Photochemistry and Photobiology

Direct contact and airborne spread are main mechanisms of transmission for SARS-CoV-2, and virus can stay viable for at least 3 hours in aerosols. Initus-V system uses a Far-ultraviolet C (UVC) system, UVC resistant textile and googles to provide virus, bacteria and spore free environments in hospitals, crowded public places and travel environments. Initus-V system may help in prevention of epidemic diseases such as Coronavirus disease-19 (Covid-19), influenza, treatment of airborne viral diseases and spread of hospital-borne resistant infections.

The role of ultraviolet radiation (UVR) exposure in the pathology of age-related macular degeneration (AMD) has been debated for decades with epidemiological evidence failing to find a clear consensus for or against it playing a role. A key reason for this is a lack of foundational research into the response of living retinal tissue to UVR in regard to AMD-specific parameters of tissue function. We therefore explored the response of cultured retinal pigmented epithelium (RPE), the loss of which heralds advanced AMD, to specific wavelengths of UVR across the UV-B and UV-A bands found in natural sunlight. Using a bespoke in vitro UVR exposure apparatus coupled with bandpass filters we exposed the immortalised RPE cell line, ARPE-19, to 10nm bands of UVR between 290 and 405nm. Physical cell dynamics were assessed during exposure in cells cultured upon specialist electrode culture plates which allow for continuous, non-invasive electrostatic interrogation of key cell parameters during exposure such as monolayer coverage and tight-junction integrity. UVR exposures were also utilised to quantify wavelength-specific effects using a rapid cell viability assay and a phenotypic profiling assay which was leveraged to simultaneously quantify intracellular reactive oxygen species (ROS), nuclear morphology, mitochondrial stress, epithelial integrity and cell viability as part of a phenotypic profiling approach to quantifying the effects of UVR. Electrical impedance assessment revealed unforeseen detrimental effects of UV-A, beginning at 350nm, alongside previously demonstrated UV-B impacts. Cell viability analysis also highlighted increased effects at 350nm as well as 380nm. Effects at 350nm were further substantiated by high content image analysis which highlighted increased mitochondrial dysfunction and oxidative stress. We conclude that ARPE-19 cells exhibit a previously uncharacterised sensitivity to UV-A radiation, specifically at 350nm and somewhat less at 380nm. If upheld in vivo, such sensitivity will have impacts upon geoepidemiological risk scoring of AMD.

Efforts are underway to develop countermeasures to prevent the environmental spread of COVID-19 pandemic caused by SARS-CoV-2. Physical decontamination methods like Ultraviolet radiation has shown to be promising. Here, we describe a novel device emitting ultraviolet C radiation (UVC), called NuvaWave, to rapidly and efficiently inactivate SARS-CoV-2. SARS-CoV-2 was dried on a chambered glass slides and introduced in a NuvaWave robotic testing unit. The robot simulated waving NuvaWave over the virus at a pre-determined UVC radiation dose of 1, 2, 4 and 8 seconds. Post-UVC exposure, virus was recovered and titered by plaque assay in Vero E6 cells. We observed that relative control (no UVC exposure), exposure of the virus to UVC for one or two seconds resulted in a >2.9 and 3.8 log10 reduction in viral titers, respectively. Exposure of the virus to UVC for four or eight seconds resulted in a reduction of greater than 4.7-log10 reduction in viral titers. The NuvaWave device inactivates SARS-CoV-2 on surfaces to below the limit of detection within one to four seconds of UVC irradiation. This device can be deployed to rapidly disinfect surfaces from SARS-CoV-2, and to assist in mitigating its spread in a variety of settings.

PurposeTo evaluate the retinal safety of repeated low-level red-light (RLRL) therapy using the Eyerising Myopia Management Device (EMMD) by analysing exposure parameters relative to established thermal and photochemical retinal injury thresholds and empirical human exposure data. MethodsEmission characteristics of the EMMD were measured in an accredited laboratory under worst-case conditions. Parameters assessed included wavelength, intraocular power, corneal irradiance, and retinal image characteristics across accommodative states. These measurements were compared with international safety standards, maximum permissible exposure limits, and experimentally derived retinal injury thresholds from animal studies and validated computational models. The effects of repeated exposures from RLRL therapy using the EMMD were evaluated using photochemical additivity principles and repair kinetics, and further contextualised using human volunteer exposure data. ResultsThe EMMD emitted red laser radiation at 654-655 nm with a maximum intraocular power of approximately 1 mW through a 7 mm pupil, placing it within Class 3R and marginally above the Class 2 limit. Corneal irradiance was approximately 26 W m- 2, well below conservative photochemical exposure limits. Thermal injury modelling indicated retinal damage thresholds above device exposure, including under worst-case assumptions of minimal retinal image size and absence of eye movements. Accounting for repeated daily exposures and photochemical additivity, safety margins remained approximately 3-fold for a 7 mm pupil and approximately 8-fold for a more realistic 4 mm pupil. Human volunteer studies demonstrated no detectable structural or functional retinal injury at exposure levels approximately five times higher than those produced by the EMMD. ConclusionExposure parameters of RLRL therapy using the EMMD remain well below conservative retinal injury thresholds under prescribed use conditions. Integration of experimental, modelling, and human data indicates substantial safety margins, supporting its safe clinical use.

