One-Year Ocular Safety Observation of Workers and Estimations of Microorganism Inactivation Efficacy in the Room Irradiated with 222-Nm far Ultraviolet-C Lamps

Interventional human ocular safety experiments for 222-nm far-ultraviolet-C lamp irradiation

The study aimed to directly assess the ocular safety of 222-nm far-ultraviolet-C (UVC) irradiation in humans, given the limited clinical trials in this area. This wavelength offers the potential for safe and effective microbial inactivation in occupied spaces, but its safety profile for human eyes requires thorough investigation. This prospective, interventional study involved five subjects aged 29-47 years, who were exposed to 222-nm UVC at doses of 22, 50, and 75 mJ/cm2. The subjects were monitored using custom-made glasses with a UV-cut filter on one eye to serve as a control. UVC irradiation was conducted using a KrCl excimer lamp, and ocular examinations were performed prior to exposure, 24 h post-exposure, and at 1, 3, and 6 months. Parameters assessed included visual acuity, refractive error, corneal endothelial density, corneal erosion scores, and conjunctival hyperemia scores. The study found no clinically significant photokeratitis or long-term eye damage across the five subjects, even at the highest dose of 75 mJ/cm2. Temporary ocular discomfort, including sensations of dryness and epiphora, was reported, but these symptoms subsided within hours after irradiation. The findings indicate that 222-nm far-UVC irradiation up to 75 mJ/cm2 does not cause "clinically significant photokeratitis" or long-term ocular damage, though it may induce temporary discomfort. This supports the safe use of 222-nm UVC for germicidal applications in occupied environments, providing a basis for revised safety guidelines.

Comparative study of inactivation efficacy of far-UVC (222 nm) and germicidal UVC (254 nm) radiation against virus-laden aerosols of artificial human saliva

Virus-laden aerosols play a substantial role in the spread of numerous infectious diseases, particularly in enclosed indoor settings. Ultraviolet-C (UVC) disinfection is known to be a highly efficient method for disinfecting pathogenic airborne viruses. Recent recommendations suggest using far-UVC radiation (222 nm) emitted by KrCl* (krypton-chloride) excimer lamps to disinfect high-risk public spaces due to lower exposure risks than low-pressure (LP) mercury lamps (254 nm). This study experimentally explored the comparative effectiveness of far-UVC (222 nm) and germicidal UVC (254 nm) in inactivating virus-laden aerosols of different protective vector media in an air disinfection chamber. The UVC inactivation performances of individual filtered KrCl* excimer lamp and LP mercury lamp were determined for inactivating the bacteriophages, MS2 (icosahedral and non-enveloped ssRNA virus) and Phi6 (spherical and enveloped dsRNA virus) aerosolized from artificial human saliva or sodium chloride and magnesium sulfate (SM) buffer as a vector media. Disinfection efficacy of filtered KrCl* excimer lamp (222 nm) and LP mercury lamp (254 nm) were evaluated for highly concentrated viral aerosols, which replicate those exhaled from infected individuals and remain suspended in air or deposited on surfaces as fomites. Our results show that using individual filtered KrCl* excimer lamp (222 nm) and LP mercury lamp (254 nm) could greatly accelerate the inactivation of the viral bioaerosols formed from artificial human saliva and SM buffer. In the case of 222 nm exposure, Phi6 exhibited significantly more susceptibility in artificial human saliva than in SM buffer whereas MS2 showed comparable vulnerability in both artificial human saliva and SM buffer. However, in the case of 254 nm exposure, both Phi6 and MS2 demonstrated significantly greater susceptibility in artificial human saliva than in SM buffer. This study offers valuable insights and improves our understanding of the influence of different vector media on UVC disinfection of exhaled virus-laden aerosols in indoor environments. These findings can guide the deployment of UVC devices which could greatly contribute to mitigating the transmission of exhaled bioaerosols in public settings.

