UVUC

LED UVC Sterilising machines and systems

We are Light years ahead of any other technology available on the market, We supply Cool-tech UVC product range, Cool-tech has been around for years specialising in lighting, LED and UVC prior to the pandemic. We are the sole distributor in the UK for Cool-tech and you will not find our technology or a spec higher than ours on the market anywhere. Majority of products available on the current market are mercury tubes or ozone. Unlike us who only supply the best machines made with top quality parts using LED, we strive to offer the best service and products with tailored solutions throughout the commercial and public sector, supplying NHS, ambulances, food production, police, Schools, large office blocks and many more.

Specialties

- Eco friendly sanatising solutions

- UVC business solutions

- Chemical free LED UVC sterilising machines and systems 

- Affordable and reliable solutions that are proven to work.

Industry

LED UVC Sterilising machines and systems

Company size

1-10

Country

United Kingdom

Website

www.uvuc.co.uk

Phone Number

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Company address

The Three BlackBirds , Sudbury Suffolk CO10 9RR

TV Production Back2Back Testimonial

600w

Cool-Tech UVC

Dr Gary Rowley

02 Nov 2021

 

 

 

Dr Gary Rowley

Associate Professor of Bacterial Pathogenesis & Metabolism

Faculty of Science

School of Biological Sciences

University of East Anglia

Norwich Research Park

Norwich NR4 7TJ

 

United Kingdom

https://people.uea.ac.uk/g_rowley

 

Opinion Report: Antimicrobial Potential of UVC

 

Background

 

Ultraviolet (UV) light covers a wavelength spectrum from 100 to 400 nm and is subdivided into three major regions: UVA (315 to 400 nm), UVB (280 to 315 nm), and UVC (100 to 280 nm) based on ISO- 21348 definitions. Ultraviolet germicidal inactivation (UVGI) was first reported in the mid to late 1800s based on initial observations by Downes and Blunt who discovered that growth of bacterial cultures could be inhibited by the ultraviolet portion of light. These initial findings were subsequently expanded by others including Koch, Ward and Geisler. However perhaps some of the most significant studies were carried out by Gates who in 1929/30 published the first UVC bactericidal action spectra of Staphylococcus aureus and Escherichia coli and pointed to a mechanistic role that involved UVC targeting of DNA rather than proteins (Reed 2010).

Investigations and application into the antimicrobial potential of ultraviolet light stalled in part due to the combined success of chemical based disinfectants and antimicrobials, vaccination programmes and overall drastic improvement in the mortality and morbidity associated with infectious diseases (albeit not for all parts of the globe). Concern regarding the impacts of ultraviolet radiation on human health was also an issue. However, treatment of infectious diseases in the 21st Century raises considerable issues which are not limited to emerging new infections (as highlighted by the SARS-CoV-2 pandemic) but also include the continued increase in multidrug resistant nosocomial and community infections and the lack of novel antimicrobials to treat them. Alongside this the cost and human/environmental toxicity associated with chemical disinfectants has led to a resurgence in UVC based UVGI. The advent of LED based UVC sources mitigates the cost, mercury contamination and ozone production associated with older technologies which were considered significant disadvantages.

Prevention of human to human and environment to human spread of infectious diseases prior to infection and subsequent treatment is desirable. UVC based technologies have been used for disinfection in a range of scenarios and sectors including, but not limited to, air purification, water treatment, surface decontamination, removal of pathogens and spoilage organisms on various fresh produce surfaces. There is also an increasing body of research associated with UVC treatment of human infections such as wounds. This report reviews the underpinning science behind UVC disinfection. Recommendations are made for future experiments that in my opinion should be undertaken to address important development criteria and knowledge gaps to support the R&D and strengthen the roll out and uptake of equipment designed for this purpose.

