Far-UV-C (222 nm) — Efficacy, Safety, and Where It Fits

Quick answer

Far-UV-C is ultraviolet light at the short end of the UV-C band, in practice 222 nm emitted by krypton-chloride (KrCl) excimer lamps. It is germicidal like conventional 254 nm UV-C, but its very short wavelength is absorbed in the outermost, non-living cell layers of skin and in the tear film of the eye — so it reaches pathogens without significantly penetrating human tissue. That opens a use case conventional UV-C cannot serve: continuous disinfection of occupied rooms.

The efficacy is real: in a room-sized chamber study, five filtered far-UV-C sources at 3 air changes per hour cut the steady-state airborne load of Staphylococcus aureus by 98.4 % (Eadie et al., Scientific Reports 2022). But far-UV-C is not a free lunch — KrCl lamps need optical filtering, can generate measurable ozone, and the long-term safety and standards picture is still maturing. This article covers the mechanism, the data, and the honest limitations.

How far-UV-C works

The KrCl excimer source

The dominant far-UV-C source is the krypton-chloride (KrCl*) excimer lamp, which emits a narrow peak at ~222 nm (Far-UVC, Wikipedia). Unlike a low-pressure mercury lamp (254 nm), an excimer lamp produces UV from a short-lived excited dimer rather than a mercury vapour discharge — so it is mercury-free. The raw emission also has weaker secondary peaks at longer (and shorter) wavelengths, which matters for both safety and ozone (see below).

For the germicidal mechanism itself — UV-C damaging microbial DNA/RNA — see Wavelengths and action spectra.

Why limited penetration enables occupied-space use

The defining property of 222 nm is shallow penetration into human tissue. Far-UV-C is absorbed in the most superficial cells of the stratum corneum of the skin and in the tear layer of the eye, limiting the dose that reaches living cells beneath (Buonanno et al., Scientific Reports 2020). The 207–222 nm range penetrates only the outermost layers of the corneal epithelium, which are naturally shed within roughly 24–48 hours through normal cell turnover (Duncan et al., Photochemistry and Photobiology 2023).

Bacteria and viruses are typically less than 1 µm across, so 222 nm readily reaches their genetic material. Human cells are roughly 10–20 µm and sit beneath protective dead-cell layers — the same dose that inactivates a virus is largely absorbed before it reaches a living human cell nucleus (Buonanno et al., Scientific Reports 2020). This biological asymmetry is the entire reason far-UV-C is considered for spaces where people are present, unlike conventional 254 nm UV-C, which requires unoccupied rooms or shielded upper-room geometry.

Efficacy data

Far-UV-C at 222 nm is broadly germicidal. Reported inactivation doses from peer-reviewed work:

Two caveats on reading these numbers. First, delivered dose depends entirely on the install — optics, fixture height, room geometry, reflectance of surfaces and the disinfection target all change real-world performance (review, Critical Reviews in Environmental Science and Technology 2022). A chamber number is not a guaranteed field number. Second, dose–response also varies with experimental and bacterial variables, so a single mJ/cm² figure should be treated as order-of-magnitude guidance, not a fixed constant (Pearson et al., antibacterial efficacy study, PMC 2024).

For how dose itself is measured and verified, see UV dosimetry fundamentals.

Safety and exposure limits

What the studies show

Filtered 222 nm light has not induced acute skin or eye reactions, nor delayed effects such as skin cancer, in animal models when longer-wavelength emissions are removed (Far-UVC, Wikipedia, summarising the Buonanno/Yamano body of work). A 36-month clinical observation of full-room 222 nm germicidal irradiation reported no ocular safety signal (Sugihara et al., ocular safety observation, PMC 2025). Interventional human ocular experiments found that direct eye exposure up to 75 mJ/cm² did not produce clinically significant photokeratitis (Sugihara et al., Photochemistry and Photobiology 2025).

In ceiling-mounted full-room deployment, the dose actually incident on the eyes is only a fraction of the dose set for the space — modelled at roughly 5.8 % of the space dose (Duncan et al., Photochemistry and Photobiology 2023), because people rarely look straight up at the fixture.

Exposure limits — a moving target

The regulatory picture changed sharply and is still settling:

The 2022 ACGIH increase reflects the accumulating evidence on shallow penetration, but ICNIRP and national bodies have not all moved in lock-step. Notably, Germany's Commission on Radiological Protection (SSK) issued a 2024 recommendation that takes a more cautious line on using far-UV-C around people, calling for careful risk assessment rather than blanket approval (SSK recommendation 2024). For German and EU occupational-safety framing, see Occupational safety norms (DE/EU).

