Validation Needed for UV Surface Disinfection Applications

Prepared by IUVA COVID-19 Task Force, UV Surface Disinfection Validation Sub-Group

Public health challenges from the SARS-CoV-2 pandemic and prior epidemics including SARS-CoV-1, MERS, Ebola and influenza highlight the scientific and engineering limitations in applying UV disinfection technologies. A limited understanding of critical UV aspects can explain some of the wide variations found in peer reviewed literature on the UV dose response of the virus. For example, highly cited publications on UV disinfection and reuse of N95 masks report required UV doses (mJ/cm2) to achieve 4-log inactivation of Influenza A virus subtype H1N1 that vary over three orders of magnitude.1-3

Similarly, literature on the common bacteriophage surrogate, MS-2, report doses between 46 and 120 mJ/cm2 to demonstrate 4-log inactivation.3,4  IUVA’s experience with UV disinfection of adenovirus demonstrated UV doses between 120 mJ/cm2 and 240 mJ/cm2 were needed to achieve 4-log inactivation.3,5  Nevertheless, the Centers for Disease Control and Prevention (CDC), National Institute for Occupational Safety and Health (NIOSH), Environmental Protection Agency (EPA) and other public and private entities promote use of UV disinfection for items ranging from personal protection equipment (PPE) (e.g., N95 masks) to medical facility surfaces including ICU and surgical suites and ambulances to drinking water.6-9

The public routinely uses UV consumer products created, at best, with an incomplete understanding of how to ensure effective UV disinfection. Putting all of this into practical context, the global UV disinfection equipment market is projected to grow from $2.9 billion (USD) in 2020 to $5.3 billion (USD) by 2025.10 These variations point to the need for standardized UV validation protocols for UV technology when applied to air or surfaces to achieve disinfection.

Ultimately, the dose distribution will govern the efficacy of any UV disinfection system. For air disinfection, this will be governed by the interplay between fluid mechanics and the fluence rate field.  For surface disinfection, the interplay of the fluence rate field, the optical properties of the surface material, and surface texture (“shadowing”) are likely to govern the dose distribution. Surfaces also will have a variety of characteristics including hydrophobicity, photochemical or photocatalytic behavior, etc., that may influence surface disinfection efficacy. The validation of UV technology for air or surface disinfection can be broken into four critical areas:

  • the UV lamp system and proper choice and calibration of radiometric systems to measure the UV irradiance
  • the geometry or complex optics between the UV lamp system and the target pathogen and the ability to accurately model that behavior with state-of-the-art ray-tracing
  • the UV transmitting nature of the media between the UV lamp system and the target pathogen
  • the microbiological aspects of the system as they relate to the sensitivity of the target pathogen or suitable surrogate test organism(s) to UV dose and/or UV wavelengths when biodosimetry validation of UV systems is performed

UV lamp system

A wide variety of UV lamps can be used in commercial devices, including but not limited to low-pressure (LP) or low-pressure high-output (LPHO) mercury vapor lamps, medium-pressure mercury vapor lamps (MP), UV LEDs at varying wavelengths, excimer lamps and pulsed UV lamps. In addition, a validation protocol needs to provide flexibility and adapt to new lamps that will inevitably be developed and enter the marketplace in the future. The validation protocol must accurately identify the lamps in the system by specific maker and serial number to provide a baseline for reference. The protocol also must identify the lamp operating conditions (e.g., power and/or number of lamps in operation) to be tested.  Measuring the lamp output spectra and the resulting irradiance (fluence rate) at each wavelength in the appropriate 3D geometry being irradiated also is a critical part of validation. This process can be advanced and optimized through state-of-the-art numerical methods and ray tracing software use, providing that appropriate protocols are established and guidance given.

The resulting irradiance map is a feature that directly allows for calculation of the UV dose distribution delivered by the UV system as a function of residence time of the air or surface at a given location while also accounting for the factors previously mentioned, such as surface characteristics. Factors such as UV lamp aging and fouling also must be considered in a validation, and it may be necessary to isolate (filter) desirable wavelengths while preventing undesirable wavelengths. If a filtered UV source is a critical part of the success of a given system being validated, the protocol must address the characteristics of the filters being employed.

UV system optical geometry

UV disinfection devices and applications often involve a complex optical system that must be well characterized with respect to light transmission, reflection, refraction and absorption. Methods used to validate the optical system will depend upon the type of UV emitters – whether they be the conventional UV lamps or systems that involve varying arrays of UV LEDs, pulsed UV sources or plasma lamps. For the conventional UV lamps, which are essentially cylindrical emitters, the decrease in UV irradiance with distance has been well studied and can be adequately predicted for a simple case with optical corrections to the Keitz equation provided by Jaworowicz.11

These first principle approximations are useful when evaluating advanced modeling efforts involving ray tracing numerical methods. It is well understood that geometry is critical to ensure that the delivered irradiance and resulting UV dose distribution can be accurately predicted at all locations in the treated area. It is recommended that the well-developed ray tracing numerical methods be employed and calibrated with UV irradiance measurements and then periodically verified by irradiance checks during UV system operation.

