the UV spectrum (the “germicidal” region) that is important for the disinfection of water and air is the range that is absorbed by DNA (RNA in some viruses). This “germicidal range” is approximately 200 – 300 nm, with a peak germicidal effectiveness at about 260 nm. The mechanism involves absorption of a UV photon photon by pyrimidine bases (principally thymine) where two pyrimidine bases are next to each other on the DNA chain. The “photochemistry” involves formation of a “dimer” that links the two bases together. This causes a disruption in the DNA chain, such that when the cell undergoes mitosis (cell division), the replication of DNA is inhibited.

What are the major commercial applications of UV?

UV has many commercial applications in society. The major ones are: UV disinfection of water and air, UV curing of inks and coatings, UV disinfection of foods, UV-based Advanced Oxidation destruction of pollutants in water and air.

What is “Advanced Oxidation”?

Advanced oxidation describes processes that induce accelerated oxidation of organic pollutants in water and air. When carried to completion, the only products are water, carbon dioxide and mineral acids from any chlorine, nitrogen or sulfur present. This process is called “mineralization”. Advanced oxidation technologies come in many varieties and usually involve the generation of highly reactive “hydroxyl radicals” (·OH), which attack and oxidize the organic contaminants. UV-based advanced oxidation technologies usually involve the generation of ·OH radicals by the photolysis of added hydrogen peroxide.