By Holger Claus, Ushio America, and Michael Clark, Far UV Technologies
This article attempts to define and describe key aspects of Germicidal UV (GUV) technology specifically for air and surface disinfection to get a clearer understanding of key values, limitations, opportunities and especially gaps in the understanding of GUV between the researchers, practitioners and regulators. It is the hope of the authors that this could become a baseline for further, much more detailed discussions and especially research, which ultimately could lead to a wider acceptance and application of GUV for surface and air disinfection.
Introduction
GUV technology has the potential to contribute significantly to lower pathogen concentrations on surfaces and in the air in many areas of human activities, in food supply and yet to be discovered areas. Consequently, this is expected to lower pathogen transmission and, therefore, infections.
However, based on its limitations as a photonic technology, an honest look at the limitations and opportunities of GUV in air and surface applications is needed. This is not only necessary to manage expectations about the envisioned results, but even more to potentially overcome critical limitations and gather greater knowledge to make the technology outcomes more predictable and, therefore, more applicable.
GUV in air and surface applications should be viewed as a supplemental technology to other, existing disinfection technologies. For surface disinfection, manual cleaning and chemical disinfection are the main technologies used today. GUV technology has the potential to complement traditional approaches, especially through the fact that this technology is less operator dependent, more energy efficient and features an environmentally clean technology. More specifically, as GUV in air disinfection has the potential to lower pathogen concentrations in rooms, it adds an energy efficient method to existing mechanical, filter-based air cleaning systems.
What is Germicidal UV?
The authors define Germicidal UV as the wavelength range between 200 and 290 nm. This is the wavelength range where the UV is damaging DNA/RNA and potentially proteins at relatively low UV doses.
Although it is known that other wavelengths (UV-A, UV-B and even IR) can lead to microbial reduction, the damaging processes are fundamentally different and require doses that are magnitudes higher than used for GUV, and therefore are excluded from this GUV definition.
Other references describe GUV as the UV-C range of 200 to 280 nm. In order to include common UV LEDs, emitting in the 280 to 290 nm range, the authors extended the definition. Shorter wavelengths than 200 nm also are not considered since they have a tendency to create substantial amounts of ozone and typically are not practical and therefore are not used in real-life applications.
GUV Applications for Air and Surface Treatment
The goal of applying GUV is first and foremost the prevention of transmission of pathogens and infections. Although there is a correlation between pathogen concentration and infection risk, exact data usually are not available. In addition, it can be confounded by several environmental variables (e.g., ventilation, temperature, humidity, concentrations of particulate matter or occupancy), device design and positioning, the output in terms of intensity by wavelength, and the transmission dynamics (pathogen type and concentration, exposure, dose-response relationship, individual susceptibility and behavior). Therefore, predictions of real-world outcomes are very uncertain and hard to quantify in the real world.
Limitations of GUV
GUV is a photon process and requires a direct line of sight to have the desired impact on surfaces and in air. This applies on a macroscopic scale (e.g., furniture obstruction/shadowing) as well as on a microscopic scale (e.g, by agglomerations of bacteria or virus aerosols and particles, surface roughness). This major limitation of GUV regarding the efficacy of air and surface disinfection will, in most practical cases, prevent a complete disinfection, i.e., a reduction of microbial entities to – or close to – zero.
Evidence-Based Strategies for Microbial Inactivation
The reduction of microorganisms (including pathogens) is directly correlated to the GUV dose that the microorganisms receive. The dose/reduction correlation is specific for each pathogen and varies with UV-C wavelength and environmental factors like humidity and temperature.
Current knowledge indicates that the reduction often follows initially a first order GUV dose/response up to certain dose levels and may show a different slope at higher doses. The slope k is based on a natural logarithmic scale.
Currently available dose response data (D90 or k factor) for specific pathogens vary widely, most likely caused by varying test procedures, varying environment and medium (water, air, surface) and measurement errors of dose.
Current science assumes that the reduction of the microorganisms follows the product of irradiation time and irradiance: In practical terms, a long irradiation time at low irradiance would give the same results as short irradiation time at high irradiance (dose reciprocity). This assumption has boundaries within the entire range of practically feasible irradiances, especially if pathogens have powerful repair mechanisms. More research in this context is urgently needed.