The practical takeaway: antimicrobial far-UV-C doses are generally well below the ACGIH skin/eye TLVs, but the applicable limit depends on jurisdiction, and a deployment must be designed against the limit that actually governs the site — not the most permissive one.

Honest limitations

Far-UV-C is promising, not finished. Four constraints to weigh:

1. Filtering is mandatory. Raw KrCl emission includes longer-wavelength tails that can harm skin and eyes, and shorter-wavelength components that drive ozone. Practical fixtures use optical filters to suppress these secondary peaks (Far-UVC, Wikipedia; ozone study, Environmental Science & Technology Letters 2023). An unfiltered or poorly filtered KrCl lamp does not carry the far-UV-C safety profile.

2. Ozone is real and power-dependent. 222 nm light can photochemically generate ozone. The magnitude depends on lamp power: low-power KrCl lamps produce modest ozone that conventional ventilation can manage, while higher-power systems can reach problematic indoor levels (Environmental Science & Technology Letters 2023). Field testing of a single conference-room lamp found no significant ozone or fine-particulate impact (Barber et al., PLOS One 2025) — but this is a per-deployment question, not a settled "non-issue".

3. Availability and cost. KrCl excimer fixtures are a newer, more specialised product class than mature low-pressure mercury or UV-C LED hardware. Buyers should expect a smaller supplier field and to scrutinise technical documentation, exposure guidance and the stated operating envelope.

4. Long-term and standards uncertainty remains. Multi-year human data is encouraging but still accumulating, and standards bodies disagree on how permissive limits should be (ACGIH vs SSK above). Far-UV-C should be treated as a layered control alongside ventilation and filtration — see Room air purifiers (UV-C) — not a standalone replacement for them.

Cross-references

• Wavelengths and action spectra — where 222 nm sits on the germicidal action spectrum and why wavelength matters.
• UV lamp technology — how KrCl excimer lamps compare to low-pressure mercury and UV-C LED sources.
• Occupational safety norms (DE/EU) — exposure-limit framing for workplaces.
• Room air purifiers (UV-C) — far-UV-C as one option in the air-disinfection toolbox.
• UV dosimetry fundamentals — measuring and verifying the delivered dose.
• Standards and certifications — how UV products are tested and documented.

Sources

• Buonanno M. et al. — Far-UVC light (222 nm) efficiently and safely inactivates airborne human coronaviruses, Scientific Reports 2020.
• Eadie E. et al. — Far-UVC (222 nm) efficiently inactivates an airborne pathogen in a room-sized chamber, Scientific Reports 2022.
• Eadie E. et al. — 222 nm far-UVC light markedly reduces the level of infectious airborne virus in an occupied room, Scientific Reports 2024.
• Ma B. et al. — UV Inactivation of Common Pathogens and Surrogates Under 222 nm Irradiation from KrCl* Excimer Lamps, Applied and Environmental Microbiology 2022.
• Pearson B.L. et al. — The Antibacterial Efficacy of Far-UVC Light, PMC 2024.
• Duncan D. et al. — Ocular and Facial Far-UVC Doses from Ceiling-Mounted 222 nm Far-UVC Fixtures, Photochemistry and Photobiology 2023.
• Sugihara K. et al. — Interventional human ocular safety experiments for 222-nm far-ultraviolet-C lamp irradiation, Photochemistry and Photobiology 2025.
• Sugihara K. et al. — Ocular safety of 222-nm far-ultraviolet-C full-room germicidal irradiation: a 36-month clinical observation, PMC 2025.
• Peng Z. et al. — Significant Production of Ozone from Germicidal UV Lights at 222 nm, Environmental Science & Technology Letters 2023.
• Barber V. et al. — Effects of germicidal far-UVC on ozone and particulate matter in a conference room, PLOS One 2025.
• Blatchley E.R. et al. — Far UV-C radiation: an emerging tool for pandemic control, Critical Reviews in Environmental Science and Technology 2022.
• ICNIRP-referenced exposure-limit data via Duncan et al., PMC 2023.
• ACGIH 2022 TLV revision (222 nm), summarised in Far-UVC, Wikipedia.
• German Commission on Radiological Protection (SSK) — Risks of using far-UVC radiation for disinfection in the presence of people, recommendation 2024.

Vendor-neutral. KrCl excimer technology is described as a class; no manufacturer is endorsed. Confidence: high — every factual claim traces to a cited, independently registered source.