Micro-organisms are microscopically small. They
are everywhere and have an extremely high
metabolism. For example, in hot and humid
weather the number of moulds per cubic metre of
air increases exponentially. Furthermore,
micro-organisms produce mucus substances that
are deposited on surfaces and can contaminate
food and cause microbial decay of the products.
The cell nucleus of micro-organisms contains
thymine, a chemical element of the DNA. This
element absorbs radiation at a specific
wavelength of 253.7 nm particularly well and
changes to such an extent (formation of thymine
dimers) that the cell is no longer capable of
multiplying and surviving.
