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How does UV light destroy microorganisms

Ultraviolet rays with wavelengths shorter than 300 nm are extremely effective in killing microorganisms. The most effective sterilizing range for UV is within the C bandwidth (UVC). This range is called the germicidal bandwidth. UVC has been used in hospitals for decades to sterilize surgical instruments, water, and the air in operating rooms. Many food and drug companies use germicidal lamps to disinfect various types of products and their containers.

All living organisms contain DNA (deoxyribonucleic acid). DNA provides the mechanism for all functions needed to sustain life. The UV light from 200 to 300 nm is easy to be absorbed by the cells, and the 253.7 nm has the strongest disinfection ability. The UV light penetrates the outer cell membranes of microorganisms, passes through the cell body, reaches the DNA and permanently alters the genetic material. The microorganisms are thereby destroyed in a non-chemical manner.

UV disinfection is a purely physical process. Micro-organisms such as bacteria, viruses, yeast, etc. that are exposed to the effective UV-C radiation are inactivated within seconds. It does not add anything to the water, such as undesirable color, odor, taste or flavor, nor does it generate harmful byproducts. It is fast, efficient, effective, economical and environmentally friendly.

The following are incident energies of germicidal ultraviolet radiation at 254 nanometers necessary to inhibit colony formation in microorganisms (90%) and for complete destruction
Organisms: Energy dosage of Ultraviolet radiation in W/cm2 needed for kill factor
Bacteria 90% 100%
 Bacillus anthracis - Anthrax 4,520 8,700
Bacillus anthracis spores - Anthrax spores 24,320 46,200
Bacillus magaterium sp. (spores) 2,730 5,200
 Bacillus magaterium sp. (veg.) 1,300 2,500
Bacillus paratyphusus 3,200 6,100
Bacillus subtilis spores 11,600 22,000
Bacillus subtilis 5,800 11,000
Clostridium tetani 3,370 6,510
Corynebacterium diphtheriae 2,140 4,100
Ebertelia typhosa 3,000 6,600
Escherichia coli 3,150 6,000
Leptospiracanicola - infectious Jaundice 6,050 12,300
Microccocus candidus 1,000 15,400
Microccocus sphaeroides 6,200 10,000
Mycobacterium tuberculosis 4,400 8,000
Neisseria catarrhalis 3,000 6,600
Phytomonas tumefaciens 5,500 10,500
Proteus vulgaris 3,500 6,600
Pseudomonas aeruginosa 4,000 7,600
 Pseudomonas fluorescens 3,200 6,100
 Salmonella enteritidis 2,150 4,100
Salmonela paratyphi - Enteric fever 8,000 15,200
Salmonella typhosa - Typhoid fever 19,700 26,400
 Salmonella typhimurium 2,420 6,160
 Sarcina lutea 2,200 4,200
 Serratia marcescens 1,700 3,400
 Shigella dyseteriae - Dysentery 1,680 3,400
 Shigella flexneri - Dysentery 4,400 6,160
 Shigella paradysenteriae 1,840 5,720
 Spirillum rubrum 2,600 6,600
 Staphylococcus albus 2,160 5,500
 Staphylococcus aerius 6,150 8,800
 Staphylococcus hemolyticus 2,000 3,800
 Staphylococcus lactis 6,150 8,800 
 Streptococcus viridans 2,000  3,800 
 Vibrio comma C Cholera 3,375; 6,500
Organisms:  
Energy dosage of Ultraviolet radiation in W/cm2 needed for kill factor
Molds 90% 100%
 Aspergillius flavus 4,520 8,700
 Aspergillius glaucus 24,320 46,200
 Aspergillius niger 2,730 5,200
 Mucor racemosus A 1,300 2,500
 Mucor racemosus B 3,200 6,100
 Oospora lactis 11,600 22,000
 Penicillium expansum 5,800 5,800
 Penicillium roqueforti 3,370 6,510
 Penicillium digitatum 2,140 4,100
 Rhisopus nigricans 3,000 6,600
Protozoa 90% 100%
 Chlorella Vulgaris 6,050 12,300
 Nematode Eggs 1,000 15,400
 Paramecium 6,200 10,000
Virus 90% 100%
 Bacteriopfage - E. Coli 3,000 6,600
 Infectious Hepatitis 5,500 10,500
 Influenza 3,500 6,600
 Poliovirus - Poliomyelitis 4,000 7,600
 Tobacco mosaic 3,200 6,100
Yeast 90% 100%
 Brewers yeast 8,000 15,200
 Common yeast cake 19,700 26,400
 Saccharomyces carevisiae 2,420 6,160
 Saccharomyces ellipsoideus 2,200 4,200
 Saccharomyces spores 1,700 3,400