Frequently Asked Questions

Far-UVC is ultraviolet light in the short end of the UV-C band, centered on 222 nanometers. It inactivates viruses, bacteria and fungi, but is absorbed by the dead outer layer of skin (the stratum corneum) and the tear film of the eye before it can reach living cells. Conventional germicidal UV-C at 254 nm penetrates tissue and is hazardous to people; filtered 222 nm far-UVC is much gentler on skin and eyes, which is why it can be used in occupied rooms.

Both bands damage the DNA and RNA that pathogens need to replicate. The difference is tissue penetration. At 222 nm the light is absorbed so strongly by proteins that it stops within the first few micrometers of tissue - the non-living outer skin and eye surface - and does not reach living cells. At 254 nm the light penetrates deep enough to cause sunburn-like skin damage and photokeratitis, so it can only be used where people are shielded or absent.

The weight of published evidence indicates 222 nm is far gentler on human tissue than 254 nm, and in 2022 the ACGIH revised its exposure limits (TLVs) for the far-UVC region substantially upward, with separate limits for the eye and skin. Studies from Columbia University's Center for Radiological Research and others report no significant DNA-damage or skin-lesion markers at 222 nm under regulated exposure. That said, long-term human data is still being gathered, and fixtures must be dosed and sited to keep occupants within the published limits.

The ACGIH Threshold Limit Values and the ICNIRP guidelines both express UV limits as an 8-hour dose at a given wavelength, and the 2022 ACGIH revision raised the permissible 222 nm exposure well above the older blanket figure, setting distinct eye and skin limits. Because the exact numbers are periodically updated and depend on eye-versus-skin and the specific standard, responsible installers measure irradiance at head height and confirm compliance with the current ACGIH / ICNIRP values rather than relying on a single quoted number.

Yes, in peer-reviewed testing. Welch, Buonanno and colleagues (Scientific Reports, 2018) reported over 95% inactivation of aerosolized H1N1 influenza at about 2 mJ/cm2, and Buonanno and colleagues (Scientific Reports, 2020) reported roughly 99.9% inactivation of aerosolized human coronaviruses at low single-digit doses. A 2022 room-sized-chamber study (Eadie, Wood and colleagues, Scientific Reports) showed far-UVC rapidly clearing an airborne bacterial pathogen from a full room.

UV-C photons are absorbed by nucleic acids and proteins. In DNA and RNA they create lesions such as pyrimidine dimers that prevent the genome from being copied, so a virus cannot infect and a bacterium cannot divide. At 222 nm there is an added effect: the light is absorbed strongly by proteins, damaging the microbe's surface proteins as well. A microbe has no protective dead outer layer to absorb the light, so it takes the full effect.

The krypton-chloride (KrCl) excimer lamps used for far-UVC emit mostly at 222 nm but also produce a small tail of longer, skin-penetrating wavelengths. An optical band-pass filter removes that tail. Every safety result for far-UVC assumes a properly filtered source. An unfiltered KrCl lamp is not safe far-UVC - the filter is the safety feature, which is also why you should never attempt to build a fixture yourself.

It can. 222 nm photons can split oxygen molecules and generate small amounts of ozone, and that ozone can react with indoor pollutants to form secondary products. Peer-reviewed indoor-air chemistry research (including work from the University of Colorado Boulder published in Environmental Science and Technology Letters) has measured this. The effect depends on fixture output and, critically, on ventilation. The guidance is to pair far-UVC with adequate air exchange and to choose fixtures characterized for low ozone - not to avoid the technology, but to deploy it carefully.

No. It is best understood as an additional layer that contributes 'equivalent air changes per hour' alongside fresh air and filtration. A layered approach - ventilation plus filtration plus far-UVC - is more robust than any single measure, and ventilation also handles CO2 and the ozone considerations above. Far-UVC is a complement to good ventilation, not a substitute for it.

Candidate and pilot settings include healthcare (waiting rooms, wards, dental operatories), schools, offices and other high-occupancy indoor spaces, and transit such as buses, trains and aircraft cabins. Traditional upper-room UVGI at 254 nm keeps the light above people's heads; whole-room far-UVC at 222 nm can illuminate the occupied space within exposure limits. Many deployments remain in evaluation and pilot stages.

Confirm it uses a filtered KrCl 222 nm source (ask to see the filtered spectrum); check the irradiance in the occupied zone and the distance it was measured at; confirm it keeps occupants within ACGIH / ICNIRP limits for the intended run time; ask for independent ozone data; verify photobiological testing to IEC 62471 and the stated risk group; and check rated lamp hours and replacement cost. Be skeptical of any '222 nm' product that cannot show its filtered spectrum and exposure data.

Named manufacturers include Ushio (its filtered 222 nm module is marketed as Care222, Japan), Far UV Technologies (KrCl fixtures, United States), and Eden Park Illumination (Champaign, Illinois, developing next-generation far-UVC sources). Many finished fixtures are built by integrators around these modules.

Regulation is still maturing. In the United States far-UVC emitters are regulated as radiation-producing electronic products, and there is not yet a single efficacy-and-safety approval specific to germicidal far-UVC. Buyers should look for testing to recognized standards such as IEC 62471, independent irradiance and ozone data, and adherence to ACGIH / ICNIRP exposure limits rather than marketing superlatives.