Ocular safety of 222-nm far-ultraviolet-c full-room germicidal irradiation: A 36-month clinical observation

The ocular safety of 222-nm far-ultraviolet-C (UV-C) irradiation, widely recognized for its germicidal properties, was evaluated in a clinical setting to assess its long-term health effects on the human eye. This prospective observational study involved a 36-month follow-up of physicians working in an ophthalmic examination room equipped with 222-nm UV-C lamps. Initially, a 12-month observation showed no signs of acute or chronic ocular damage. To further substantiate these findings, the study period was extended to 36 months, during which four participants underwent regular ocular examinations, including assessments of visual acuity, refractive error, and corneal endothelial cell density. The irradiation dose was meticulously controlled to remain within the previous threshold limit of 22 mJ/cm2 over an 8-h period, as advised by the ACGIH prior to 2022. Results indicated no significant changes in these parameters, suggesting no clinically significant ocular hazards associated with prolonged exposure to 222-nm UV-C irradiation under real-world conditions. Additionally, no delayed side effects, such as pterygium, keratopathies, or cataracts, were observed. Our study supports the safe use of 222-nm UV-C for microbial disinfection in occupied environments and provides a robust foundation for updated safety guidelines.

Experimental study on the effect of light source arrangements on the disinfection performance of upper-room 222 nm Far-UVC

The air disinfection efficacy of upper-room 222 nm Far-UVC was experimentally investigated in a real-size chamber under well-mixed air conditions. Two bacteria (Escherichia coli, Staphylococcus epidermidis) and two bacteriophages (MS2, and P22) were selected for the test. The study considered different lamp source arrangements, including single and double sources, stationary and rotating operating modes, and an overlapping mode with a 45° irradiation angle. A numerical view-factor model was developed to analyze the irradiance distributions. Four irradiation angles, 30°, 45°, 60°, and 90°, were chosen. The results show that the lamps operating with an irradiation angle of 45° provide the highest chamber-averaged irradiance. This suggests an optimal irradiance level for a given room dimension, as inferred from the view factor model. Experimental results indicated that the overlapping mode with a 45° irradiation angle consistently outperformed both the stationary mode and rotating mode in disinfection. This can be attributed to the higher chamber-averaged irradiance, which is also supported by the numerical model predictions. The increment ratios ranged from 14.9 % to 42.9 % compared to the stationary mode. The susceptibility constants of Escherichia coli, Staphylococcus epidermidis, MS2, and P22 were measured as 0.572 m2/J, 0.099 m2/J, 0.060 m2/J, and 0.081 m2/J respectively.

Far-Ultraviolet C Disinfection Reduces Oxidative Damage to the Cornea Compared to Povidone-Iodine Disinfection

Far-ultraviolet C (far-UVC) light shows promise for pathogen control but its safety and efficacy for corneal disinfection remain unclear. In this study, safe far-UVC dosages were investigated for corneal disinfection and its germicidal performance and oxidative damage potential to 5% povidone-iodine (PVP-I) were compared. Rat corneas were exposed to varying 222 nm far-UVC doses (3-60 mJ/cm2) and assessed for ocular damage, apoptosis, and oxidative stress to determine the safe dose of far-UVC. Far-UVC at 30 mJ/cm2 induced corneal apoptosis and oxidative damage, but 15 mJ/cm2 caused no apoptosis or oxidative damage. At this optimized dose (9 mJ/cm2), far-UVC achieved 90.5% sterilization, exceeding 5% PVP-I (80.8%), with significantly less oxidative damage and cell death in the cornea. In conclusion, our study demonstrates that the use of 5% povidone-iodine (PVP-I) for disinfection results in significant oxidative damage to the corneal tissue. However, a safe dosage of far-UVC light exhibited a promising disinfection effect without causing oxidative damage to the corneal tissue. Far-UVC offers a promising alternative for corneal disinfection but requires careful dosage control (≤30 mJ/cm2) to avoid ocular surface harm.

Organic aerosol formation from 222 nm germicidal light: ozone-initiated vs. non-ozone pathways