BackgroundThe SARS-CoV-2 pandemic demonstrated the vulnerability of our societies to aerosol transmitted pathogens. With no less than 260mio known cases and > 5mio deaths, SARS-CoV-2 is a global catastrophe leading to human and economic losses unprecedented in recent history. Thus, effective methods to limit the spread of aerosol transmitted pathogens are needed. Universal masking and curfew laws are effective but no permanent solution. MethodsA mass producible LED light source emitting homogeneous parallel UV-C light was used as a "light-barrier" to block the spread of infectious aerosols. In an aerosol test channel, Gram-negative and Gram-positive bacteria as well as coronavirus were nebulized and inactivation rates were determined. FindingsWith air speeds of 0.1 m s-1 an exposure time of 1 s in the UV-C light is obtained. Reduction in CFU for E. coli was >3log10 and for S. aureus [~]2.8log10. Plug-forming-units of the murine coronavirus (Mouse Hepatitis Virus, MHV) were reduced by about 3log10. InterpretationThe concept of a UV-C light barrier to ward off infectious aerosols if feasible and possible with a light element as described here. Coupled with sensor based activation/deactivation, such a technology could greatly reduce the transmission rates of aerosol transmitted pathogens while not disturbing natural human behaviour. This is an interesting technology allowing a "new normal" in societies after/with SARS-CoV-2.

For more than 100 years, germicidal lamps emitting 254 nm ultraviolet (UV) radiation have been used for drinking-water disinfection and surface sterilization. However, due to the carcinogenic nature of 254 nm UV, these lamps have been unable to be used for clinical procedures such as wound or surgical site sterilization. Recently, technical advances have facilitated a new generation of germicidal lamp whose emissions centre at 222 nm. These novel 222 nm lamps have commensurate antimicrobial properties to 254 nm lamps while producing few short- or long-term health effects in humans upon external skin exposure. However, to realize the full clinical potential of 222 nm UV, its safety upon internal tissue exposure must also be considered. Type I collagen is the most abundant structural protein in the body, where it self-assembles into fibrils which play a crucial role in connective tissue structure and function. In this work, we investigate the effect of 222 nm UV radiation on type I collagen fibrils in vitro. We show that collagens response to irradiation with 222 nm UV is fluence-dependent, ranging from no detectable fibril damage at low fluences to complete fibril degradation and polypeptide chain scission at high fluences. However, we also show that fibril degradation is significantly attenuated by increasing collagen sample thickness. Given the low fluence threshold for bacterial inactivation and the macroscopic thickness of collagenous tissues in vivo, our results suggest a range of 222 nm UV fluences which may inactivate pathogenic bacteria without causing significant damage to fibrillar collagen. This presents an initial step toward the validation of 222 nm UV radiation for internal tissue disinfection.

Hospital-acquired infections are a known and growing problem worldwide. Far-UVC is a novel disinfection method that inactivates bacteria with limited penetration into human skin or eyes. A clustered, unmatched, randomized control trial (RCT) will be implemented in two Bolivian hospitals. The intervention arm will receive functioning Far-UVC lamps, whereas the control arm will receive identical lamps that do not emit UV light (shams). Based on baseline data, 40 lamp fixtures will be installed above hospital sinks, 10 per arm per hospital. Environmental samples (air and surface swabs) will be collected and analyzed via culture and sequencing. Simultaneously, air chemical monitoring data will be collected.

