Why one narrow band of ultraviolet light inactivates viruses, bacteria, and fungi — yet is stopped by the dead outer layer of skin and the tear film of the eye before it can reach a single living cell.
“UV” spans roughly 100 to 400 nanometers, and biology responds very differently across that range. The germicidal sweet spot has long been the UV-C band (100–280 nm), where photons carry enough energy to damage the nucleic acids that pathogens need to replicate. For nearly a century, germicidal ultraviolet (often called UVGI) has relied on low-pressure mercury lamps that emit almost entirely at 254 nm. That light works — it is used in water treatment, HVAC coils, and upper-room fixtures — but 254 nm penetrates human tissue deeply enough to cause sunburn-like skin damage and painful corneal inflammation (photokeratitis). For that reason conventional UV-C can only be used where people are shielded or absent.
Far-UVC refers to the shorter end of the UV-C band, around 200–235 nm, with commercial systems centered on 222 nm. The distinction is not academic: those extra tens of nanometers change how the light interacts with living tissue almost completely.
Shorter-wavelength photons are absorbed more strongly by proteins — specifically by the peptide bonds and aromatic amino acids that fill every cell. At 222 nm the absorption is so high that the light penetrates only a few micrometers into biological material before it is extinguished. Human skin is covered by the stratum corneum, a layer of dead, keratin-filled cells typically 5–20 micrometers thick. The eye is protected by the tear film and the outermost corneal epithelium. Far-UVC at 222 nm is absorbed within these non-living barriers before it can reach the living basal cells of the skin or the sensitive cells deeper in the eye.
A microbe has no such shield. Bacteria are roughly a micrometer across; viruses are far smaller still. There is no protective dead layer to sacrifice, so 222 nm photons pass straight into the organism and reach its genetic material. This is the core of the far-UVC premise, articulated by David J. Brenner and colleagues at the Center for Radiological Research, Columbia University: a wavelength short enough to be blocked by our own dead cells is still long enough — and energetic enough — to destroy a pathogen.
UV-C photons are absorbed by nucleic acids (DNA and RNA) and by proteins. In DNA and RNA they drive the formation of lesions such as pyrimidine dimers, which fuse adjacent bases and jam the molecular machinery of replication. A virus that cannot copy its genome cannot infect; a bacterium that cannot repair the damage cannot divide. Far-UVC adds a second mechanism: because 222 nm is absorbed so strongly by proteins, it also damages the structural and functional proteins on a microbe’s surface, an effect that is comparatively minor at 254 nm.
Buonanno, Ponnaiya, Welch and colleagues (Radiation Research, 2017) reported that 222 nm efficiently inactivated methicillin-resistant Staphylococcus aureus yet produced none of the DNA-damage or skin-lesion markers that 254 nm caused in exposed mammalian skin models.
Welch, Buonanno et al. (Scientific Reports, 2018) showed that low doses of 222 nm inactivated aerosolized H1N1 influenza virus — more than 95% at about 2 mJ/cm² — establishing far-UVC as a tool against airborne transmission, not just surfaces.
Buonanno, Welch, Shuryak and Brenner (Scientific Reports, 2020) found that 222 nm inactivated ~99.9% of aerosolized human coronaviruses (229E and OC43) at low doses on the order of 1–2 mJ/cm².
Eadie, Wood and colleagues (University of St Andrews with Columbia; Scientific Reports, 2022) ran far-UVC in a full room-sized chamber and reduced airborne S. aureus by well over 90% in minutes — an effect the authors equated to a very high number of “equivalent air changes” per hour.
Germicidal effect depends on dose, the product of light intensity and time. Intensity (irradiance) is usually quoted in microwatts per square centimeter (µW/cm²); dose is quoted in millijoules per square centimeter (mJ/cm²), where 1 mJ/cm² equals 1,000 µW·s/cm². The airborne-pathogen studies above achieved 90–99.9% inactivation at single-digit mJ/cm² doses — low enough that a ceiling fixture running continuously at exposure levels considered safe for occupants can keep clearing the air all day. That combination — effective at low dose, tolerable to people at those same levels — is what makes 222 nm distinct from every germicidal lamp that came before it.
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