Germicidal ultraviolet lamps outputting 222 nm light (GUV222) have the potential to reduce the airborne spread of disease through effective inactivation of pathogens, while remaining safe for direct human exposure. However, recent studies have identified these lamps as a source of ozone and other secondary pollutants such as secondary organic aerosol (SOA), and the health effects of these pollutants must be balanced against the benefits of pathogen inactivation. While ozone reactions are likely to account for much of this secondary indoor air pollution, 222 nm light may initiate additional non-ozone chemical processes, including the formation of other oxidants and direct photolytic reactions, which are not as well understood. This work examines the impacts of GUV222 on SOA formation and composition by comparing limonene oxidation under GUV222 and O3-only control conditions in a laboratory chamber. Differences between these experiments enable us to distinguish patterns in aerosol formation driven by ozone chemistry from those driven by other photolytic processes. These experiments also examine the influence of the addition of NO2 and nitrous acid (HONO), and investigate SOA formation in sampled outdoor air. SOA composition and yield vary only slightly with respect to GUV222vs. ozone-only conditions; NO2 and HONO photolysis do not appreciably affect the observed chemistry. In contrast, we observe consistent new particle formation under high-fluence 222 nm light (45 μW cm-2) that differs substantially from ozone-only experiments. This observed new particle formation represents an additional reason to keep GUV222 fluence rates to the lowest effective levels.

Evaluation of an automated far ultraviolet-C light technology for decontamination of surfaces and aerosolized viruses in bathrooms

BACKGROUND

Aerosols generated during toilet flushing are a potential source for transmission of viral and bacterial pathogens in bathrooms. However, manual decontamination of bathrooms after each use is not feasible.

METHODS

We tested the efficacy of a wall-mounted far ultraviolet-C (UV-C) light technology that only delivers far UV-C when people are not present for decontamination of surfaces and aerosolized viral particles in an unoccupied hospital bathroom. A quantitative disk carrier test method was used to test efficacy against organisms on steel disk carriers placed in 9 sites in the bathroom with an exposure time of 45 min and 2 h; Clostridioides difficile spores were also exposed for 24 h. Efficacy against aerosolized bacteriophage MS2 was tested with a 45-minute exposure.

RESULTS

The far UV-C technology reduced methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE), Candida auris, and bacteriophage MS2 on steel disk carriers by ≥ 1.2 log10 (range, 1.2 to 4.2 log10) at all test sites after 2 h of exposure. The technology reduced C. difficile spores by < 1 log10 after 2 h exposure, but 4 of 9 test locations had ≥ 2 log10 reductions after 24 h exposure. Aerosolized bacteriophage MS2 was reduced by 4 log10 plaque-forming units in 45 min.

CONCLUSIONS

The far UV-C light technology could potentially be useful for automated decontamination of air and surfaces in bathrooms in healthcare and community settings.

A far ultraviolet-C light technology is effective for decontamination of items in proximity to sinks and is enhanced by a far UV-C reflective surface

BACKGROUND

Dispersal of gram-negative bacilli from sink drains has been implicated as a source of transmission in multiple outbreaks.

METHODS

In an acute care hospital, we assessed how often patient care supplies and other frequently touched items were within 1 meter of sink drains. We tested the efficacy of a ceiling-mounted far ultraviolet-C (UV-C) light technology for decontamination of sink bowls and surfaces near sinks with and without a wall-mounted film that reflects far UV-C light.

RESULTS

Of 190 sinks assessed, 55 (29%) had patient care supplies or other frequently touched items within 1 meter of the drain. The far UV-C technology reduced Pseudomonas aeruginosa, Enterobacter cloacae and Candida auris on steel disk carriers by ≥1.5 log10 colony-forming units (CFU) in 45 minutes. On inoculated real-world items, ≥1.9 log10 CFU reductions in P. aeruginosa were achieved on sites in line with the light source versus 0.4-1.8 log10 CFU reductions on shaded surfaces. The addition of the reflective surface significantly enhanced efficacy in shaded sites (P < 0.01).

CONCLUSIONS

In a hospital setting, patient care supplies and other frequently touched items were often in proximity to sinks. The far UV-C light technology could potentially be useful for sink decontamination in high-risk areas.

The public-health significance of far-UVC-induced indoor ozone and its associated secondary chemistry

There has been much recent interest in whole-room far-UVC (wavelength around 222 nm) to markedly and safely reduce overall levels of airborne pathogens in occupied indoor locations. Far-UVC light produces very low levels of ozone-in real-world scenarios induced ozone levels of less than 10 ppb, and much less in moderately or well-ventilated rooms compliant with US far-UVC dose recommendations, and very much less in rooms compliant with international far-UVC dose standards. At these very low ozone levels, there is no epidemiological evidence of increased health risks from any of the very large outdoor ozone studies, whether from ozone alone or from ozone plus associated pollutants. Indoors, at the low ozone concentrations of relevance here, ozone does not react rapidly enough with preexisting airborne volatile organic compounds to compete with even extremely low levels of room ventilation, so significant ozone-induced ultrafine particle production is very unlikely. Direct measurements in real-life room scenarios are consistent with these conclusions. A potential exception is the cleaning material limonene, which has an unusually high ozone interaction cross-section; in the far-UVC context, turning off far-UVC lights during cleaning with limonene products would be reasonable.

