Visual radiation
Of the radiation energy to which we are exposed on a daily base, we are only aware of a small part as light or warmth. The far greater part of this electromagnetic energy, however, remains unnoticed. This also includes ultraviolet radiation.
The energies can also be explained with a wavelength model. Radiation differs through frequency, for example, radio waves are of long wavelength, while visual radiation is in the short wave range. The frequency range of UV radiation only includes a very small part of the electromagnetic range.
UV radiation
Ultraviolet radiation (UV) is a type of short wave energy and beside visible light and infrared rays is in the group of optical radiation. It is therefore possible to bend, deflect, refract and reflect this radiation.
The term ‘ultraviolet’ (in the sense of ‘beyond violet’) is based on the fact that the UV range with the shortest wavelength starts with those wavelengths that the human eye sees as blue-violet colour. Due to this fact UV rays are invisible for the human eye.
UV radiation is divided into three areas:
| UV-A (long wavelength): UV-B (medium wavelength): UV-C (short wavelength): |
400 - 315 nm 315 - 280 nm 280 - 200 nm |
The long wave UV-A radiation strikes the surface of the earth as part of the rays of the sun. They cause a number of photo-chemical processes, have a brief pigment-building effect (sun tan) and can indirectly cause DNA damage and melanomas. UV-A radiation can penetrate glass and transparent plastics.
The medium wave UV-B radiation shows a delayed pigment-building effect, resulting in increased melanin production. In addition, it can cause erythms in form of sunburn on the skin.
UV-B is also responsible for building the pre-vitamin D in the human body. This type of radiation is used for therapeutic purposes, amongst other things, since it has an anti-rachitic effect. At sea level, the proportion of UV-B radiation is lower than in Alpine areas. Normal window glass is not permeable for UV-B radiation or shorter wave radiation.
UV-C radiation has a short wavelength and contain more energy than UV-A- and UV-B radiation. It includes the greater part of the entire UV range and has a strong germicidal effect in the range of 260 nm. Like the visible wavelength of light, UV-C radiation moves only directly and loses its intensity in proportion to the distance from the source.
UV-C radiation does not essentially penetrate cloth or window glass.
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.
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 (UV-C).
Since the Fifties of the last century, with the discovery of the structure of DNA as a double helix by the researchers James Watson and Francis Crick, the secret of this explanation was found.
Why does UV-C 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 UV-C 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.
Connection between dosage and effect
The effectiveness of a disinfection method based on UV-C 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.
Effect on humans
In the case of a higher radiation dosage, UV-C 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 UV-A- and UV-B radiation, the depth of penetration of UV-C radiation into the human skin is very small. The risk of skin cancer is therefore very low, even when exposed to intensive UV-C radiation (unprotected body parts). Scientific evidence concerning a direct relation could not be found yet.
Harmless use of UV-C
UV-C-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, UV-C 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.
Do you have questions?
Contact our specialist advisors today:
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sterilAir AG
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sterilAir EU-Lager
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sterilAir UK Ltd
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