The advent of mass-market Light Emitting Diodes (LEDs) has seen considerable interest in potential dermatological applications of LED light photobiomodulation (PBM) for a range of conditions, with a thriving market for direct-to-consumer LED treatments, including red light, blue light, and yellow light wavelengths. Evidence of efficacy for many conditions is however decidedly mixed, with starkly different outcomes reported by different authors. Due to the wide range of irradiances and wavelengths used, interpretation, comparison, and even efficacy evaluation is often impossible or prohibitive, impeding evidence synthesis. This work establishes a framework for objectively cross-comparing patient dose in terms of fluence, and a model for contrasting received dose to typical solar dose at ground level to facilitate interpretation of results and evidence synthesis. This allowes direct cross-comparison of patient skin fluence from LED PMB treatments under different regimes, and a means for evidence synthesis. This was applied to LED PMB data from 27 clinical trials to examine fluences and patient-equivalent solar exposure from LED light-sources for dermatological conditions, including acne vulgaris, wrinkle-reduction, wound-healing, psoriasis severity, and erythemal index. The results of this analysis suggest that fluences, wavelengths, and solar exposure equivalent differed by orders of magnitude in he studies analysed, with effective doses often comparable to typical daily solar exposure. Better dose quantification and plausible biological justification for various wavelengths and fluences are imperative if LED therapy studies for dermatology are to be informative and research replicability improved.

Germicidal UV devices offer a promising solution to mitigate surface-mediated pathogen transmission, providing effective disinfection without material corrosion. This study evaluated the surface inactivation kinetics of two bacteria and two bacteriophages using a low-pressure (LP) mercury UV lamp (254 nm) and a filtered krypton chloride (KrCl*) excimer lamp (222 nm). Three deposition methods (Spray, Spread, and Pipette) and two extraction methods (Swab and Elute) were compared. The UV dose response on surfaces followed a two-region non-linear model due to shielding from dried deposition constituents, primarily through UV absorption. KrCl* excimer exhibited similar bacterial inactivation but slightly lower viral inactivation than LP UV lamp (maximum inactivation [~] 1 log lower), but its safety profile makes it compelling in occupied spaces. Compared to aqueous conditions, bacteria were more UV sensitive on surfaces, whereas viruses were more resistant. The deposition methods affected the inactivation results, with the Spray method resulting in higher bacteria inactivation. While the extraction methods had limited effect on inactivation efficacy, the Swab method provided higher inactivation detection limits ([~] 2 log higher) and more consistent extraction efficiency. This study provides mechanistic insights into the effects of deposition conditions, UV wavelengths, and microbial characteristics on UV surface disinfection and contributes to standardization of testing methods. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=104 SRC="FIGDIR/small/734141v1_ufig1.gif" ALT="Figure 1"> View larger version (24K): org.highwire.dtl.DTLVardef@11db511org.highwire.dtl.DTLVardef@15aa3faorg.highwire.dtl.DTLVardef@1c39ac9org.highwire.dtl.DTLVardef@e726ed_HPS_FORMAT_FIGEXP M_FIG C_FIG

AbstractO_ST_ABSAimsC_ST_ABSThe dosages and efficacy of 14 ultraviolet (UV) decontamination technologies were measured against a SARS-CoV-2 surrogate virus that was dried on to different materials for lab and field testing. Methods and ResultsA live enveloped, ribonucleic acid (RNA) virus surrogate for SARS- CoV-2 was dried on stainless steel 304 (SS304), Navy Top Coat-painted SS304 (NTC), cardboard, polyurethane, polymethyl methacrylate (PMMA), and acrylonitrile butadiene styrene (ABS) at > 8.0 log10 plaque-forming units (PFU) per test coupon. The coupons were then exposed to UV radiation during both lab and field testing. Commercial and prototype UV- emitting devices were measured for efficacy: 4 handheld devices, 3 room/surface-disinfecting machines, 5 air-disinfection devices, and 2 larger custom-made machines. UV device dosages ranged from 0.01-729 mJ cm-2. Anti-viral efficacy among the different UV devices ranged from no decontamination up to nearly achieving sterilization. Importantly, cardboard required far more dosage than SS304. ConclusionsEnormous variability in dosage and efficacy was measured among the different UV devices. Porous materials limit the utility of UV decontamination. Significance and Impact of the StudyUV devices have wide variability in dosages, efficacy, hazards, and UV output over time indicating that each UV device needs independent technical measurement and assessment for product development, prior to, and during use.