The Antibacterial Efficacy of Far-UVC Light: A Combined-Method Study Exploring the Effects of Experimental and Bacterial Variables on Dose-Response

Far-ultraviolet C light, with a wavelength of 200-230 nm, has demonstrated broad-spectrum germicidal efficacy. However, due to increased interest in its use as an alternative antimicrobial, further knowledge about its fundamental bactericidal efficacy is required. This study had two objectives. Firstly, it investigated experimentally the Far-UVC dose-response of common bacteria suspended at various cell densities in transparent buffer, ensuring no influence from photosensitive suspending media. Increasing doses of Far-UVC were delivered to Enterococcus faecium, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus in PBS at 101, 102, 103, 105 and 107 CFU·mL-1, with surviving colony-forming units enumerated (n ≥ 3). Secondly, through a systematised literature review, this work sought to explore the impact of genus/species, Gram type, cell form, cell density and irradiance on dose-response. The screening of 483 publications was performed with 25 included in the study. Data for 30 species were collated, analysed and compared with the experimental results. Overall, Gram-positive species showed greater resilience to Far-UVC than Gram-negative; some inter-species and inter-genera differences in resilience were identified; endospores were more resilient than vegetative cells; the results suggested that inactivation efficiency may decrease as cell density increases; and no significant correlation was identified between irradiance and bactericidal dose effect. In conclusion, this study has shown Far-UVC light to be an effective decontamination tool against a vast range of bacterial vegetative cells and endospores.

Microbial contamination of spittoons and germicidal effect of irradiation with krypton chloride excimer lamps (Far UV-C 222 nm)

BACKGROUND

In dentistry, instruments, appliances, and body fluids such as saliva or blood are possible sources of infection. Although conventional antiseptic procedures effectively prevent infection, spittoons cannot be sanitized between each treated patient and are usually washed only with running water. However, there is currently no fast and efficient disinfection method that can be implemented between treatments. An optically filtered krypton chloride excimer lamp using ultraviolet light (Far UV-C) in the 200-230 nm wavelength range (innocuous to humans) has been recently used as a virus- and bacteria-inactivating technology. This study aimed to identify the bioburden of a dental spittoon and examine the susceptibility of two oral Streptococcus and two Enterococci to 222-nm Far UV-C by irradiating the spittoon with 222 nm Far UV-C for 5 min before evaluating the disinfection effect.

METHODS

Bacterial analysis and real-time polymerase-chain reaction testing was used to confirm the spittoon's biological contamination. Bacterial susceptibility to a 222-nm Far UV-C was determined with a graded dose irradiation test. After each treatment, the spittoon was irradiated with 222-nm Far UV-C for 5 min, and the disinfecting effect was evaluated. Microbial analysis of the spittoon's surface was performed using the Silva database.

RESULTS

We found that > 97% of the microbes consisted of six bacterial phyla, whereas no viruses were found. Pseudomonas aeruginosa was frequently detected. The 1-log reduction value of two oral-derived Streptococci and two Enterococci species at 222-nm Far UV-C was 4.5-7.3 mJ/cm2. Exposure of the spittoon to 222-nm Far UV-C at 3.6-13.5 mJ/cm2 significantly decreased bacterial counts (p < 0.001).

CONCLUSIONS

Irradiation with 222-nm Far UV-C at 3.6-13.5 mJ/cm2 significantly eliminates bacteria in spittoons, even when they are only rinsed with water. Hence, 222-nm Far UV-C irradiation may inhibit the risk of bacterial transmission from droplets in sink surfaces.