An example of this approach for air disinfection, particularly for upper room air, is provided by Yang and others.12-14 In addition, for air disinfection system validation the ability to accurately model the air flow and mixing will play a critical role in calibrating and verifying models to predict the given UV dose distribution as function of time and space.

Significant complexity is introduced when developing validation protocols for UV disinfection of surfaces due to the wide array of potential surface textures and/or geometries of items that commercial and consumer UV disinfection products are used to disinfect. Original research presented by Jaffe et al.15 at the 2020 IUVA Americas Conference demonstrated the “Canyon Wall Effect.” Consider a minimally textured surface with “valley” depths only 1/10th of a human hair, or about 10 microns. The size of the SARS-CoV-2 virus is 0.15 microns. This is the equivalent of a supine person sunbathing in a canyon with 1,000-foot walls. Just as the morning sun cannot reach the canyon floor, UV applied perpendicular to the surface will not reach into the crevices of a textured surface, allowing germ survival. Jaffe et al.15 showed that UV-C from lamps oriented horizontally (parallel) over a horizontal surface were more than 500 times more germicidal than the same lamps oriented vertically (perpendicular) at the same UV dose, time and distance to the horizontal surface.

The study by Simmons, et al.16 reported effectiveness of a vertically oriented UV device in deactivating SARS-CoV-2 on test surfaces “placed vertically to be parallel with the lamp” at 1 m. Test surfaces placed horizontally to be perpendicular to the lamp, where viral particles are far more likely to land, would show hundreds of times less effectiveness, extrapolating the Jaffe et al.15 conclusions. The Simmons et al.16 publication did not report viral reduction data on horizontal test surfaces. Future device validation studies should include horizontal test surfaces to allow real-world interpretation and applicability.

In addition to these surface aspects, product geometries can range from car keys to face shields to N95 FFRs and beyond. It may well be impossible to choose a conservative surrogate item or product to validate a UV device for a wide array of applications, which would suggest that UV devices may need to be validated for the specific items they are going to be sold to disinfect.   

Given the immediate need to apply UV for disinfection of N95 FFRs and equivalents, developing a specific validation approach for those products will be the priority. This complexity lends itself to a biodosimetry approach using a suitable test organism or test organisms. The test organism(s) are used to develop UV dose response curves in a carefully controlled collimated beam experiment. These curves would be analogous to the typical standard curves developed for chemical analyses. Next, the biodosimetry approach would involve treating (inoculating) the item to be disinfected with an aerosolized test organism at a high enough titer to ensure the desired log inactivation can be experimentally determined.

The biodosimetry steps and protocols must be carefully specified and tested to ensure a worst-case scenario is being validated and to ensure that there is a specific, reproducible inoculation procedure that is practical and relevant. The initial number of organisms would be determined by control runs in which steps are taken to ensure all experimental conditions are held constant but a delivered UV irradiance and, hence UV dose, of zero or below detection is applied.

Carefully designed and analyzed controls are critical to the validity of the process and also will determine the minimum detectable performance since a certain log loss of viable organisms will be quantified during the control runs, and this must be accounted for when reporting overall UV system performance. The validation report for the UV system must report control results, experimental results and the measured UV log inactivation and/or percent inactivation for the specific organisms tested. Validation experience in the development of the EPA UVDGM17 suggests that a minimum of two representative organisms spanning a practical range of UV sensitivity be used in the validation study.

UV transmittance

In almost all UV applications, the UV transmittance (UVT) is a critical consideration. It is acknowledged in the literature that some prefer to work with the mathematically related UV absorbance. Conversion from one to another is relatively simple, and this paper will use UVT throughout.

From the moment the UV-C photon is generated, the consideration of UVT for each layer it passes through can be evaluated.  For practical simplicity, discussion during validation normally begins after the UV lamp’s internal components – coating on the glass and/or lamp sleeves – already have been accounted for in the initial measurements made by the UV radiometer readings recorded at the outer lamp/sleeve surface. The change in irradiance is impacted by the distance, the overall optical geometry and the UVT of the air can be accounted for by verifying the UV irradiance with a radiometer at the surface of the object to be irradiated.  Experimental conditions also should maintain a constant temperature and a constant relative humidity (RH) to avoid confounding the validation results.