UVC – activity spectra and mode of action

The UVC region is well known to possess a powerful broad-spectrum germicidal effect capable of inactivating a wide range of microorganisms (bacteria, viruses, protozoa, fungi, yeasts and algae) (Hijnen, Beerendonk et al. 2006). Although UVC is classified as broad spectrum, the efficiency of UVC killing can vary drastically by species, and even strains within a species. Understanding this activity spectrum of any given organism, a measurement of UV inactivation as a function of wavelength for a given dose, is important for validation of UV sources (something that is lacking in most of the marketing material I have reviewed). Generally speaking, Gram negative bacteria (E.g. E. coli, Salmonella) are considered to be more sensitive to UVC than Gram positive bacteria (E.g Staphylococcus aureus), then viruses, fungi and spores are, in that order, increasingly more resistant to killing than bacteria. That said, Adenovirus is recognized as the most UV-resistant waterborne pathogen of public health concern, an enteric virus primarily responsible for respiratory and gastrointestinal illness in humans. A UVC dose of ~40 mJ/cm2 results in ~4-log inactivation of most waterborne viral, bacterial and protozoan pathogens (e.g., Hepatitis A, Salmonella, Cryptosporidium), considerably higher doses 120-200 mJ/cm2 are required to achieve the same level of inactivation of adenoviruses (Woo, Beck et al. 2019). Because of this resistance, the US Environmental Protection Agency (USEPA) has used adenovirus as benchmark, to set UVC dosage requirements.

The 250–280nm wavelengths are considered to be the dominant germicidal region as these wavelengths targets the nucleic acids of microbial cells (Dai, Vrahas et al. 2012); 260 nm is the maximum absorption wavelength for UVC photons by a DNA molecule (Koutchma, Popovic et al.

2016) and the maximum growth inactivation peak of E. coli is reported at 268nm (Von Sonntag and Schuchmann 1992). It is worth making a specific note at this stage: the vast majority of published UVC based inactivation and mechanistic studies have been carried out at 254nm. Primarily because of the use of low-pressure mercury vapour lamps which emit solely at this wavelength, rather than this being the optimum wavelength for UVC based UVGI. There are relatively fewer studies at other wavelengths within the UVGI region which in my opinion is something that needs to be addressed and is a major attraction and advantage for the use of LED based technologies.

Based on studies with E. coli strains (with different DNA repair deficiencies, see below), UVC (254nm), unlike UVA and UVB, is considered to cause cell death solely as a result of the damage to intracellular DNA rather than damage to cell membranes or protein damage, which has been reported for mammalian cells exposed to UVC (Gurzadyan, Gorner et al. 1995). UVC DNA damage is a result of DNA cross linking and the formation of dimers between pyrimidine residues, particularly Thymine, in the nucleic acid strands. There are two main types of dimerization products: 5–6-cis-syn-cyclobutane (CPD) and pyrimidine-6–4 pyrimidones (6-4PP). The consequence of the formation of pyrimidine dimers is altered structure of the DNA molecule, halting DNA replication and transcription and subsequently reproduction (Douki, Court et al. 2000). DNA damage is therefore without question the primary biological effect caused by UVC in microbial cells, however photochemical reactions in proteins can also occur near 280 nm or below 240 nm (Harm 1980). Absorption of UV light by proteins may be significant for large microorganisms like Fungi, Protozoa, and Algae (Kalisvaart 2001), and recent research suggests it may also be important for Viruses (Eischeid and Linden 2011). This may also be proven to be the case for bacteria at the correct wavelength and when tested under appropriate parameters. However, much of the data that is referenced on this point is dated, and again is based on 254nm experiments. These observations should be revisited to ensure that secondary mechanisms, as seen with UVA and UVB, are not also involved with UVC; particularly at wavelengths elsewhere in the UVC region that might be achieved with LED sources.

The selected wavelength is also important when considering the point of developing resistance to UVC. There are some concerns regarding the photoreactivation or resistance of bacteria to repeat cycling of suboptimal UVC doses, as might for example occur with air purification systems. The DNA

pyrimidine dimers caused by direct absorption of UVC photons can be repaired leading to the reactivation of bacterial cells. The main two mechanisms are photoreactivation by visible light which requires the enzyme photolyase, or the dark repair mechanism (aka nucleotide excision repair mechanism, NER, SOS response) (Oguma, Katayama et al. 2002) by the UvrABC protein complex (Verschooten, Claerhout et al. 2006, Sánchez-Navarrete, Ruiz-Pérez et al. 2020). Alcantara-Diaz et al. investigated the adaptation of E. coli (strain PQ30) to 80 repeated cycles of UVC irradiation at 254 nm (Alcantara-Diaz, Brena-Valle et al. 2004). Cultures were found to give rise to different degrees of UVC resistance as a consequence of mutations arising in genes related to repair and replication of DNA. A single study in fungi undergoing a similar, but not as intensive cyclical regime, did not observe any selection for resistance (Dai, Tegos et al. 2008). Whether this UVC resistance phenomenon is species specific requires further experimentation but in summary, excessive repetition of UVC irradiation may induce resistance of microorganisms to UVC.