Far-ultraviolet irradiation at 222 nm destroys and sterilizes the biofilms formed by periodontitis pathogens

BACKGROUND

Periodontal disease is the leading cause of tooth loss, and an association between periodontal disease and non-oral systemic diseases has been shown. Formation of biofilm by periodontal pathogens such as Fusobacterium nucleatum, Porphyromonas gingivalis, and Streptococcus mutans and their resistance to antimicrobial agents are at the root of persistent and chronic bacterial infections.

METHODS

The bactericidal effect of far-ultraviolet (F-UV) light irradiation at 222 nm on periodontal bacteria was assessed qualitatively and quantitatively. The effect of biofilm disruption by F-UV light on periodontal bacteria was examined by crystal violet staining, and the morphologic changes of the biofilm after F-UV irradiation were explored by confocal laser microscopy and scanning electron microscopy. We developed a thin fiber-type 222 nm F-UV irradiator and studied its safety and effect of reducing bacteria in rodent models.

RESULTS

F-UV light at 222 nm had a bactericidal effect on F. nucleatum, P. gingivalis, and S. mutans. Irradiation with F-UV light reduced the biofilm formed by the bacteria and sterilized them from within. Confocal laser microscopy showed a clear reduction in biofilm thickness, and scanning electron microscopy confirmed disintegration of the biofilm architecture. F-UV irradiation was less damaging to DNA and less cytotoxic than deep-ultraviolet light, and it reduced bacterial counts on the tooth surface.

CONCLUSION

F-UV irradiation has the potential to destroy biofilm and act as a bactericide against pathogenic bacteria in the biofilm.

Evaluation of an Automated Wall-mounted Far Ultraviolet-C Light Technology for Continuous or Intermittent Decontamination of Candida auris on Surfaces

Background

Technologies that provides safe and effective decontamination of surfaces and equipment between episodes of manual cleaning could be an important advance in efforts to prevent transmission of the emerging fungal pathogen Candida auris.

Methods

We tested the efficacy of a novel wall-mounted far ultraviolet-C (UV-C) light technology that delivers far UV-C, when people are not detected within the field of illumination, against C. auris isolates from clades I, II, III, and IV using a quantitative disk carrier test method. In an equipment room, we examined the efficacy of the technology in reducing an isolate of C. auris from clade IV inoculated on multiple sites on portable devices.

Results

The far UV-C technology reduced isolates from all 4 clades of C. auris by >3 log10 colony-forming units (CFU) aſter an 8-hour exposure on steel disks. For the clade IV isolate, similar reductions were achieved on glass and plastic carriers. In the equipment room, the technology reduced C. auris inoculated on multiple sites on portable equipment by >2 log10 CFU in 4 hours.

Conclusions

The far UV-C technology could be useful for decontamination of surfaces and equipment between episodes of manual cleaning. Additional studies are needed to evaluate the use of the technology in clinical settings.

Assessing the safety of new germicidal far-UVC technologies

The COVID-19 pandemic underscored the crucial importance of enhanced indoor air quality control measures to mitigate the spread of respiratory pathogens. Far-UVC is a type of germicidal ultraviolet technology, with wavelengths between 200 and 235 nm, that has emerged as a highly promising approach for indoor air disinfection. Due to its enhanced safety compared to conventional 254 nm upper-room germicidal systems, far-UVC allows for whole-room direct exposure of occupied spaces, potentially offering greater efficacy, since the total room air is constantly treated. While current evidence supports using far-UVC systems within existing guidelines, understanding the upper safety limit is critical to maximizing its effectiveness, particularly for the acute phase of a pandemic or epidemic when greater protection may be needed. This review article summarizes the substantial present knowledge on far-UVC safety regarding skin and eye exposure and highlights research priorities to discern the maximum exposure levels that avoid adverse effects. We advocate for comprehensive safety studies that explore potential mechanisms of harm, generate action spectra for crucial biological effects and conduct high-dose, long-term exposure trials. Such rigorous scientific investigation will be key to determining safe and effective levels for far-UVC deployment in indoor environments, contributing significantly to future pandemic preparedness and response.

The new 222-nm far ultraviolet-C lowers bacterial contamination to endoscopists during esophagogastroduodenoscopy

Objectives

This study aimed to clarify the disinfectant efficacy of the 222-nm far ultraviolet-C (UV-C) during esophagogastroduodenoscopy using bacterial cultures.