Behavioral and medical control measures are not effective in containing the spread of SARS-CoV-2. Here we report on the effectiveness of a preemptive environmental strategy using UV-C light to prevent airborne transmission of the virus in a hamster model and show that UV-C exposure completely prevents airborne transmission between individuals

Photodynamic therapy (PDT) is an emerging antimicrobial strategy that uses light to activate photosensitizers, generating reactive oxygen species (ROS) that induce microbial cell death. This study investigates the antimicrobial efficacy of curcumin-mediated PDT against the microbial pathogens Escherichia coli, Pseudomonas fluorescens, and Candida auris. The minimum inhibitory concentration (MIC) for each microorganism is determined at different light fluence rates within a clinically relevant range. Our results demonstrate that curcumin-mediated PDT effectively inactivates E. coli and C. auris, while exhibiting only partial inhibition against P. fluorescens at curcumin concentrations up to 60 {micro}M. Notably, within the tested fluence rate range, the total light dose appears to be a more critical determinant of antimicrobial efficacy than the specific fluence rate itself. The observed variations in microbial susceptibility highlight the importance of species-specific factors, such as cell wall/membrane structure. These findings provide guidelines for the application of curcumin-mediated PDT in combatting antimicrobial resistance.

The therapeutic application of blue light (380 - 500nm) has garnered considerable attention in recent years as it offers a non-invasive approach for the management of prevalent skin conditions including acne vulgaris and atopic dermatitis. These conditions are often characterised by an imbalance in the microbial communities that colonise our skin, termed the skin microbiome. In conditions including acne vulgaris, blue light is thought to address this imbalance through the selective photoexcitation of microbial species expressing wavelength-specific chromophores, differentially affecting skin commensals and thus altering the relative species composition. However, the abundance and diversity of these chromophores across the skin microbiota remains poorly understood. Similarly, devices utilised for studies are often bulky and poorly characterised which if translated to therapy could result in reduced patient compliance. Here, we present a clinically viable micro-LED illumination platform with peak emission 450 nm (17 nm FWHM) and adjustable irradiance output to a maximum 0.55{+/-}0.01 W/cm2, dependent upon the concentration of titanium dioxide nanoparticles applied to an accompanying flexible light extraction substrate. Utilising spectrometry approaches, we characterised the abundance of prospective blue light chromophores across skin commensal bacteria isolated from healthy volunteers. Of the strains surveyed 62.5% exhibited absorption peaks within the blue light spectrum, evidencing expression of carotenoid pigments (18.8%, 420 - 483 nm; Micrococcus luteus, Kocuria spp.), porphyrins (12.5%, 402 - 413 nm; Cutibacterium spp.) and potential flavins (31.2%, 420 - 425 nm; Staphylococcus and Dermacoccus spp.). We also present evidence of the capacity of these species to diminish irradiance output when combined with the micro-LED platform and in turn how exposure to low-dose blue light causes shifts in observed absorbance spectra peaks. Collectively these findings highlight a crucial deficit in understanding how microbial chromophores might shape response to blue light and in turn evidence of a micro-LED illumination platform with potential for clinical applications.

The skin microenvironment created by keratinocytes (KC) influences stress responses of melanocytes (MC) to UVB insult. Here, we investigated paracrine factors involved in the regulatory role of microenvironment created by KC in UVB-mediated MC responses using RNA sequencing analysis as well as in vitro and in vivo models. RNA-Seq showed that G-CSF and CCL20 genes were highly upregulated in UVB-irradiated KC and their levels best correlated with paracrine protective effects of KC on stress responses of MC to UVB. Recombinant G-CSF and CCL20 treatment revealed the strongest modulatory effects on UVB-induced MC responses by mitigating apoptosis and ROS formation and upregulating tyrosinase and tyrosinase-related protein-1 (TRP-1) involved in the melanogenic pathway. A similar correlation between G-CSF and CCL20 expression in KC and the tyrosinase level in MC was also observed in the UVB-irradiated mouse skin. Our study reports for the first time that G-CSF and CCL20 might play a regulatory role in the KCs paracrine effects on UVB-mediated MC damage and also provides translational insights for the development of biomarkers for predicting susceptibility to photodamage. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=185 SRC="FIGDIR/small/523939v1_ufig1.gif" ALT="Figure 1"> View larger version (58K): org.highwire.dtl.DTLVardef@113588org.highwire.dtl.DTLVardef@1d1af40org.highwire.dtl.DTLVardef@1489df0org.highwire.dtl.DTLVardef@7935c5_HPS_FORMAT_FIGEXP M_FIG C_FIG