Synergy of UVC wavelengths – inactivation efficiency and photoreactivation inhibition

There is some evidence that combining UV light treatment of different wavelengths has enhanced microbial inactivation by repressing the repair of damaged DNA (Poepping, Beck et al. 2014). When considering the source of UVC, direct comparisons of UVGI between single and dual wavelength LEDs, versus traditional low and medium pressure mercury based vapour lamps have been made. In one such study from Beck et al, 2017, the energy efficiency and inactivation potentials of single and dual wavelength LEDs at 260nm, 280nm and 260/280nm were directly compared with low and medium pressure mercury vapour lamps for UVGI of water samples (Beck, Ryu et al. 2017). When solely focusing on inactivation of microorganisms they found that killing of E. coli was similar across all UVC sources with 260 and 280nm LEDs causing a 3-log reduction in CFUs at UV doses of 12mJ/cm2. At higher UV doses some tailing off of inactivation is observed and there was no synergy between 260 and 280nm LEDs. For other organisms tested, the 260nm LED was the most effective against MS2 coliphages and medium pressure vapour lamps most effective for killing Human Adenovirus and Bacillus pumilus spores. Synergy of two different UVC wavelengths have been reported elsewhere, with the simultaneous application of combined 259/289‐nm UV‐LED treatment, at a UV dose of 14 mJ/cm2, synergistically reducing E. coli and Listeria recovery (Green, Popović et al. 2018). This same study demonstrated that UVC-LEDs emitting at approximately 280 nm exhibit the greatest trade-off between germicidal efficacy, power output and cost.

However, it is not a case of considering inactivation alone when looking at synergy of multiple wavelengths, and perhaps given the mechanisms of inactivation one would not expect significant synergy at this level. The synergy is however an attractive concept when considering development of resistance to UVC, without being detrimental to overall inactivation spectra. As discussed above, microorganisms, particularly bacteria, can reactivate via repairing their DNA damage after exposure to UV light. To counteract the destructive effects of DNA lesions, a mechanism of DNA repair is mainly photoreactivation and excision repair (dark repair) taking place within the microbial cells (Rastogi, Richa et al. 2010). This phenomenon is considered one of the significant drawbacks of UV treatment as it decreases its efficiency; this is where wavelength synergy may be advantageous. The repair systems are dependent on enzymes within the cell, so proteins, and synergy between wavelengths that damage the DNA as well as the proteins that stop the photoreactivation or DNA repair would go a long way to reduce resistance and should be tested with key pathogens. Photoreactivation appears the most important mechanism of reactivation based on UV‐LED treatment of E. coli in water. After processing with different dual combinations of wavelengths, UVC/UVC‐LED (267/275 nm) treatment, the photoreactivation was significantly repressed compared to those treated with 267‐nm UV‐LED alone. This might be due to a crucial role of the wavelength of 275 nm in repressing photoreactivation through protein damage (Nyangaresi, Qin et al. 2018).

Parameter Considerations

SARS-CoV-2 is highly susceptible to irradiation with ultraviolet light. High viral loads of SARS-CoV-2 can be inactivated in 9 minutes by UVC irradiation (Heilingloh, Aufderhorst et al. 2020) and the pandemic reinvigorated UVC based technologies for room and air sterilisation to reduce viral load. However, in my opinion there is a significantly broader and brighter future for UVC disinfection beyond COVID-19. The most attractive advantages of UV-based antimicrobial therapies lie in their ability to eradicate most microbes in the absence of costly and hazardous chemicals regardless of antibiotic resistance. A major concern with regards to health care associated infection are the spread of drug resistant pathogens. Studies examining the UVC inactivation of antibiotic-resistant bacteria have found them to be as equally susceptible as their naive counterparts. In a side by side comparison measuring UVC killing of antibiotic resistant and sensitive isolates of Staphylococcus aureus and Enterococcus faecalis, the Enteroccci were significantly more susceptible to killing than

S. aureus (Conner-Kerr, Sullivan et al. 1998). However, in a more recent study, some multidrug resistant pathogens including Salmonella were more resistant to UVC 254nm than the antibiotic sensitive parents (Narita, Asano et al. 2020). This may be the difference between Gram positive and Gram negative bacteria, and these experiments need repeating with a variety of drug sensitive and resistant Salmonella strains and the associated mechanisms behind this, if replicated, need resolving.

