The search for safe and effective antimicrobial strategies has intensified in recent years, driven both by global viral outbreaks and the growing burden of antimicrobial resistance. Ultraviolet radiation is a well-established germicidal modality. Conventional germicidal UV-C at 254 nm causes profound DNA photodamage and photocarcinogenesis, precluding its use in occupied environments. In contrast, far-UVC within the 200–235 nm range demonstrates a markedly different biological profile. Because of its strong absorption by proteins and the stratum corneum [9], far-UVC penetrates only the outermost non-viable cell layers, while retaining potent antimicrobial efficacy against bacteria, fungi, and viruses [1–3, 16]. This distinctive profile—potent germicidal action combined with restricted skin penetration—has established far-UVC as a candidate for safe disinfection in occupied spaces. Importantly, its limited biological reach also raises the possibility of dermatological applications. Given the role of microbial dysbiosis in conditions such as atopic dermatitis (AD), cutaneous T-cell lymphoma (CTCL), and polymorphic light eruption (PLE), far-UVC may offer a novel approach to targeted microbiome modulation.

Robust inactivation of gram-positive and gram-negative bacteria, fungi, and viruses has been consistently reported at 222 nm and 233 nm. Far-UVC is effective against multidrug-resistant organisms such as methicillin-resistant Staphylococcus aureus (MRSA), extended-spectrum β-lactamase producing Escherichia coli (ESBL), and opportunistic pathogens including Candida species [2, 4, 5]. Interestingly, antimicrobial action appears to be independent of resistance phenotype, with equal susceptibility demonstrated in resistant and non-resistant strains [4, 5]. Inactivation is rapid: 233 nm LED systems achieved 5-log reductions within clinically relevant doses (20–60 mJ/cm²), while 222 nm excimer lamps eliminated airborne and surface pathogens with comparable efficiency [2, 7]. Nevertheless, efficacy is reduced in protein- and mucin-rich matrices, highlighting the need for contextual dose adjustments [7, 16].

The central concern is whether repeated far-UVC exposure induces genotoxicity or photocarcinogenesis. Multiple preclinical models, including reconstructed 3D skin models, ex vivo studies, and murine experiments, consistently show that 222 nm and 233 nm irradiation at doses up to 100 mJ/cm² induces only superficial and transient cyclobutane pyrimidine dimers (CPDs) without basal keratinocyte involvement [1, 2, 9, 10]. Long-term murine experiments demonstrate absence of photocarcinogenesis after chronic 222 nm far-UVC exposure, in contrast to conventional UVC or narrowband-UVB [8]. An extreme human self-exposure pilot study up to 18 000 mJ/cm² found no erythema or basal DNA lesions [11]. Importantly, interindividual factors influence susceptibility. Recent investigations demonstrate that phototypes with low melanin content exhibit slightly higher CPD levels after far-UVC exposure, although still markedly below those induced by UVB [6, 14]. The thickness of the stratum corneum has been identified as the principal protective factor, and demographic variables such as age and sex do not significantly alter barrier thickness [14]. However, safety is also critically dependent on spectral purity. Early devices with residual >230 nm emission induced DNA damage [12]. Band-pass filtering eliminates this hazard [12, 15], though inter-device variability remains [15]. Recently, the concept of the lamp exposure limit (HLEL) has been proposed as a practical safety metric, enabling calculation of safe exposure times and installation parameters [13]. Advances in LED-based systems at 233 nm may further improve spectral control, efficacy, and integration flexibility [2, 7]. These data collectively support a favorable safety margin when devices are adequately filtered.

Dysbiosis is central to several inflammatory skin diseases. In AD, S. aureus dominates lesional skin and exacerbates inflammation via superantigens and proteases; targeted microbial modulation improves disease control [17]. In CTCL, S. aureus colonization is linked to disease progression through toxin-driven proliferation of malignant T cells [18]. In PLE, altered microbial composition has been reported, with significant shifts in bacterial communities after ultraviolet exposure [19]. Recent mechanistic data show that eradication of the skin microbiota restores cytokine production in PLE, highlighting a causal link between dysbiosis and abnormal immune responses [20]. Current antimicrobial strategies rely on systemic or topical antibiotics, with attendant risks of resistance and collateral disruption of commensals. Far-UVC irradiation, by contrast, penetrates only superficial epidermis, enabling selective suppression of surface-associated pathogens while sparing deeper commensal reservoirs [2, 10]. Experimental studies demonstrate effective reduction of Candida and bacterial pathogens on skin models without compromising tissue viability [2, 5]. This raises the hypothesis that far-UVC could be developed as a microbiome-modulating therapy. Controlled, local exposures might transiently suppress dominant pathogens such as S. aureus, permitting restoration of diversity and barrier function (Fig. 1).

Despite current data on far-UVC are promising in terms of safety, several controversies remain, due to strong industrial involvement, underscoring the need for independent long-term studies [16]. Ocular safety is insufficiently characterized, with limited human data available. Likewise, chronic low-dose exposure in diverse skin phototypes warrants further investigation. Translation into clinical dermatology—such as targeted use for wound antisepsis and beyond—will require controlled clinical trials. In AD, endpoints could include microbial diversity, barrier recovery, and clinical severity scores. In CTCL, lesion-directed far-UVC could be tested for effects on microbial load and malignant T-cell activation. In PLE, prophylactic modulation of microbial composition may influence disease onset.

Far-UVC offers a compelling balance between broad antimicrobial efficacy and dermatological safety. Current data indicate minimal risk of basal DNA damage or carcinogenesis within exposure limits when properly filtered, though long-term human data remain unknown. Beyond antisepsis, a transformative possibility is the use of far-UVC as a targeted microbiome-modulating approach in dysbiosis-driven dermatoses such as AD, CTCL, and PLE [17–20]. Realizing this potential will require dermatology-led, independent clinical trials to define optimal doses, microbial and host outcomes, and long-term safety. If successful, far-UVC could inaugurate a new therapeutic class in dermatology, positioned at the intersection of photobiology and microbiome science.