In the world of germicidal ultraviolet (UV) technologies, far-UVC light at 222 nm has been generating a lot of excitement. Studies continue to show that 222 nm light, when properly filtered, cannot penetrate beyond the outer layer of the skin (stratum corneum) or the tear layer of the eyes. This scientific consensus has led leading industrial hygiene authorities—particularly the American Conference of Governmental Industrial Hygienists (ACGIH)—to significantly increase recommended exposure limits for far-UVC radiation.
Below, we’ll explore how and why these limits changed, what this means for safety and practical applications, and what the future may hold for far-UVC disinfection technology.
The ACGIH’s Updated Threshold Limit Values (TLVs)
Historically, all UVC sources were treated very conservatively, often using TLVs derived from the well-studied 254 nm mercury lamps. At that time, far-UVC at 222 nm was lumped together with longer wavelengths, which penetrate more deeply and can cause harm to sensitive tissues.
However, new research in recent years demonstrated that properly filtered 222 nm light is absorbed in the most superficial cells of the stratum corneum and in the tear layer, limiting the risk to deeper tissues. In 2022, the ACGIH responded by raising the allowable daily exposure to far-UVC sources:
- Eyes:
- Old Limit: 23 mJ/cm² (grouped with standard UVC)
- New Limit: 161 mJ/cm²
- Increase: ~7 times higher
- Skin:
- Old Limit: 23 mJ/cm²
- New Limit: 479 mJ/cm²
- Increase: ~20 times higher
This dramatic change underscores the growing confidence in the safety profile of well-filtered far-UVC.
Why Far-UVC Is Different
Far-UVC (222 nm) differs from the more traditional germicidal UVC (254 nm) often used in healthcare and laboratory settings. Unlike 254 nm, which can penetrate deeper into tissues and pose a higher risk of erythema (skin reddening) or photokeratitis, properly filtered 222 nm has the following advantages:
- Minimal Tissue Penetration:
Far-UVC at 222 nm is largely absorbed by the topmost layer of dead skin cells. The same goes for the tear layer covering the eyes, which helps to block deeper penetration. - Strong Germicidal Efficacy:
Though it doesn’t penetrate deeply into human tissue, it effectively inactivates viruses and bacteria on surfaces and potentially in the air. - Reduced Risk Profile:
Because it doesn’t reach living cells in the skin or deeper ocular structures, it poses far fewer long-term risks compared to broader-spectrum or higher-wavelength UVC.
Practical Applications: A Safer Way to Disinfect?
The higher permissible exposure limits have immediate practical implications. Far-UVC’s shorter penetration depth and robust germicidal action open the door to disinfection applications in occupied environments, potentially including:
- Hospitals & Clinics: Operating rooms, waiting areas, and patient rooms.
- Public Transit: Buses, trains, and airports.
- Commercial & Residential Spaces: Shared offices, grocery stores, gyms, and restaurants.
By installing well-filtered far-UVC lamps, these environments can continuously disinfect the air and surfaces without posing undue risk to staff or patrons.
Regulatory Recognition of Far-UVC Safety
The ACGIH’s updated TLVs serve as a bellwether for other regulatory and advisory bodies. As more research emerges, we are likely to see:
- Further refinement of exposure guidelines.
- Growing acceptance of 222 nm lamps across industries and jurisdictions.
- Expanded development of far-UVC technologies (including improved filtering techniques to ensure minimal emission above 230 nm).
Overall, these revisions highlight a shifting paradigm in industrial hygiene, where specific wavelength ranges can no longer be treated as universally dangerous. Instead, the safety of narrow-band far-UVC has found increasing recognition.
Key Points to Remember
- Filtered vs. Unfiltered:
- Filters that remove any peaks above 230 nm are crucial. If a far-UVC lamp is not adequately filtered, it may emit slightly higher wavelengths, potentially reducing the permissible exposure limits.
- Monitoring and Verification:
- Always verify actual irradiance in a space—using meters or dosimeters—to ensure that you stay within the recommended exposure thresholds.
- Continuous Research:
- Though safety data is promising, far-UVC is a relatively new field. Continuous monitoring, real-world studies, and peer-reviewed research all contribute to evolving guidelines.
- Look to the ACGIH TLVs:
- The latest ACGIH publication provides detailed spectral weighting factors, daily exposure limits, and best practices. These guidelines are regularly updated as new data emerge.
The Road Ahead
Far-UVC technology stands at the intersection of public health, industrial hygiene, and emerging photonics. By recognizing how shallowly 222 nm light penetrates tissue, regulatory bodies are embracing a science-driven approach to disinfection. This shift not only underscores the potential safety of far-UVC but also accelerates innovation in safer, more effective disinfection solutions.
As more facilities adopt far-UVC to reduce pathogen transmission, we can expect ongoing improvements in lamp quality, filtration, and monitoring systems. Meanwhile, the lessons learned here could pave the way for other wavelength-specific UV technologies to undergo similar scrutiny and acceptance.
Conclusion
The massive increase in ACGIH’s Threshold Limit Values for far-UVC at 222 nm—from 23 mJ/cm² to 161 mJ/cm² for the eyes and 479 mJ/cm² for the skin—reflects an important breakthrough. These higher limits acknowledge compelling evidence that filtered 222 nm light does not penetrate beyond the skin’s surface or through the eye’s tear layer in significant amounts, thus presenting a much lower risk of harm than previously assumed.
For stakeholders—building managers, healthcare administrators, and safety officers—this development marks a milestone. By adopting properly filtered 222 nm systems and complying with these guidelines, we can provide continuous, effective, and people-friendly disinfection solutions. As research continues, the future of far-UVC shines bright indeed.
Disclaimer: While recent studies and official TLV updates strongly support the relative safety of far-UVC at 222 nm, all UV deployments should follow best industrial hygiene practices. Always consult up-to-date ACGIH TLVs, relevant regulatory guidelines, perform risk assessments, and ensure devices are properly filtered and validated.