Sensitivity to UV disinfection can vary for a certain species of microorganism according to strain, growth medium, growth phase of the organism (exponential phase, stationary phase or biofilm), and influences of medium on repair of sublethal damage (Chang, Ossoff et al. 1985). These variances should be considered when making specific recommendations for individual sectors, based on the types of microorganisms that are most likely to be encountered and the media that they exist on or in. The physiological state of the bacterial cultures used in the majority of studies I have reviewed, rely on liquid bacterial cultures in a vegetative state, which are more sensitive to killing than spores. These studies also do not address the state of microorganisms in environments such as biofilms which are notoriously more difficult to remove from surfaces. Biofilms are communities of bacteria protected by a polysaccharide matrix that allows them to attach to each other and surfaces extremely well and are often more resistant to killing by antibiotics and chemical disinfectants (Costerton, Lewandowski et al. 1995). Studies using LED generated UVC to reduce biofilms are few and far between, however, based on these it would suggest that disinfection of surfaces where biofilms are present must incorporate a multimodal approach, as bacteria deeper in the biofilm are protected against the UV rays, and biofilms can endure high levels of ultraviolet light (Elasri and Miller 1999). Therefore, UV-C must be part of a process that first includes the physical removal of biofilm. However, preventing biofilms is easier than their removal – so inline UVC LEDs in areas that are notorious for biofilms to establish would certainly be attractive such as drainage systems, tanks (dishwashers). Medical equipment and high touch surfaces, like personal electronics, medical carts, and diagnostic equipment would also come under this umbrella.

Summary & Key Messages

Use of LEDs as a source of UVC based UVGI has many advantages from an engineering stance. LEDs are compact, shock-resistant, have no warm up time, are user friendly for operators and require relatively lower energy and have longer life span than lamps (Banas, Crawford et al. 2005). However, from a biological stand point they also have a major advantage over monochromatic mercury lamps including the ability to precisely dial the wave-length emitted which allows the potential benefits of coupling dual wavelengths. Ideally, a tailored UV disinfection system would target bacteria and viruses by combining a wavelength from the dominant germicidal region (250 nm - 280 nm) with a wavelength from the polypeptide absorbance region below 240 nm (i.e. 220 - 230 nm). Proteins also absorb at 280nm which could be factored into future experiments as there is little in the literature at this wavelength alone or in synergy.

The vast majority of data in the literature, and current dogma for UVC antimicrobial potential, is based on experiments conducted with mercury lamps at a single 254nm wavelength. Studies directly comparing, monochromatic and polychromatic lamps against LEDs are limited. LED applications, with multiple synergistic wavelengths are the future for UVC based UVGI and a thorough analysis of the application of LED UVC across the wavelength spectra will determine how LED UV-C can be the most effective germicidal source in a given project. In support of this statement it would be beneficial to have key datasets derived for application scenarios as these should impact the specification and design of UV-C-based sterilization and disinfection products. Currently, complete action spectra for many pathogens are lacking and such information could increase the inactivation efficacy of UV-LEDs. For example spectra for foodborne pathogens and spoilage organisms could lead to improved uptake in the food processing industry. As an example, UV-LEDs at approximately 280 nm were found to be the best choice in terms of bacterial inactivation for 3 foodborne pathogens (Salmonella, Listeria and E.coli ) compared to a mercury lamp (Green, Popović et al. 2018). This requires further investigation to find conditions for UV disinfection without impacting on food quality (Gayan, Serrano et al. 2012).

This report deliberately focuses on the biological aspects of UVC based UVGI and steers away from the engineering parameters raised in the literature such as shadowed areas, distance to source,

reflection and surface penetration. Although in my opinion there is no question for the future potential of LED UVC based UVGI, the review of the literature revealed that many studies lack specific information on dosimetry, microbial quality, or experimental details that would allow comparative analysis of UVC data. Perhaps this is because these are studies carried out from an engineering stance. Surrogate organisms are often used in place of real pathogens in a number of studies, as they are easier to handle in the laboratory, which is also problematic. From my experience with Salmonella, real pathogens tend to be more resistant to external stress and have different resistance mechanisms when compared to more inert lab strains. As there is such enormous variation in study design and information reporting, extrapolating data from one study to the next is therefore extremely challenging and lends itself to cherry picking of studies to suit needs. This is perhaps why the same references and quotes get repeatedly utilised in marketing material.

In my opinion, LED system specific parameters therefore need to be experimentally validated against representative strains of the most likely microbes encountered, in the correct media (e.g. surface, water, or air) and also take into consideration the potential of drug resistant strains and biofilm formation. These studies should explicitly state UVC dosimetry and distance from source and should be designed and carried out in collaboration between engineers and microbiologists, and not one without the other. It is hoped that future studies will include as many of these details as is reasonable within experimental constraints, in order that the results may be of use to a wide audience, and is something I would recommend to anyone that is advancing their R&D in this area.