Long-term exposure to high-energy visible (HEV) blue light and infrared-A (IR-A) radiation accelerates oxidative stress, inflammation, and transepidermal water loss (TEWL), leading to photoaging and damage to the skin barrier. In this study, we developed Raybloc(R), a marine bioactive silica microsponge formulation, and evaluated its protective effects against combined high-energy visible (HEV; 410-480 nm) and infrared-A (IR-A; 700-1400 nm) exposure in a preclinical model. We divided 36 nude BALB/c-nu/nu mice into six groups: one that didnt get any treatment, one that got Raybloc(R) (no radiation), one that got Raybloc(R) 5%, one that got Raybloc(R) 8%, one that got HA 0.5%, and one that got HA 0.8%. Animals underwent topical treatment for 14 days under regulated exposure to HEV (410-480 nm, 100 J/cm2/day) and IR-A (700-1400 nm, 30 mW/cm2). We examined transepidermal water loss (TEWL), skin hydration, oxidative stress, inflammatory cytokines (IL-1{beta}, IL-6, TNF-, IL-10), and histological indicators of collagen preservation through biophysical, biochemical, and histopathological techniques. In the Raybloc(R) 8% group, TEWL dropped by 48.3 {+/-} 4.6% (p < 0.001), and skin hydration went up by 62.7 {+/-} 5.1%. The levels of ROS and MMP-1 expression decreased by 63.4% and 57.2%, respectively, while collagen I increased by 2.1 times compared to HA 0.8%. There was a big drop in the pro-inflammatory cytokines IL-1{beta}, IL-6, and TNF- (-54%, -49%, and -46%), and a big rise in IL-10 (+38%). Histological analysis demonstrated well-preserved epidermal integrity and dense collagen bundles in Raybloc(R)-treated mice, whereas irradiated controls exhibited dermal disorganization and inflammatory infiltration. Raybloc(R) showed better photoprotective, antioxidant, and moisturizing effects than HA-based products. It also helped reduce oxidative and inflammatory skin damage caused by blue light and IR-A. These results support Raybloc(R) as a next-generation multifunctional dermocosmetic that can help stop photoaging caused by digital and solar radiation. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=127 SRC="FIGDIR/small/713389v1_ufig1.gif" ALT="Figure 1"> View larger version (70K): org.highwire.dtl.DTLVardef@54e046org.highwire.dtl.DTLVardef@502f87org.highwire.dtl.DTLVardef@6088daorg.highwire.dtl.DTLVardef@1b8c241_HPS_FORMAT_FIGEXP M_FIG C_FIG

Clostridioides difficile infections (CDI) pose a significant threat to patient safety in healthcare facilities. In this study, we investigated the effectiveness of an ultraviolet-C (UV-C) disinfection system in reducing C. difficile surface contamination in a simulated patient room. The results showed an overall 98.17% reduction in C. difficile spores after UV-C treatment of directly exposed and shadowed areas, an efficiency that translated to a 4.91% reduction per minute of room vacancy. This study demonstrates that UV-C disinfection is an effective method for reducing C. difficile spores in healthcare settings, and that efficiency can be improved with shorter setup times and optimal device placement.

Techniques for pathogen inactivation have been employed by laboratories to help ease the financial, physical, and health strains associated with (A)BSL-3 work. Exposure to radiation is the most common and useful of these methods to inactivate pathogens grown in large-scale culture. While robust protocols exist for radiation exposure techniques, there are variances in methods used to determine the radiation dose and dose rate required to inactivate pathogens. Furthermore, previous studies often do not include radiation dosimetry verification or address corresponding dosimetry uncertainties for dose response-assays. Accordingly, this study was conducted with the purpose of completing a dosimetry assessment of the radiation field within the sample chamber of a sealed source irradiator, to subsequently determine the radiation dose required to inactivate pathogenic cultures. Physical dosimetry techniques (Fricke dosimetry, ion chamber measurements, and measurements with thermoluminescent dosimeters) were used to measure dose rate and rate variances within the sample chamber. By comparing the variances between the dosimetry methodologies and measurements, an estimated dose rate within the sample chamber was determined. The results of the dosimetry evaluation were used to determine the radiation dose samples of Mycobacterium tuberculosis received, to accurately associate biological markers of inactivation to specific doses of ionizing radiation. A D10 value and dose-response curve were developed to describe the inactivation of Mtb from increasing doses of ionizing radiation. The D10 value is experimentally relevant for comparative analysis and potentially provides a biological baseline for inactivation verification. This methodology can also easily be translated to other pathogen models. ImportanceThis work set out to give us a better understanding of how much radiation is required to inactivate Mycobacterium tuberculosis, the bacteria that causes tuberculosis disease. Radiation dose from a source is not something that can just be inputted, it must be calculated, so we also determined the approximate dose from the source to address ambiguities that had previously existed while inactivating microbes. We were able to generate an accurate description of inactivation of Mycobacterium tuberculosis by correlating it with a value representing 90% death of the treated cells. We also unexpectedly discovered that very low levels of radiation increase certain activity within the cell. This is important because it allows us to better understand how radiation kills Mycobacterium tuberculosis, and gives us a value to compare to other organisms. It also offers other researchers a method to use under their own specific conditions.