Validation experiments, at a minimum, should be performed at a worst-case temperature and RH. The complexity increases once at the surface of the object since the UV photons need to next be transmitted through the surface deposit, such as the droplet, and, if applicable, through the different layers of a porous surface. Work by Baribeau18 and others has shown that deposits on the surface from substances such as saliva, health and beauty aid residues (e.g., cosmetics or sunscreen) or crystals of salt from dried perspiration can exert a large UV absorbance and significantly impact the effectiveness of UV disinfection.

The literature often provides a discussion of the importance of droplet size distribution in the aerosols deposited on a surface.8, 19 A limited number of studies also have indirectly addressed the importance of the chemistry or UVT of the droplet itself by varying the droplet composition from extremely pure laboratory water to a variety of natural or artificial saliva, beef extracts, etc.

The validation protocol should specify a reproducible approach to preparing the composition of the droplets. In some cases, preparing a high enough titer of the test organisms will result in a droplet that has a relatively low UV; this should be documented and accounted for in the testing report. Similarly, experiments have addressed different types of soiling or staining of the surfaces impacting UV transmission.

It is recommended that the validation protocol employ a standard approach to soiling the test surface to represent a range of conditions or a worst-case condition as a minimum. An efficient number of validation experiments should hold these independent variables constant during testing or vary them over an agreed upon practical range to account for effects on required UV dose needed to achieve a given log inactivation.

Microbiological aspects

Establishing the UV dose response and the UV action spectra20 for the target pathogen is critical, but the complexity of those efforts especially for novel pathogens may take considerable time due to BSL-3 hazards and other factors. Therefore, initial work is often performed using appropriate surrogates. Once reliable, statistically valid correlations have been established in laboratory studies between the target pathogen and several appropriate surrogates,

UV device validations typically employ the surrogates to reduce the risks to personnel and the costs of UV device validations. In addition, surrogates that can be inexpensively grown to high titers and easily handled often make it easier to do large-scale validations.

For target pathogens or surrogates, carefully controlled laboratory UV dose response experiments are often conducted at 254 nm (or other appropriate single wavelength) initially and then later, due to costs and complexity, the action spectra of the pathogen over the entire practical germicidal range (200 nm and 280 nm with some studies reporting the value of examining impacts of wavelengths as high as 300 nm to capture significant output of MP UV lamps or UV LEDs in that range).

UV validation literature suggests that it often is prudent to reduce uncertainty in RED determinations by using two surrogate organisms that tend to bracket the dose response range of the target pathogen. This approach coincides well with current FDA guidelines for verifying UV DECON of N95 masks that discuss 3-log inactivation of virus and 6-log inactivation of spores and mycobacteria.

Typical surrogates that have been used in UV validation include B. subtilis spores, a wide array of bacteriophages with MS-2 being used the most. Although somewhat counterintuitive, UV validations can induce significant RED bias if a surrogate is much more resistant to UV than the target pathogen.

This relates to the fact that the UV dose distribution of the device to be validated is the key characteristic that is being validated.

In testing of air and surfaces, there are practical experimental approaches related to statistical design of how and where to sample or the use of two surrogates that bracket the target pathogen UV sensitivity can be used to reduce RED bias. In either case, it is an important consideration to address in designing the validation procedure.

Action spectra and RED bias considerations emphasize the importance of carefully selecting test surrogates. Ideally, the test surrogates should have UV dose response and action spectra that closely mimic the target pathogen or using two surrogates that bracket its range.

Experience suggests that using viral surrogates is a better choice when the target organism is a virus rather than using bacterial spores as a surrogate for virus and vice versa.

Additional validation guidance

After conducting all necessary experimentation, the goal is to produce a concise validation report detailing the specific device, the materials and methods used in the validation, the range of operating conditions (UV dose, relative humidity, type of item or product treated) to achieve a given log inactivation of a particular target pathogen (e.g., viral inactivation credit). The report also should detail the conclusions, recommendations and limitations of the validation.

A critical item to be detailed in the report is what conditions or changes to the validated device would require the need for revalidation.

These would include items such as changing the type of product to be treated by the UV device, significantly changing the UV lamp types or arrangements, the reflective surfaces or the overall optical geometry of how the system is operated (for example changing the distance from the lamp surface to the surface of the item or product being disinfected). It has been well proven that UV dose exponentially decreases with distance from the UV lamp source.

This white paper was reviewed by a diverse group of International Ultraviolet Association (IUVA) members to ensure scientific accuracy and a fair representation of general consensus; however, it does not necessarily reflect a unanimous agreement from all IUVA members and it is intended to be a dynamic and living document responsive to the latest peer reviewed information available.

Chairperson: Jim Malley

Contributing members: Castine Bernardy, Hadas Mamane Steindel, Yoram Gerchman, Richard Simons, John Boyce, Matthew Hardwick, Ernest (Chip) Blatchley III, Sara Beck, Arthur Kreitenberg, Joel Ducoste and Benoit Baribeau.

REFERENCES

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