It is clear that more research is necessary to evaluate the potential of LED units emitting at multiple UV wavelengths in the germicidal range. This should occur against a variety of pathogens under appropriate growth parameters to optimize pathogen inactivation and understand potential in reducing resistance. Furthermore, future whole genome based omics studies are encouraged to unveil the global cellular mechanisms behind the relationship between stress responses and increased UV-C resistance and wavelengths outside of 254nm. Empirical data from such experiments would support any LED based product design and commercial strategy.

Finally, short wavelength UVC at 222nm is absorbed by proteins in the membrane and cytosol, and fails to reach the nucleus of human cells. UVC 222nm inactivates a wide spectrum of microbial pathogens without the associated risks to human health (Narita, Asano et al. 2020) and are also effective against 2 tested human coronaviruses (Buonanno, Welch et al. 2020). If LEDs with wavelengths in the 222nm range could be developed – which would be extremely attractive - a revisit to all datasets would be required.

References

Alcantara-Diaz, D., M. Brena-Valle and J. Serment-Guerrero (2004). "Divergent adaptation of Escherichia coli to cyclic ultraviolet light exposures." Mutagenesis 19(5): 349-354.

Banas, M. A., M. H. Crawford, D. S. Ruby, M. P. Ross, J. S. Nelson, A. A. Allerman and R.

Boucher (2005). Final LDRD report: ultraviolet water purification systems for rural environments and mobile applications, Sandia National Laboratories.

Beck, S. E., H. Ryu, L. A. Boczek, J. L. Cashdollar, K. M. Jeanis, J. S. Rosenblum, O. R. Lawal and K. G. Linden (2017). "Evaluating UV-C LED disinfection performance and investigating potential dual-wavelength synergy." Water Res 109: 207-216.

Buonanno, M., D. Welch, I. Shuryak and D. J. Brenner (2020). "Far-UVC light (222 nm) efficiently and safely inactivates airborne human coronaviruses." Sci Rep 10(1): 10285.

Chang, J. C., S. F. Ossoff, D. C. Lobe, M. H. Dorfman, C. M. Dumais, R. G. Qualls and J. D. Johnson (1985). "UV inactivation of pathogenic and indicator microorganisms." Appl Environ Microbiol 49(6): 1361-1365.

Chevremont, A. C., A. M. Farnet, B. Coulomb and J. L. Boudenne (2012). "Effect of coupled UV-A and UV-C LEDs on both microbiological and chemical pollution of urban wastewaters." Sci Total Environ 426: 304-310.

Conner-Kerr, T. A., P. K. Sullivan, J. Gaillard, M. E. Franklin and R. M. Jones (1998). "The effects of ultraviolet radiation on antibiotic-resistant bacteria in vitro." Ostomy Wound Manage 44(10): 50-56.

Coohill, T. P. and J. L. Sagripanti (2008). "Overview of the inactivation by 254 nm ultraviolet radiation of bacteria with particular relevance to biodefense." Photochem Photobiol 84(5): 1084-1090.

Costerton, J. W., Z. Lewandowski, D. E. Caldwell, D. R. Korber and H. M. Lappin-Scott (1995). "Microbial biofilms." Annu Rev Microbiol 49: 711-745.

Dai, T., G. P. Tegos, G. Rolz-Cruz, W. E. Cumbie and M. R. Hamblin (2008). "Ultraviolet C inactivation of dermatophytes: implications for treatment of onychomycosis." Br J Dermatol 158(6): 1239-1246.

Dai, T., M. S. Vrahas, C. K. Murray and M. R. Hamblin (2012). "Ultraviolet C irradiation: an alternative antimicrobial approach to localized infections?" Expert Rev Anti Infect Ther 10(2): 185-195.

Douki, T., M. Court, S. Sauvaigo, F. Odin and J. Cadet (2000). "Formation of the main UV-induced thymine dimeric lesions within isolated and cellular DNA as measured by high performance liquid chromatography-tandem mass spectrometry." Journal of Biological Chemistry 275(16): 11678-11685.

Eischeid, A. C. and K. G. Linden (2011). "Molecular indications of protein damage in adenoviruses after UV disinfection." Applied and Environmental Microbiology 77(3): 1145-1147.

Elasri, M. O. and R. V. Miller (1999). "Study of the response of a biofilm bacterial community to UV radiation." Applied and environmental microbiology 65(5): 2025-2031.