Although standards exist for measuring surface disinfection, it is recognized that most standards originated from chemical disinfection testing. Taking actual goals and limitations of GUV technology into account, specific pathogen reduction standards for GUV disinfection should be developed.
Modeling GUV Efficacy: From Irradiance Distribution to Infection Risk Reduction
In order to predict pathogen reduction in practical applications, the distribution of irradiance (for surfaces) or fluence rate (for air) is needed. Irradiance and fluence rate distributions at varying distances can be accurately (enough) simulated by modern lighting design software. The models can take reflections of walls etc. into account but cannot predict surface effects on a microscopic scale (for example, roughness or reflection of target surface) or air movement. More research on this topic is advised, and results potentially could be used as correction factors when predicting pathogen reduction.
The prediction of pathogen reduction in room air appears to have significant uncertainties as well. There are further parameters contributing to the overall uncertainties, which suggest further research (such as more accurate k factors of pathogens in air and the development of a standardized measurement setup). Also, it should be verified whether the assumptions of well-mixed room air and the use of average fluence rate in a room is acceptable.
Validation and Measurement of Models
Irradiance on surfaces can be measured with existing irradiance meters. It must be noted that the quality of available meters in the market varies widely, and meters typically are specific to the GUV wavelength. This may create measurement errors in practical applications, but potentially also in research. A minimum quality standard for GUV meters would be helpful in this context.
The measurement of fluence rate in rooms needs significantly more research and practical solutions.
GUV Technologies: Safety and Commissioning
It is recognized that GUV, in general, is hazardous to human (and animal) eyes and skin. The hazard strongly depends on the wavelength and dose received by humans during their exposure. The hazard levels (TLV) are described in the ACGIH TLV tables and are widely accepted in the USA. Internationally, lower TLV values typically are used based on IEC62471.
The safety validation of installations typically is described in national and international standards (such as in the US, RP27.1 and RP44, and internationally, IEC62471-6).
It is uncertain if and how such standards are being applied in practice. More international information exchange and harmonization are highly recommended. It also would be meaningful that training procedures and potentially “training certifications” would be developed for the training of installers on the commissioning, verification, quality control and maintenance of GUV.
GUV: Other Concerns
It is known that the high photon energy of GUV does create photochemical effects that must be considered and potentially are limiting factors in the usefulness of GUV.
- Effects on plants can be severe and can damage them permanently. More research is needed.
- Byproducts of GUV are being created, especially with shorter wavelengths, but also in the 250 to 270 nm range. This includes ozone generation by lamps (especially 222 nm), and the creation of particle matter (PM) and secondary organic aerosols (SOA). This field needs substantial research and attention, especially in regards of a risk/benefit analysis for practical applications.
- Modification of surface materials has to be considered.
GUV: Risk/Benefit Considerations
To assess the benefits, but also to perform an appropriate risk/benefit analysis, further international discussion and pointed research are required. The ultimate goal of the application of GUV is a reduction in risk (of transmission or infection), but the measurements (or models) typically are tied to either irradiance or pathogen reduction measurements. It is necessary to develop metrics which are widely accepted to forecast the risk reduction, especially when striving to reduce airborne infections.
Future, more advanced risk/benefits analyses need to include additional potential risks from ozone, volatile organic compounds (VOCs) and ultrafine particles (UFP) to achieve a comprehensive, widely accepted assessment, as well as to reduce the liability of users, such as facility managers, and manufacturers. To further increase the acceptance of GUV, these risk/benefit approaches and decisions need to be based on standardized metrics to enable GUV manufacturers, users, regulators and the health community to generate scientifically founded decisions when applying GUV technology.
The authors would like to encourage readers to submit more specific articles and presentations about the topics mentioned above and support the IUVA community as it proposes action plans and updates to already existing related projects. IUVA currently has a Task Force focused on this topic – Technology and Research of Air and Surface Treatment (TRAST) – which meets virtually on a monthly basis. For information on joining, contact gcohen@iuva.org.
