Dosage, impact and harmless use of UVC-radiation
Which types of radiation are germicidal?
Healing powers have been attributed to the sun since ancient times. The scientists Arthur Downes and Thomas P. Blunt discovered as far back as 1878, that micro-organisms do not reproduce when exposed to direct sunlight ('On the influence of light upon protoplasm', Proc. Roy. Soc. 28, 1878: p. 199-213). After this discovery, it took some time before the correlation between a specific wavelength and the maximum reaction was recognised at 250 to 270 nm, being part of the UV-radiation from the short wave C range (UVC).
Since the Fifties of the last century, with the discovery of the structure of DNA (Deoxyribonucleic Acid) as a double helix by the researchers James Watson and Francis Crick ('Molecular structure of nucleic acids. A structure for deoxyribonucleic acid', Nature. Bd. 171, No. 4356, 1953: p. 737–738), the secret of this explanation was found.
Why does UVC radiation have disinfecting properties?
The double helix structure of the DNA is based on a purine and pyrimidine base pairing. The pairing of these bases are really carriers of DNA information; the four bases adenine, thymine, guanine and cytosine can be distinguished. Research in later years showed that the short wave and the strong energy of the UVC radiation mainly causes a photo-chemical effect in the thymines. These dimerise (meaning that the two information carriers lying next to each other form a chain or close up. This molecular change makes the DNA unusable for the essential biological process of transcription (metabolism) and replication (cell division). A cell sufficiently damaged in that way will eventually die away.
This inhibiting function of the UVC technology makes it different from the mostly chemical, oxidative disinfection processes. This is the main reason why a mutation-caused resistance is impossible.
Connection between dosage and effect
The effectiveness of a disinfection method based on UVC radiation is directly connected to the dosage used (= duration x energy / surface). High intensity during a short duration, or low intensity over a long time period are practically interchangeable and almost have the same disinfecting effects. The dosage as an important element is demonstrated as µW*s/cm², and frequently also in J/m².
As a rule, the following applies: the simpler a microorganism is formed, the easier it can be inactivated by UV radiation. This is why viruses or bacteria (procaryotic cells) in general can be much more easily destroyed than complex microorganisms such as yeasts and vegetative fungi (eucaryotic cells). Especially mould spores, which have a DNA that is protected through a pigmented cell wall and concentrated cytoplasm, do need a huge amount of UV energy to be combated in this manner.
It is very important to take this into consideration when demanding an efficient ultraviolet germicidal installation (UVGI). Beside the pure power definition of germicidal emitters there alway should be a clear definition of the target organism.
Effect on humans
In the case of a higher radiation dosage, UVC radiation causes red skin (erythms) and painful eye infection (conjunctivitis) to humans.
This is why the threshold value of 6 mJ/cm², and/or 60 J/m² daily radiation dosage respectively, is recommended by the EU (EU Directive 2006-25-EC) (with 254 nm), which should not be exceeded. Sufficient protection is paramount.
Other than UVA- and UVB radiation, the depth of penetration of UVC radiation into the human skin is very small. The risk of skin cancer is therefore very low, even when exposed to intensive UVC radiation (unprotected body parts). Scientific evidence concerning a direct relation could not be found yet.
Harmless use of UVC
UVC-radiation does not permeate solid bodies - even no window glass (borosilicate, duran) or transparent plastics (acrylic glass, polystyrene, etc.).
As with the visible wavelengths of light, UVC radiation only moves in a direct line and decreases in intensity by increasing distance to the source. Consequently, the further the distance towards the UV source the less dangerous it is. Equipment with protective slats or fully enclosing vessels therefore never can be hazardous.
If protracted direct eye or skin contact with a freely emitting UV source is unavoidable, simple precautions such as protective glasses or suntan cream with a high protection factor are sufficient.