UVC light has been used for decades to disinfect objects, water, various surfaces and even air. However, since natural UVC light does not occur on earth, it has to be generated artificially. For a long time, this was done with so-called low-pressure mercury lamps. Here, too, lamp technology is making great strides in a new direction. For some time now, LEDs have been gaining ground in the UVC sector, just as they have in the conventional lighting industry. But what impact do these little lamps actually have? How high is the disinfection effect, what about the costs and can they already keep up with the previous UVC lamps? The UVIS hygiene experts have taken a look at this topic and summarised it here.
UVC light has been used for a long time in medicine, water treatment and the food industry to disinfect surfaces, objects, water and air. However, the high-energy part of natural sunlight is completely filtered out by the ozone layer, so that UVC light has to be generated artificially by lamps.
Disinfecting light
Light is divided into different spectra - infrared light, visible light and UV light, which then changes into X-rays. Within UV radiation, we divide into vacuum UV and the well-known UVA, UVB and UVC rays. All are extremely harmful to our cells and likewise to organisms of all kinds, including disease-causing germs. UVC radiation is so highly energetic that the DNA or RNA in the cell nucleus is destroyed, and the organism is unable to survive. With the UVC lamp in the UVIS Surface, which is at a wavelength of 254 nanometres (nm - millionths of a millimetre), a disinfection level of 6 log* can be achieved, for example, which means that only one germ out of a million survives, and this is such a small amount that it is theoretically no longer possible to become infected.
In addition to UVC radiation, there is also a range of radiation that lies within the visible light range that is harmless to humans, but which also has a disinfecting effect - the so-called Blue Light in the wave range of 400-500 nm.
Light as a deadly weapon - not a Star Wars fantasy
We have known for a long time that UV light is harmful from sunscreen advertisements. We get sunburn or, if we overdo it, the inside of the cell, the DNA, is attacked and the cells may mutate into cancer or, if the eyes get the radiation, we can go blind.
But what happens to germs when they are irradiated by UVC light? Basically exactly the same. DNA and also RNA consist of four basic building blocks (adenine, thymine/uracil, guanine and cytosine) which in the case of DNA are assembled in the double helix.
The UVC light splits the connection to the opposite strand, for example when two thymines are next to each other, and connects them to form so-called dimers. As a result, the strands can no longer be read in reproduction and are thus useless. The cell cannot divide or reproduce and the cell functions are blocked. The cell or the entire microorganism dies.
UVC lamps - spoilt for choice
Our aim is to damage the cells so that they die.
So to achieve disinfection with 4 or 5 log or even sterilisation with 6 log, the object, or in our case the germ, must be irradiated with the right UV dose. This depends on the right lamp, in the right wavelength, the irradiation time and the distance to the object. Low-pressure mercury lamps emit most of their photons at 254 nm, at the maximum of DNA destruction. Therefore, these are still used for disinfection and are still the most efficient with an efficiency of 17 percent.
Since the corona pandemic, however, UVC LEDs, or UVEDs, have been gaining ground, with a radiation bandwidth of 260 to 280 nm. This means that they are not within the previous optimum of 254 nm for disinfection. Here, however, we are making great strides, just as we did years ago with the development of alternatives to the light bulb.
UV LEDs are already much more efficient (>65%) than mercury vapour lamps (8%) at the wavelength of 365 nm, but in terms of their effect on the DNA of microorganisms, the lamps are still the undisputed technological leader.
It is assumed, however, and the development of UV LEDs is also moving in this direction, that the microbiological effect on DNA is somewhat stronger at 278 nm than at 254 nm. However, the rapid development and increase in efficiency is also accompanied by a rapid price development, which is positive for the processing industry because the price is halved approximately every twelve months.
Challenges in development
It is undisputed that UVEDs are on the advance. The pandemic of recent years has also contributed to this. However, there is another major challenge in the development process. In addition to achieving the optimal wavelength and the appropriate UV dose to kill germs, a UVED equivalent to existing UVC lamps would generate high heat. Currently, 35 UVEDs of 70 mW each would be needed to replicate the output of the UVC mercury lamp installed in the ESCALITE module, for example. To keep the UVEDs at a temperature that is compatible with the rest of the technology, they would have to be deheated with a 90 W line.
This is similar to the heat generated by current high-performance processors in the PC sector, which can only be actively cooled with fans and air-conditioning systems in the server rooms. Even in this area, it is very difficult to keep the temperature below 60 °C permanently. Therefore, further research is needed not only on the radiation dose and thus the efficiency of UVEDs, but also on their heat generation.