It is acknowledged that the inactivation of ultraviolet has been widely used in various fields. Much literature has been reported that ultraviolet C caused DNA damage to achieve inactivation of microorganisms. There is a lack of unified dose calibration and related parameters in this field. In this study, we used a device consisted of the LED of 272 nm to conduct sterilization experiments against A. baumanii. We confirmed the effectiveness of ultraviolet C sterilization for both sensitive and drug resistance strains and explored the relationship between bactericidal rate and ultraviolet doses under various irradiance. Dose requirements of various irradiance were clarified. High irradiance improved sterilization efficiency greatly. The overall damage to the total genome was observed though gel electrophoresis. Ultrastructure of damaged bacteria were investigated by transmission electron microscope in detail. The study revealed that damage to DNA and to the cytoplasm matrix and ribosomes. The study has yielded the possible effects of ultraviolet light on cells by amplifying the energy. The radiation significantly promoted the production of cell wall and cellular membrane. Significance StatementThe statistical results of bactericidal efficiency are influenced by the quantity of bacteria on the medium. Irradiance on the target surface affects the sterilization efficiency directly. And doses required in low irradiance are much more than high. A high irradiance reduced dosage efficiently to achieve the sterilization which improves the sterilization efficiency. There is a difference between low and high UVC dosage damage to the structure of bacteria. Less energy can make DNA coagulation solidified or be dispersed to the edge. Meanwhile the cytoplasm matrix is ruined. When the energy was enough, there is a boost of cell wall and cellular membrane production. The invisible light causes comprehensive damage to bacteria.

Mold infestations in buildings pose significant challenges to human health, affecting both private residences and hospitals. While molds commonly trigger asthma and allergies in the immunocompetent, they can cause life-threatening diseases in the immunocompromised. Currently, there is an unmet need for new strategies to reduce or prevent mold infestations. Far-UVC technology can inactivate microorganisms while remaining safe for humans. This study investigates the inhibitory efficacy of far-UVC light at 222 nm on the growth of common mold-producing fungi, specifically Penicillium candidum, when delivered in low-dose on-off duty cycles, a configuration consistent with its use in real-world settings. The inhibitory effect of the low-dose duty cycles was assessed on growth induced by i) an adjacent spore-producing P. candidum donor and ii) P. candidum spores seeded directly onto agar plates. In both setups, the far-UVC light significantly inhibited both vertical and horizontal growth of P. candidum, even when the UV doses were below the Threshold Value Limit of 23 mJ/cm2. These results suggest that far-UVC light holds the potential to improve indoor air quality by reducing or preventing mold growth, also when people are present.

Despite evidence that visible light (VL) has similar effects on human skin as those of UVA, VL is often viewed as harmless. High SPF sunscreen prevents erythema but can lead to overexposure to UVA and VL, with unknown consequences. To explore the impact of chronic blue light exposure, we irradiated (50 J/cm{superscript 2}, {lambda}= 408 nm, three times a week) human immortalized keratinocytes under acute (3 irradiations), intermediate (14 irradiations), and chronic (42 irradiations) blue-light exposure, monitoring phenotypic and gene expression changes. Chronically exposed keratinocytes exhibit increased nuclei area, chromatin alterations, higher proliferation, and apoptosis resistance, mirroring the consequences of chronic UVA exposure. While acute exposure upregulated keratinization and downregulated tissue repair and apoptosis genes, chronically exposed cells had upregulated genes involved with energy metabolism and oxidative phosphorylation and downregulated genes were enriched for immune and inflammatory responses. Specific transcriptional factors were identified in the acute and chronic stages, some of them had been associated with UVB exposure. We identified some changes in chronically irradiated keratinocytes similar to the malignant transformation, emphasizing the need for further research on the long-term impacts of blue light exposure on human skin.