Ultraviolet C (UVC) light has been used for decades as a powerful germicide for disinfecting surfaces and air in unoccupied environments. This high-energy light is effective against a broad spectrum of pathogens, including bacteria, viruses, and spores. Recent advancements have focused on a specific, narrow band of the UVC spectrum known as Far-UVC, which operates primarily within the 200 to 222 nanometer (nm) range. This technology is being developed as a potential alternative that can safely deliver continuous disinfection even in spaces occupied by people.
Defining the Spectrum Difference
Traditional germicidal lamps typically emit UVC light at 254 nm, a wavelength highly effective at damaging the genetic material of microorganisms. However, this 254 nm light also penetrates deep into human tissue, posing a risk of skin damage and eye injury, thus limiting its use to unoccupied rooms.
Far-UVC sources, most commonly Krypton-Chloride (KrCl) excimer lamps, are engineered to emit light centered at 222 nm. This minor difference in wavelength profoundly changes the light’s penetration depth into biological cells. The radiation at 222 nm is absorbed much more strongly by proteins than the longer 254 nm wavelength, stopping the energy at the non-living outer layers of human cells.
The Mechanism of Pathogen Inactivation
Far-UVC light is effective at inactivating pathogens because it targets nucleic acids. When the light strikes the genetic material, it creates molecular lesions known as pyrimidine dimers, specifically cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts.
These dimers are molecular kinks that form a covalent bond between two adjacent pyrimidine bases, such as thymine or cytosine. This structural distortion prevents the microorganism’s polymerase enzymes from accurately reading the genetic code, halting replication and transcription. By preventing the pathogen from reproducing, the Far-UVC light renders the microbe harmless and non-infectious. The 222 nm light can easily penetrate the outer protein coats and cell walls of viruses and bacteria to reach the genetic material within.
Why Far-UVC is Safe for Human Exposure
The primary reason for Far-UVC’s safety profile is its limited penetration depth into living human tissue compared to conventional UVC light. The short 222 nm wavelength is intensely absorbed by peptide bonds and other biomolecules present in the protective outer layers of the body. This absorption prevents the radiation from reaching the underlying, live cells where genetic damage could occur.
On the skin, the light is blocked almost entirely by the stratum corneum, the thin, outermost layer composed of dead, keratinized skin cells. Since this layer is non-living, the energy is absorbed and dissipated before it can reach the viable epidermal cells. Studies using human skin models have demonstrated that exposure to 222 nm light produces negligible amounts of pre-mutagenic DNA lesions, unlike the damage caused by 254 nm light.
Similarly, the safety for the eyes is attributed to the light’s inability to penetrate beyond the most superficial layers of the cornea. Far-UVC light at 222 nm only reaches the corneal epithelium, the outermost layer that undergoes a rapid, natural turnover process. In contrast, longer UVC wavelengths can penetrate deeper, potentially damaging the basal stem cells of the cornea.
The scientific consensus on safety has led regulatory bodies to reconsider exposure limits. For example, the American Conference of Governmental Industrial Hygienists (ACGIH) revised its Threshold Limit Values (TLVs) for 222 nm light, raising the permissible daily exposure for the eyes and skin. This revision was based on mounting evidence from animal and clinical studies that confirmed the non-hazardous nature of Far-UVC at germicidal doses.
Current Applications and Implementation Challenges
The unique combination of germicidal effectiveness and human safety positions Far-UVC as a technology for continuous disinfection in shared indoor spaces. Its most promising application is the active decontamination of air and surfaces in occupied areas like hospitals, schools, public transit vehicles, and commercial office buildings. By operating while people are present, the technology can reduce the concentration of airborne pathogens, such as influenza and coronaviruses, in the breathing zone.
The widespread adoption of Far-UVC faces several logistical and economic challenges. The Krypton-Chloride excimer lamp is currently complex and expensive to manufacture compared to traditional mercury vapor lamps. This higher unit cost presents a barrier to large-scale installation across public and private facilities.
Regulatory hurdles remain because the specific dosage limits set by organizations like ACGIH must be strictly adhered to, requiring careful calibration and monitoring of installed systems. Long-term material degradation is also a concern, as UVC light can sometimes yellow or weaken certain plastics and fabrics. Manufacturers must validate both efficacy and material compatibility before widespread deployment.