222 nm far-UVC vs 254 nm conventional UV-C: what changes in practice

Same family, different jobs

Ultraviolet C covers wavelengths from about 100 to 280 nm. Germicidal lamps have used 254 nm light for decades because it is strongly absorbed by the DNA and RNA of microorganisms. Far-UVC refers to the shorter end of the band, roughly 200 to 235 nm, and most devices use krypton-chloride excimer lamps with a peak at 222 nm.

Both wavelengths inactivate bacteria, viruses and fungi. What separates them is how deeply they penetrate human tissue, and that decides how they can be used in a hospital.

Penetration and safety

Light at 254 nm passes through the outer layer of the skin and the surface of the eye and reaches living cells. That is why conventional UV-C causes skin redness and painful eye inflammation, and why rooms must be empty during treatment.

Light at 222 nm is absorbed so strongly by proteins that it is largely stopped by the outer, non-living layer of the skin and by the tear film. Microorganisms are far smaller than human cells, so the light still reaches and damages them. Filtering matters: unfiltered lamps also emit longer wavelengths that penetrate further, so hospital devices should use filtered lamps.

Exposure limits

Occupational exposure limits reflect this difference. The ACGIH threshold limit value for 254 nm is 6 mJ/cm² over eight hours. In 2022, ACGIH revised its limits for 222 nm to 161 mJ/cm² for the eyes and 479 mJ/cm² for the skin over eight hours. Far-UVC devices for occupied rooms are designed and positioned to keep people below these limits, which is why placement and settings are checked during installation.

How each is used

Conventional UV-C works best as a high-intensity, short-cycle step when a room is empty: after discharge, between theatre cases or at the end of the day. Mobile robots and cabinets deliver a large dose quickly, and cycle records show what was treated.

Far-UVC works as a continuous, low-intensity layer while people are present: during surgery, in clinic rooms and waiting areas. It adds disinfection of the air and surfaces over hours, often described as equivalent air changes.

Practical points

• Neither replaces cleaning, ventilation or hand hygiene. Both add a layer.
• Some far-UVC lamps generate small amounts of ozone. It should be assessed alongside room ventilation.
• Shadows reduce the dose for both. Placement is part of the design, not an afterthought.
• Choose devices with published testing and, where relevant, regulatory clearance.

Using both together

Many facilities combine the two: a UV-C robot for terminal disinfection between patients and far-UVC fixtures that run during the working day. The combination covers the empty-room and occupied-room hours that each technology alone cannot.

Related: Zener far-UVC, THOR UVC