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More than 100 years ago, scientists realized that some components of strong sunlight were capable of disinfecting and sterilizing items by killing micro-organisms (such as bacteria, spores, molds and viruses). One such portion of the electromagnetic spectrum was identified as being ultraviolet radiation of wavelengths between 200 nanometers (nm) and 300 nanometers. The region of ultraviolet radiation is often called UV C, short-wave UV, or germicidal UV. The DNA in most life forms is double stranded with hydrogen bonds connecting parts between the two strands. When micro-organisms are exposed to UV C radiation, the energy is absorbed in the hydrogen bonds in the DNA, causing some of the bonds to rupture and also causing portions of the DNA to fuse.

This disruption of the DNA chain prevents the cell from replicating and the micro-organism ceases to grow. The DNA disruption is well known and many studies have been done to determine the amount of UV C needed to kill particular micro-organisms. In very general terms, the more complicated the micro-organism is, the more UV will be required to kill it. For instance, the simplest bacteria usually have only a cell wall with the DNA inside, so they are easy to kill. Yeasts have the cell wall, plus a cell nucleus, with the DNA inside the nucleus, so more UV is needed to kill yeasts. Fungi have all of those parts, and also contain pigments so even more UV is required to kill them than for the simpler micro-organisms.

The UV dose is the product of the intensity of the UV and the amount of time. Although in mathematical terms, a small amount of UV for a long period of time can equal the same dose as a large amount of UV in a short time, there is some evidence that a high dose in a small amount of time kills more micro-organisms than the reverse. The dose of UV is often given in terms of micro watt seconds per centimeter squared. The relationship between the dose of UV and the deactivation rate of micro organisms is a log scale. An important concept is called the D10 value, which is defined as the UV dose needed to deactivate 90% of any given micro-organism. If the D10 value is 500 micro watt seconds per centimeter squared (micro watt s/cm2), then to deactivate 99% will require 1000 micro watt s/cm2, to deactivate 99.9% will require 2,000 micro watt s/cm2.

Table A shows the dose in micro watt seconds per centimeter squared required to kill 99.9% of various micro-organisms.



Bacillus anthracis 8,500

Legionella dumoffil 5,500

Mycobaterium tuberculosis 10,000

Clostridium Tetani 22,000

Sarcina Lutea 26,400


Baker's yeast 8,800

Brewer's yeast 6,600

Common yeast cake 13,200


Penicillum expensum 22,000

Penicillum roqueforti 26,400


Chlorella vulgaris 22,000


Bacteriophage (E. col) 6,600

Hepatitis virus 8,000

Influenza virus 6,600

Using data of this nature and given the UV output of a particular UV C tube/lamp, we can figure out the amount of time needed to deactivate 99.9% of any particular micro-organism. For example, if we choose the mold Penicillum roqueforti (one of the most UV resistant molds), and a single UV C tube/lamp that emits 340 micro watts per centimeter squared at one meter, then at 1 meter, the time required to kill 99.9% of the Penicillum roqueforti is calculated by the formula: [26,400 micro watt seconds per centimeter squared] divided by [340 micro watts per centimeter] which equals [77.65 seconds].

If the UV dose is doubled by adding a second tube/lamp, the time reduces to 38.8 seconds, and for the use of four lamps, the time reduces to 9.7 seconds. In practical terms, this means if we want to be able to kill 99.9% of even the toughest micro-organisms in 10 seconds, then we simply calculate the number of UV C lamps that will be required to supply the proper dose and an apparatus can be designed accordingly.