Methods

The endoscopists performed esophagogastroduodenoscopy wearing a gown with a tryptic soy agar medium plate on their epigastric region and were divided into two groups: 222-nm far UV-C irradiation (UV group) and non-UV irradiation (non-UV group). As a control group, tryptic soy agar medium plates were placed about 110 cm above the floor. The incidence of bacterial contamination was determined by positive bacterial culture. The cultured bacteria were identified by 16S rRNA sequencing. Additionally, the actual UV exposure dose was measured using the UV-indicator card which changed colors upon exposure to 222 nm far UV-C.

Results

The bacterial culture positivity in the UV group (5.03%) was significantly lower than that in the non-UV group (25.76%), p < 0.0001. Most of the bacteria identified in the UV and non-UV groups were normal constituents of the oral flora, including Streptococcus salivarius and Staphylococci. Conversely, pathogenic microbes were found in the control group. The actual exposure doses of 222-nm far UV-C at the endoscopists' face, neck, and epigastric region were 2.09 ± 0.29, 5.89 ± 0.49, and 7.36 ± 0.58 mJ/cm2, respectively.

Conclusions

The 222-nm far UV-C irradiation reduced bacterial contamination for endoscopists. It can be used with conventional physical coverings to provide more effective infection control.

222 nm far-UVC light markedly reduces the level of infectious airborne virus in an occupied room

An emerging intervention for control of airborne-mediated pandemics and epidemics is whole-room far-UVC (200-235 nm). Laboratory studies have shown that 222-nm light inactivates airborne pathogens, potentially without harm to exposed occupants. While encouraging results have been reported in benchtop studies and in room-sized bioaerosol chambers, there is a need for quantitative studies of airborne pathogen reduction in occupied rooms. We quantified far-UVC mediated reduction of aerosolized murine norovirus (MNV) in an occupied mouse-cage cleaning room within an animal-care facility. Benchtop studies suggest that MNV is a conservative surrogate for airborne viruses such as influenza and coronavirus. Using four 222-nm fixtures installed in the ceiling, and staying well within current recommended regulatory limits, far-UVC reduced airborne infectious MNV by 99.8% (95% CI: 98.2-99.9%). Similar to previous room-sized bioaerosol chamber studies on far-UVC efficacy, these results suggest that aerosolized virus susceptibility is significantly higher in room-scale tests than in bench-scale laboratory studies. That said, as opposed to controlled laboratory studies, uncertainties in this study related to airflow patterns, virus residence time, and dose to the collected virus introduce uncertainty into the inactivation estimates. This study is the first to directly demonstrate far-UVC anti-microbial efficacy against airborne pathogens in an occupied indoor location.

Investigation of Far-UVC (222 nm) disinfection of bioaerosols deposited on surfaces with different material properties

Far-ultraviolet C (UVC) light has demonstrated its ability to inactivate microbes on surfaces. However, the factors influencing the efficacy of far-UVC surface disinfection remain unclear. This study aimed to explore the effects of material properties on far-UVC disinfection of bioaerosols (represented by Escherichia coli (E. coli)) deposited on surfaces. The susceptibility constants (Z-values) of E. coli on 14 common materials were measured and analyzed. Additionally, five possible influencing factors (roughness, pores, electrostatic charge, wetness, and temperature) related to surface properties were investigated by control experiments. The results show that far-UVC light effectively disinfected E. coli on the 14 materials, with disinfection efficiencies ranging from 69.1% to 98.9% under a dose of 100.8 J/m2. Surface roughness and electrostatic charges had negligible influence on far-UVC disinfection of E. coli on surfaces. However, for porous materials, pore sizes larger than the E. coli size resulted in lower Z-values. Higher surface wetness decreased both the Z-value and natural decay rate. Meanwhile, a higher surface temperature of 40 °C resulted in a higher Z-value and natural decay rate. The results can improve our understanding of far-UVC disinfection of microbes on surfaces, and the database can be used for numerical models.

