Out of a considerable amount of energy we are exposed to on the daily basis, we are only aware of a small fraction of it, such as light or heat. The greater part of that electromagnetic energy, however, goes unnoticed. Ultra-violet radiation is also part of that.
Energy can be explained by a wavelength model. Radiation varies in frequency, for instance, radio waves have a long wavelength, whereas optical radiation belongs to the short-wave spectrum. The frequency area of UV radiation includes just a tiny fraction of electromagnetic spectrum.
Ultra-violet radiation ( UV ) is a kind of short-wave energy, and, together with visible light and infra-red rays, belongs to optical radiation. Such radiation has an ability to bend, to refract and to reflect. The name “ultra-violet” ( meaning “beyond violet”) is based on the fact that UV spectrum starts at the wavelengths which are shorter than the wavelengths that the human eye can perceive as the blue-violet colour. Thus, UV rays are invisible to the human eye.
UV radiation is divided into three areas:
- UV-A (long wave): 400 – 315 nm
- UV-B (medium wave): 315 – 280 nm
- UV-C (short wave): 280 – 100 nm
- VUV (vacuum UV ): 10 – 200 nm
Long-wave UVA radiation hits the Earth’s surface as a part of solar radiation. It triggers many photo-chemical reactions, it affects the short-term development of pigmentation ( skin-tanning in the sun ), and it can indirectly cause DNA damage and melanoma. UVA radiation penetrates glass and transparent plastic.
UVB medium-wave radiation can trigger a delayed increase of pigmentation, which results in the increased production of melanin. In addition, it can cause erythema and sunburns on the skin. UVB radiation is also responsible for the creation of provitamin D in human body. This kind of radiation is used for therapeutic purposes, among other things, for the prevention of rickets. Close to the sea-level, the portion of UVB radiation is smaller than in the Alpine region. The common windowpane is impervious to UVB and short-wave radiation.
UVC radiation is short-wave and contains more energy than UVA and UVB radiation. It takes the greater part of the UV spectrum, and has a strong germicide effect at the frequency of 260 nm. Like the visible light wavelengths, UVC radiation also acts directly and loses intensity in proportion to the distance from the source. UVC radiation does not penetrate textile fabric or windowpanes.
Vacuum UV radiation
VUV radiation is the abbreviation for vacuum ultra-violet radiation and it indicates the spectrum of electromagnetic radiation in the short-wave area at the ultra-violet border. The name derives from the fact that air absorbs radiation and creates the vacuum necessary for expansion. The spectral range is not precisely determined, since there are different definitions, e.g. 10-200 nm, or 100-200 nm. According to some definitions, the edge of the short-wave area borders on the extreme ultra-violet radiation, or on soft X-ray radiation.
Wavelengths and disinfection effect
By analysing the disinfection effect of UVC rays having the wavelengths of 200 to 300 nm ( yellow line ), we notice the greatest efficiency of the wavelengths between 240 and 280 nm, with the maximum effect at 260 nm. If we compare it with the absorption spectrum of genetic material ( DNA, blue line ), we notice the identical effect. Therefore, the more UVC radiation is absorbed by DNA, the more damage it does – the degree of disinfection increases. The UVC disinfection tube achieves its maximum effect at 253.7 nm ( violet column ), and, in accordance with that, the maximum DNA destruction. The efficiency of the necessary electrical energy at these wavelengths is above 30%, and that is why this technique is highly efficient.
Thymine dimers and death of cells
DNA is a long molecule shaped as a double helix. The two chains are linked by base pairs of four building units ( adenine, thymine, guanine and cytosine, abbreviated as A, T, G and C ).
Two pairs with one thymine each stand next to one another, having a precisely determined frequency. The action of UVC breaks the link with the opposite adenine, and the two thymines get linked. They form a thymine dimer which prevents polymerisation and further DNA replication.
The more damaged spots on the DNA there are, the greater the impact on the life of a cell is, e.g. in micro-organisms. This principle confirms the dosage and impact; the greater the UVC dosage is, the more frequent the damages are, and so is the mortality rate. The lack of enzymes and the blocks formed by the thymine dimer prevent the cell division, and the cell dies.
Species and mortality rates
Generally speaking, all forms of life on the Earth are based on DNA and RNA. This means that UVC radiation can do damage to any organism. The simpler an organism is, the smaller its cell volume, the larger the lethal effect of UVC is. The UVC radiation dose that destroys 90% of organisms belonging to one species is called LD₉₀ dose.
To achieve a mortality of 90%, for simple bacteria we need a dose of 1.2 to 6 mJ/cm², for bigger moulds with a cell nucleus we need between 6 and 10 mJ/cm², and for fungal spores that can survive in extreme conditions as many as 20 to maximum 120 mJ/cm². Within a group of organisms there may be some variations, depending on the colour of the cell membrane of DNA transposition. We have at our disposal many charts and resources in order to calculate the necessary doses, depending on the species. When doing projects on disinfection solutions and positioning UVC products, what is really important is the exposure and number of micro-organisms, because opacity, reflection and dispersion affect the penetration of UVC radiation into the DNA. Another important factor which determines the LD₉₀ dose, and therefore also the success of UVC disinfection, is the cellular cycle of an organism. The cells with a prominent protein synthesis, necessary in the division process, get damaged more quickly, because their DNA separates, and thus gets more exposed to the radiation.
Protection through knowledge
Solar UV radiation penetrates the atmosphere differently, depending on the wavelength. UVA rays of long wavelength reach us to a large extent, and they cause skin-tanning. UVB radiation partially reaches the Earth’s surface and penetrates relatively deeply into our skin and eyes. UVC radiation is filtered through the ozone layer situated in the Earth’s atmosphere.
Were it not so, that high-energy radiation would cause skin and eye burns. Since it penetrates very shallowly into the outer skin layers, the skin-cancer risk is very low, compared with UVB radiation. With artificial sources of UVC radiation, skin and eye protection is mandatory. For that purpose, clothes, gloves and goggles are sufficient. In practice, in order to protect humans and animals against UVC radiation, metal and glass protection is mostly used, as well as electrical safety systems.
The proper installation of UVC equipment and systems, as well as measuring radiation according to certain protocols in production areas, are part of our consulting and project design work. UVC-LOG measuring equipment enables a personal control of workplaces, with a sensitivity of 0.01 μW/cm².