Gayan, E., M. J. Serrano, J. Raso, I. Alvarez and S. Condon (2012). "Inactivation of Salmonella enterica by UV-C light alone and in combination with mild temperatures." Appl Environ Microbiol 78(23): 8353-8361.

Green, A., V. Popović, J. Pierscianowski, M. Biancaniello, K. Warriner and T. Koutchma (2018). "Inactivation of Escherichia coli, Listeria and Salmonella by single and multiple wavelength ultraviolet-light emitting diodes." Innovative Food Science & Emerging Technologies 47: 353-361.

Gurzadyan, G. G., H. Gorner and D. Schulte-Frohlinde (1995). "Ultraviolet (193, 216 and 254 nm) photoinactivation of Escherichia coli strains with different repair deficiencies." Radiat Res 141(3): 244-251.

Harm, W. (1980). "Biological effects of ultraviolet radiation."

Heilingloh, C. S., U. W. Aufderhorst, L. Schipper, U. Dittmer, O. Witzke, D. Yang, X. Zheng, K. Sutter, M. Trilling, M. Alt, E. Steinmann and A. Krawczyk (2020). "Susceptibility of SARS-CoV-2 to UV irradiation." Am J Infect Control 48(10): 1273-1275.

Hijnen, W. A., E. F. Beerendonk and G. J. Medema (2006). "Inactivation credit of UV radiation for viruses, bacteria and protozoan (oo)cysts in water: a review." Water Res 40(1): 3-22.

Kalisvaart, B. F. (2001). "Photobiological effects of polychromatic medium pressure UV lamps." Water science and technology 43(4): 191-197.

Koutchma, T., V. Popovic, V. Ros-Polski and A. Popielarz (2016). "Effects of Ultraviolet Light

and High-Pressure Processing on Quality and Health-Related Constituents of Fresh Juice Products." Compr Rev Food Sci Food Saf 15(5): 844-867.

Lim, W. and M. A. Harrison (2016). "Effectiveness of UV light as a means to reduce Salmonella contamination on tomatoes and food contact surfaces." Food Control 66: 166-173.

Narita, K., K. Asano, K. Naito, H. Ohashi, M. Sasaki, Y. Morimoto, T. Igarashi and A. Nakane (2020). "222-nm UVC inactivates a wide spectrum of microbial pathogens." J Hosp Infect.

Nyangaresi, P. O., Y. Qin, G. Chen, B. Zhang, Y. Lu and L. Shen (2018). "Effects of single and combined UV-LEDs on inactivation and subsequent reactivation of E. coli in water disinfection." Water Res 147: 331-341.

Oguma, K., H. Katayama and S. Ohgaki (2002). "Photoreactivation of Escherichia coli after low- or medium-pressure UV disinfection determined by an endonuclease sensitive site assay." Appl Environ Microbiol 68(12): 6029-6035.

Poepping, C., S. E. Beck, H. Wright and K. G. Linden (2014). "Evaluation of DNA damage reversal during medium-pressure UV disinfection." Water Res 56: 181-189.

Rastogi, R. P., Richa, A. Kumar, M. B. Tyagi and R. P. Sinha (2010). "Molecular mechanisms of ultraviolet radiation-induced DNA damage and repair." J Nucleic Acids 2010: 592980.

Reed, N. G. (2010). "The history of ultraviolet germicidal irradiation for air disinfection." Public Health Rep 125(1): 15-27.

Sánchez-Navarrete, J., N. J. Ruiz-Pérez, A. Guerra-Trejo and J. D. Toscano-Garibay (2020). "Simplified modeling of E. coli mortality after genome damage induced by UV-C light exposure." Scientific reports 10(1): 1-15.

Verschooten, L., S. Claerhout, A. Van Laethem, P. Agostinis and M. Garmyn (2006). "New strategies of photoprotection." Photochemistry and photobiology 82(4): 1016-1023.

Von Sonntag, C. and H.-P. Schuchmann (1992). "UV disinfection of drinking water and by-product formation- some basic considerations." Aqua- Journal of Water Supply: Research and Technology 41(2): 67-74.

Woo, H., S. E. Beck, L. A. Boczek, K. Carlson, N. E. Brinkman, K. G. Linden, O. R. Lawal, S. L. Hayes and H. Ryu (2019). "Efficacy of inactivation of human enteroviruses by dual-wavelength germicidal ultraviolet (UV-C) light emitting diodes (LEDs)." Water (Basel) 11(6): 1-1131.

 

 

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