Gentle light for disinfection
Both technologies, UVED and UVC lamps, or rather the light wavelengths are, as we know, very harmful to humans. But there is also disinfecting UVC radiation that is not supposed to be harmful to human cells. One is Far-UVC, where the radiation is produced by eximer lamps with a wavelength of 222 nm, and the other is the so-called "Blue Light" with a wavelength of 400 to 500 nm, i.e. above the UV wave range, in the visible range of light.
But why does the light in these ranges of the UV spectrum also have a disinfecting effect, but is not harmful to humans? At the wavelength of 222 nm, it is not DNA that is affected, but primarily proteins. The outermost layer of the skin consists mainly of dead skin cells that contain keratin. The radiation is absorbed by the keratin to such an extent that only a very small proportion of the radiation reaches deeper layers of the skin where it could cause damage.
The "blue light", on the other hand, can kill all forms of bacteria, yeast and mould because the microbes contain light-sensitive compounds in their cells. This allows them to absorb the light and use the energy to produce enzymes or proteins and increase their growth and vitality. However, when these photosensitive compounds are now exposed to certain wavelengths of disinfecting blue light and a certain light intensity, a chemical reaction is set in motion in which various types of reactive oxygen radicals (ROS) are formed. These are also called free radicals. The ROS damage everything organic with which they come into contact - and if they are maintained long enough, they also destroy the microbe from within. So here it is not directly the light that is the culprit, but the ROS.
However, experts are not yet in complete agreement about the harm to humans from both technologies, because long-term studies are lacking that could prove their harmlessness.
As with everything - it's the dose that counts
How quickly which organism is rendered harmless depends on the so-called lethal dose, or more simply: How long must one irradiate in order to reach a dose that kills the germ? This in turn depends on the type of lamp used, i.e. the wavelength, the distance to the irradiated object and the duration of irradiation.
For each type of micro-organism, there is a specific lethal dose (LD) when disinfecting with UVC light. This means that there is also no resistance. The LD90, i.e. the dose at which 90 percent of a population (1 log) is killed, serves as a guideline.
Different germs have different susceptibilities. Bacteria and viruses are simple cells whose DNA (RNA) is hardly protected, so they also have a low LD90 of 1 to 6 mJ/cm2 (micro-joule/square centimetre). These include, for example, the influenza virus with a dose of 2.1 mJ/cm2 or the rotavirus with 7.8 mJ/cm2 as well as the bacteria E.coli with 2.5 mJ/cm2 and Bacillus subtilis with 6.8 mJ/cm2 .
Somewhat more resistant are yeasts and vegetative fungal cells, which already have a more complex cell with a nucleus and the DNA is protected in it as chromosomes. With these, the lethal dose is on average 4 to 10 mJ/cm2. One of the best-known yeasts is Candida albicans even with an LD90 of 11 mJ/ccm2, which can cause very unpleasant fungal infections mostly in babies, small children and women.
Moulds, however, have the highest resistance. Black mould (Aspergillus niger) is the most resistant with an LD90 of 98 mJ/cm2. Moulds have a robust cell wall as well as a dense cell volume and their DNA is complexly packed in a cell nucleus. Therefore, the LD90 is also 8 to 100 mJ/cm2.
A study by the University of Tübingen showed how effective disinfection with UVC radiation is against SARS-CoV-2 on surfaces. Even during the rapid movement with the handheld device (zone HP) at a distance of twenty centimetres over the surfaces, which was supposed to simulate a disinfection of common surfaces, no more infected cells could be detected in the sample. The calculations showed that for all irradiated samples an infection reduction of at least 99.9999 percent (6 log) was achieved, which corresponds to complete inactivation.
How long the surface has to be irradiated with an appropriate type of lamp also depends on the distance to the surface. Which dose is reached at which distance can be determined very well with the help of UVC reactive test points and the corresponding dose scale.
Creating clarity
Keeping track of the jungle of UVC technology and possible applications is not so easy. Which comprehensive disinfection options are already available today? Where is the development heading? How and with whom can individual solutions be developed? With these questions, you are welcome to contact the UVIS hygiene experts or browse through our FAQs.
Conclusion
The disinfection of surfaces and objects as well as air or water with UVC light will certainly continue to be available in the future not only to medicine and industry, but also to the general public in public areas. It will no longer be possible to avoid this resource-saving and sustainable variant of disinfection. Where and how the modern technologies will then be used will be shown by the increasing demand for environmentally and climate-friendly alternatives.
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Adapted from UV pro
Published: 4. July 2023