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Which rays are bactericidal?
Since time immemorial the sun has been said to have healing powers. But it was not until 1878 that researchers Arthur Downes and Thomas P. Blunt realised that micro-organisms did not multiply under strong sunlight. After this discovery, however, it took quite some time before a specific wavelength dependence was recognised with a reaction maximum, a part of the UV radiation from the short-wave C-band (UV-C).

 

Only since the 1950s, with the discovery of the structural make-up of DNA (deoxyribonucleic acid) as a double helix by researchers James Watson and Francis Crick, has the key to an explanation been available.

 

Why is UV-C radiation germicidal?
The double helix structure of DNA is based on a puri- and pyrmidine base pairing. These base pairings are the actual information carriers of the DNA and a distinction is made between the four bases adenine, thymine, guanine and cytosine. Research in the following years showed that the short-wave and high-energy UV-C radiation primarily causes a photochemical effect on the thymines. This dimerisation (i.e.: linking or sticking together two information carriers lying next to each other) This molecular change makes the DNA virtually unusable for the essential biological process of transcription (maintenance of metabolism) and replication (cell division). The primary difference to chemical, mostly oxidative, disinfection processes is the inhibitory function of the UV-C technology. It is the central reason why a mutation-related formation of resistance is excluded.

 

Connection between dose and effect
The effectiveness of a disinfection method based on UV-C radiation is directly related to the doses used. The dose as the decisive determining factor is given in µW*s/cm², frequently also in J/m².

 

As a general rule, the simpler the structure of a micro-organism, the easier it is to inactivate with UV-C radiation. Viruses or bacteria (prokaryotic cells) are therefore much easier to destroy than complex micro-organisms such as yeasts and vegetative fungal cells (eukaryotic cells). Fungal spores, whose DNA is protected by a pigmented cell wall and a concentrated cytoplasm, can only be inactivated by applying substantial doses of UV-C.

Effect on people
With an increased radiation dose, UV-C radiation causes reddening of the skin (erythema) and painful inflammation of the eyes (conjunctivitis) in humans. For this reason, the limit values set by the EU (EU Directive 2006-25-EC) should not be exceeded with 6 mJ/cm², or 60 J/m² daily radiation dose (at 254 nm). Unlike UV-A or UV-B radiation, the penetration depth of UV-C radiation into the human skin is very low. The risk of skin cancer is therefore classified as extremely low even with intensive UV-C radiation (unprotected parts of the body). A scientific proof of the direct connection could not be provided so far.

Safe use of UV-C
UV-C rays do not penetrate solid materials – not even window glass (borosilicate, duran) or transparent plastics (acrylic glass, polystyrene, etc.) Like the visible wavelengths of light, UV-C rays only travel in a straight line and their intensity decreases significantly with increasing distance from the source. The further you are away from a UV source, the less dangerous it is. Therefore, devices with viewing protection slats or fully enclosed housings can never pose an immediate danger. If direct eye or skin contact with a freely radiating UV source is unavoidable, simple measures such as protective goggles or sunscreen with a high sun protection factor are completely sufficient.

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Post time: Dec-20-2021