Germicidal efficacy of continuous and pulsed ultraviolet-C radiation on pathogen models and SARS-CoV-2

Ultraviolet radiation's germicidal efficacy depends on several parameters, including wavelength, radiant exposure, microbial physiology, biological matrices, and surfaces. In this work, several ultraviolet radiation sources (a low-pressure mercury lamp, a KrCl excimer, and four UV LEDs) emitting continuous or pulsed irradiation were compared. The greatest log reductions in E. coli cells and B. subtilis endospores were 4.1 ± 0.2 (18 mJ cm-2) and 4.5 ± 0.1 (42 mJ cm-2) with continuous 222 nm, respectively. The highest MS2 log reduction observed was 2.7 ± 0.1 (277 nm at 3809 mJ cm-2). Log reductions of SARS-CoV-2 with continuous 222 nm and 277 nm were ≥ 3.4 ± 0.7, with 13.3 mJ cm-2 and 60 mJ cm-2, respectively. There was no statistical difference between continuous and pulsed irradiation (0.83-16.7% [222 nm and 277 nm] or 0.83-20% [280 nm] duty rates) on E. coli inactivation. Pulsed 260 nm radiation (0.5% duty rate) at 260 nm yielded significantly greater log reduction for both bacteria than continuous 260 nm radiation. There was no statistical difference in SARS-CoV-2 inactivation between continuous and pulsed 222 nm UV-C radiation and pulsed 277 nm radiation demonstrated greater germicidal efficacy than continuous 277 nm radiation. Greater radiant exposure for all radiation sources was required to inactivate MS2 bacteriophage. Findings demonstrate that pulsed irradiation could be more useful than continuous UV radiation in human-occupied spaces, but threshold limit values should be respected. Pathogen-specific sensitivities, experimental setup, and quantification methods for determining germicidal efficacy remain important factors when optimizing ultraviolet radiation for surface decontamination or other applications.

Efficacy of a far-ultraviolet-C light technology for continuous decontamination of air and surfaces

A wall-mounted, far-ultraviolet-C light technology reduced aerosolized bacteriophage MS2 by >3 log10 plaque-forming units within 30 minutes. Vegetative bacterial pathogens on steel disk carriers in the center of the room were reduced by >3 log10 after 45 minutes of exposure, but Candida auris and Clostridioides difficile spores were not.

222 nm Far-UVC from filtered Krypton-Chloride excimer lamps does not cause eye irritation when deployed in a simulated office environment

Far-UVC, from filtered Krypton-Chloride lamps, is promising for reducing airborne transmission of disease. While significant research has been undertaken to investigate skin safety of these lamps, less work has been undertaken on eye safety. There is limited data on human eye safety or discomfort from the deployment of this germicidal technology. In this pilot study, immediate and delayed eye discomfort were assessed in a simulated office environment with deployment of Krypton-Chloride lamps, located on the ceiling and directed downwards into the occupied room. Discomfort was assessed immediately postexposure and several days after exposure using validated, Standard Patient Evaluation Eye Dryness (SPEED) and Ocular Surface Disease Index (OSDI) questionnaires. Our results show no significant eye discomfort or adverse effects from the deployment of Far-UVC in this simulated office environment, even when lamps were operated continuously with participants receiving head exposures of up to 50 mJ cm-2 . In addition, a statistically significant reduction in bacteria and fungi of 52% was observed. Far-UVC in this simulated office environment did not cause any clinically significant eye discomfort and was effective at reducing pathogens in the room. These results contribute an important step to further investigation of the interaction of Far-UVC with the human eye.

Continuous surface and air decontamination technologies: Current concepts and controversies

Effective and safe continuous surface and air decontamination technologies could be a useful adjunct to routine cleaning and disinfection in health care settings. Continuously active quaternary ammonium disinfectants that provide residual antimicrobial activity on undisturbed surfaces for up to 24.ßhours have been shown to reduce the recovery of clinically important pathogens in some but not all real-world studies. Although quaternary ammonium-based supplemental coatings have been reported to provide prolonged residual efficacy in patient care settings, there is concern that some of these products may be removed by routine cleaning and disinfection. To address this concern, the Environmental Protection Agency has recently issued updated guidance requiring demonstration of efficacy after multiple abrasion and chemical exposures for registration of supplemental residual antimicrobial coatings. Far-ultraviolet-C and direct irradiation below exposure limits are promising technologies for continuous air and surface decontamination in occupied spaces, but additional studies are needed to evaluate their long-term safety and efficacy. Given the increasing use of electronic air cleaning technologies in community and health care settings, there is a need for studies to assess real-world efficacy and safety.