UV Light and How UV radiation Kills Microbes

UV Light: Ultraviolet is an electromagnetic radiation which has a wavelength between 100nm to 400nm. UV radiation is not visible to human eye. Lamps, bulb, crackling fire are some examples of objects that emit UV radiation.

source: http://www.lightlab.com/smarter-uv-light/what-is-uv-light

UV region is divided into three bands

  • Ultra Violet A(315-400nm): UVA possess the lowest energy compared to UVB and UVC. It enters deep into the skin causing various skin damages such as wrinkling of the skin, skin cancer and ageing.
  • Ultra Violet B(280-315nm): UVB has the highest energy compared to UVA and UVC. It affects the outer layer of skin causing sunburn, skin cancer and tans.
  • Ultra Violet C (100-280): Highest energy than UVA and UVB. But it doesn’t reach the earth’s surface, it is absorbed by the ozone layer.
source: http://www.arpansa.gov.au/understanding-radiation/what-is-radiation/non-ionising-radiation/ultaviolet-radiation

According to World Health Organization, as sunlight travels through the atmosphere all UV-C and about 90% of UV-B energy are drawn by ozone, water vapour, oxygen and CO2. UV-A radiation is less affected by the atmosphere compared to other two radiation. Therefore, the UV radiation touching the earth’s surface is mainly composed of UV-A with a small amount UV-B factor.

Ultraviolet germicidal irradiation (UVGI) is a decontamination method that uses shorter wavelength UV radiation to inactivate microbes by terminating nucleic acids and disrupting their DNA which makes them powerless to perform certain vital cellular purposes. UVGI is used in a various applications such as food, air purification and water purification.

Killing the microorganisms with UV radiation needs the germicidal wavelengths of between 185 to 254 nanometers. UVC light on earth’s surface is very weak, since the ozone layer blocks it. UVGI devices produces sufficient UVC radiation in air or water systems which breaks apart the derm DNA to make them inactive in environments to microbes such as bacteriaviruses other pathogens. UVC has the strongest germicidal outcome which is very effective in killing or inactivating bacteria, viruses and other pathogens.

A group of single celled microbes which reproduce through the process of cell division are called Bacteria. This single cell feature of bacteria makes it decontaminated by UV radiation as the major form of reproduction in bacteria happens through the process of cell division. Any form of modification in the cell structure of the bacteria can affect the method of cell division.


DNA is made up of two complementary strands that are twisted together into a double helix structure. This structure is made up of four nucleotides Adenine(A), Thymine(T), Guanine(G) and Cytosine(C). UV radiation kills the cells or the process of cell division by damaging their DNA. When UV light falls on bacteria it initiates a reaction between two thymine molecules, it is one of the bases that form DNA. The subsequent thymine dimer produced is very stable but fixing this DNA damage is by removing the two thymine bases and replacing with new nucleotides, which is not an efficient method.

[Dimers: It’s a chemical compound made up of two identical subunits or monomers]

Extensive exposure to UV radiation results in the formation of more thymine dimers in the DNA which produces an incorrect repair or missing dimer. If the process of cell division is interrupted due to an incorrect repair, missing dimer or any damage then the cell cannot do its normal functions. At this point the cell will die or directs the cell to apoptosis.


The unit of brightness is typically expressed in microwatt per centimeter (μW/cm2) and the exposure time employed for UV sterilization is directly proportional to UV dosage. The mathematical calculations for the same are as shown below.

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