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Germicidal Efficiency

It has been found out that the germicidal action of ultraviolet rays is maximal at the wave length of 250-260nm. There is an abrupt decrease in the efficiency at 290-300nm and continues upto the visible range.

At a fixed UV intensity, the contact time required to achieve a 99.9% kill for certain organisms is given below.

E.coli - 60 sec.Shigella - 47 sec.
Streptococcus faecalis - 165 sec
Bacillus subtilis - 240 sec.
Bacillus Spores - 369 sec.

UV radiation has also been found to be effective in the inactivation of viruses. There is no reference to the ability of UV radiation to destroy cysts. It is the consensus that UV radiation will not kill any organism which can be seen with naked eye.